Contents Overview

Final Grant Report 2018: Virtual Planetary Laboratory at The University of Washington

Reporting Period: January 1, 2013 – December 31, 2018 PI: Victoria Meadows (UW)

Contents Overview ...... 2 Our Research: Year 1 ...... 3 Our Research: Year 2 ...... 8 Our Research: Year 3 ...... 16 Our Research: Year 4 ...... 25 Our Research: Year 5 ...... 35 Our Research: Year 6 (NCE) ...... 49 New Technology ...... 56 Publications (398 Total) ...... 57

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Overview

The Virtual Planetary Laboratory’s interdisciplinary research effort focuses on answering a single key question in astrobiology: If we were to find a terrestrial planet orbiting a distant star, how would we go about recognizing signs of habitability and life on that planet? This question is relevant to the search for life beyond our Solar System, as outlined in NASA's Astrobiology Strategy. VPL research addresses many of the Strategy Key Research Areas and questions including How can we enhance the utility of biosignatures to search for life in the Solar System and beyond (Section 5.4 Research Area II) and How can we identify habitable planets and search for life beyond the Solar System? (Section 5.4 Research Area IV).

Recent observations have brought us much closer to identifying extrasolar environments that could support life. The successful Kepler Mission has discovered most of the now over 3,500 known extrasolar planets – many of them smaller than twice the diameter of the Earth – and several residing in the habitable zones of their parent stars. Ground-based telescopes and the K2 mission are now finding potentially habitable planets around nearby M dwarfs, including a potentially Earth-mass planet in the habitable zone of Proxima Centauri, the Sun’s nearest neighbor, and the seven Earth- sized planet TRAPPIST-1 system, where three of these planets are likely in the habitable zone. These targets and others found by the Transiting Exoplanet Survey Satellite (TESS) mission – slated for launch in 2018 – will provide potentially habitable targets for the planned James Webb Space Telescope (JWST), which could probe the atmospheric composition of at least one habitable zone terrestrial planet. In the longer term, we anticipate large spaceborne telescopes, such as NASA’s LUVOIR or HabEx concepts, that can directly image and obtain spectroscopy of a larger sample of potentially habitable terrestrial extrasolar planets.

The VPL provides a scientific foundation for interpreting data from extrasolar terrestrial planet detection and characterization missions such as Kepler, K2, TESS, JWST and LUVOIR/HabEx. To provide this foundation, the VPL uses data from NASA's Earth observing and planetary exploration programs, and information from field and laboratory studies and Earth's geological history, to validate and develop more comprehensive models of terrestrial planets. These models allow us to simulate and explore the likely diversity of extrasolar planet environments in advance of the more challenging spacecraft observations. VPL's models can be used to understand the radiative and gravitational effects of stars on the planets that orbit them. Combinations of model, lab, and field work are also used to understand which biologically-produced gases can generate a detectable biosignature in globally-averaged planetary observations. Finally, models and instrument simulators tell us how best to extract planetary environmental parameters from astronomical observations that have no direct spatial resolution, and that may also be limited in sensitivity, spectral resolution and temporal sampling.

The team required to develop and run these models is necessarily large, and highly interdisciplinary. Our research encompasses single-discipline efforts that produce results pertinent to our overarching habitability and biosignatures focus, to highly interdisciplinary efforts where stellar astrophysicists, NAI Information Management System 2 of 91 planetary climate modelers, orbital dynamicists, atmospheric chemists and biologists work together to determine the effects of stellar radiation and gravitation on the habitability of terrestrial planets.

Our research can be divided into five main tasks: Solar System Analogs for Exoplanets, Early Earth, The Habitable Planet, The Living Planet and The Observer. The first four tasks explore known and simulated environments to understand the factors that affect habitability and the global impact of life on its environment, and to expand the plausible range of terrestrial planet environments – both inhabited and uninhabited. This knowledge can be used to help prioritize newly discovered potentially habitable planets for more-detailed observational follow-up, and to generate new biosignatures to be sought in planetary spectra. The fifth task uses the models and data generated and gathered in the first four tasks to assess the remote detectability of newly identified potential global signs of habitability and life.

Our Research: Year 1

Earth as an Extrasolar Planet.

In this task we use observations of our home planet to explore the detectability of signs of habitability and life on terrestrial planets. In collaboration with LCROSS mission scientists, this year Robinson, Meadows and Sparks performed a comparison of predictions from the VPL 3-D spectral Earth model with UV to infrared spectra of the Earth obtained by the LCROSS mission (Robinson et al., submitted). This comparison was used to validate our predictions of the detectability and spectral dependence of glint from the Earth’s ocean, and it also revealed an error in the spectral calibration of data from the LCROSS mission, which we were able to help correct. We also discuss using the UV Hartley band of ozone as a biosignature. This year we also added N2-N2 collisionally-induced absorption (CIA) to the VPL Earth model. The updated model has been used to demonstrate that N2- N2 CIA is required to fit the spectral region near 4.1um in Earth spectra taken by the NASA/EPOXI mission (Schwieterman et al., in prep). Detection of N2-N2 CIA in a planet’s spectrum can help constrain surface pressure and, thus, surface habitability. We also completed spectral libraries of the Earth’s appearance through a Lunar month as a simulated dataset for studies of observations of the Earth from a lunar platform.

Early Earth and Mars:

In this task we work to understand the early Earth and Mars environments, both of which serve as potential analogs for habitable environments unlike those seen on Earth today. In our early Mars studies on surface environments, David Catling and collaborators completed work on understanding the origin and abundance of carbonates on Mars (Niles et al., 2013), and the environmental implications of clay minerals (Ehlmann et al., 2013). Photochemical modeling of the formation of salts from the oxidation of gases in the atmosphere of early Mars was also performed (Smith et al., 2013). Ongoing efforts include work on the possibility of oceans on Mars and the formation of salts in the soil. Conrad participated extensively in work using instrumentation on the Curiosity Rover to NAI Information Management System 3 of 91

assess the current and past habitability potential for Mars (e.g. Mahaffey et al., 2013; Leshin et al., 2013). Kasting, Ramirez, Kopparapu, Robinson, Freedman and Zugger collaborated on studies of the warming of Early Mars by using CO2 and H2 (Ramirez et al., 2013). They modeled the origin, abundance and lifetime of CO2 in the early Martian environment, and investigated the possible abundance of H2 on early Mars, since H2 can act as a secondary greenhouse gas via collision-induced absorption.

For studies of the early Earth’s environment, we made progress in the areas of Earth’s geochemical history and its implications for life, climate evolution, and the evolution of atmospheric oxygen levels. Studying the Earth’s geochemistry, Buick and colleagues discovered a soluble and reactive phosphorus species (phosphite) in early Archean carbonates (Pasek et al., 2013) that was likely of meteoritic origin. Phosphite’s delivery during the Late Heavy Bombardment may have driven chemistry to form cell membranes and make nucleotides, a precursor to the RNA World hypothesis for the origin of life. Team members also explore the effectiveness of H2 as a greenhouse gas on the early Earth (Wordsworth and Pierrehumbert, 2013) with commentary on the paper from team member Kasting (Kasting 2013c). In understanding the history of O2 in the Earth’s atmosphere we outlined possible causes for the first rise of oxygen at ~2.5 Ga. (Kasting, 2013a, 2013b), and a framework for understanding both the first and second rise of oxygen was provided (Catling, 2013a, 2013b). Zahnle reviewed the importance of hydrogen escape for the oxygenation of the Earth’s atmosphere (Zahnle et al., 2013). Catling and Krissansen-Totton are currently re-examining the multi-billion year record of marine carbon and sulfur isotopes to derive optimal statistical estimates of their implications for oxygen fluxes into the atmosphere and ocean. Domagal-Goldman and Robinson are working to generalize our 1-D atmospheric chemistry and climate models and automate changes in boundary conditions. This will allow us to rapidly run the models for a variety of conditions, to quantify the effects of specific metabolism on the atmospheric evolution of the Earth.

The Habitable Planet

VPL’s core research is in the area of planetary habitability, and this task explores the effect of interactions between the potentially habitable planet, its star, other planets in the system, and the host galaxy. This year, exploration of these factors that affect habitability included work on planet formation, internal properties, atmospheric evolution, orbital dynamics, and the limits of the habitable zone. Raymond provided in-depth reviews of planet formation for the periodic international meeting Protostars and Planets 6. (Raymond et al., 2013a; Davies et al., 2013). He and his students also considered numerous accretional phenomena such as a final accumulation of volatiles (Raymond et al., 2013b), the distribution of dust left over from formation (Bosnor et al., 2013), and planetary migration (Pierens, et al., 2013). Additionally, Timpe, Barnes, Kopparapu, Raymond and colleagues demonstrated that the observed orbital dynamics of multi-planet systems naturally arises from the ejection of fully formed planets at the very end of planet formation (Timpe et al., 2013).

VPL also modeled the environments at the planetary system-galaxy interface. These regions are far from the host star and hence features can be more easily detected as the starlight can be separated. We showed that systems that eject planets can have cometary reservoirs that are closer in than that in NAI Information Management System 4 of 91

the Solar System (Raymond & Armitage, 2013). We also showed that distant stellar companions can disrupt planetary systems on long timescales after close stellar passages perturb the binary star’s orbit (Kaib et al., 2013). Ongoing work by Deitrick and Barnes explored the stability of planets on mutually-inclined orbits (Dietrick et al., in prep).

We also explored the direct effect of the star’s gravity on planetary habitability for a number of different scenarios. In Barnes et al (2013), Barnes, Goldblatt, Meadows, Kasting and colleagues showed that tidal heating can trigger a runaway greenhouse on planets in the habitable zones of M dwarf stars, and especially for those that have planetary companions that can maintain the planet’s orbital eccentricity. A "tidal greenhouse" can be maintained for upwards of 1 Gyr prior to circularization and hence planets can evolve into the habitable zone after losing all their primordial water content. Barnes & Heller (2013) showed that a tidal-heating generated greenhouse is also possible on planets in the habitable zones of white dwarfs and brown dwarfs. For these planets, eccentricities as small as 10-6 can be sufficient to trigger the runaway greenhouse, severely limiting the chances that they may support life. Heller & Barnes (2013) showed that exomoons are also in danger of a tidal greenhouse. Luger and Barnes explored the evolution of atmospheric loss for small gas-dominated planets under the combined effects of stellar radiation and orbital evolution (Luger et al., in prep). Armstrong, Barnes, Domagal-Goldman and Meadows worked together on understanding the coupled effects of obliquity and orbital evolution on planetary climate and surface ice coverage (Armstrong et al., submitted).

VPL also looks at internal planetary processes and how these may impact habitability, and in turn be impacted by life. This year Driscoll wrote a numerical program to model the internal evolution of the mantle and core of a rocky terrestrial planet. This model includes radioactive heat generation, heat loss due to mantle melting, inner core growth, and magnetic field generation in the outer core. Work is underway to add tidal heating to explore the effect of this process on outgassing and habitability. Sleep et al., (2013, submitted) discuss the state of the mantle and crust soon after the moon-forming impact, the influence of lunar induced earth-tides on these zones, and the fate of CO2 in the mantle and atmosphere in the Hadean. Sleep and colleagues also looked at the partitioning of radionuclides in the Earth’s mantle, crust and hydrosphere as the result of biologically mediated geochemical evolution (Sleep et al., 2013). Bolton is working on code efficiency improvements for quantifying CO2 draw-down from the atmosphere by weathering of soils derived from idealized granite and basalt rock types. This modeling is being done to find the influence of atmospheric composition, temperature, and infiltration rates on CO2 consumption.

We also continued to refine the climatic limits of planetary habitability and the habitable zone. Walkowicz continued work assessing flare rates in Kepler data for potential planetary host stars. Kopparapu, Ramirez, Robinson, Kasting and collaborators updated and validated a 1-D radiative- convective climate model with new water and carbon-dioxide absorption coefficients to recalculate habitable zone limits around stars of different spectral types (Kopparapu et al., 2013). Using these results Kopparapu estimated the occurrence rate of Earth-size planets around Kepler M-stars to be between 40%-50% (Kopparapu, 2013). Shields, Meadows, Bitz, Pierrehumbert and collaborators studied the near-IR ice-albedo effect on planets in the middle of the habitable zone of M-stars, and NAI Information Management System 5 of 91

found that planets orbiting M stars are more stable against sustained low-latitude glaciation than planets orbiting F or G stars (Shields et al., 2013). Goldblatt, Robinson, Zahnle and Crisp also used updated spectral databases to calculate a new limit for a planet undergoing a runaway greenhouse, and showed that the Earth is closer to this limit than previously thought (Goldblatt et al., 2013). Claire and colleagues recast the habitable zone equations to take into account stellar evolution, and used estimates of stellar age to obtain improved determinations of the habitable zone for known potentially habitable planets (Rushby et al., 2013). Pierrehumbert and colleagues continued to work on development of a generalized 3-D GCM model for exoplanet atmospheres.

The Living Planet

In this research area, VPL team members use modeling, laboratory and field work to understand the co-evolution of the biosphere with its environment, and the limits of life. Kiang, Mielke and colleagues continued testing the photon use efficiency of the bacterium Acarychloris marina, which photosynthesizes at longer than usual wavelengths, serving as an example of an organism adapted to the longer wavelength light environments that might be found around M dwarf stars. They used models to identify the likely trap wavelengths (Mielke et al., 2013) and further work being prepared for publication has confirmed dips in efficiency between the trap wavelengths. Blankenship and Kiang conducted field expeditions to gather further long-wavelength-using cyanobacteria, which are currently being cultured. Hoehler and Parenteau are initiating studies of the impact of anoxygenic photosynthesis on its environment, Parenteau conducted field reconnaissance to Yellowstone to locate anoxygenic mats that will subsequently be analyzed for biogenic gases and volatile organics. A new anaerobic chamber has been purchased and installed at NASA Ames and small vessels are being developed to allow testing of the mats under different environmental parameters such as starting atmosphere and radiation environment. The Biological Pigments Database of the VPL Spectral Library was also officially launched this year and is intended for community use: (http://vplapps.astro.washington.edu/pigments). Seifert continues to study the biology of Cuatro Cienegas, in collaboration with the NAI ASU team and astrobiologists in Mexico. They conducted two field trips last year with several papers in progress. Conrad participated in field work to sulfur caves to assess gas isotopic biosignatures. Black and Keller performed laboratory studies that suggest that the joining of RNA and fatty acids to form the first cells may have been assisted by a natural chemical affinity between these components (Black et al., 2013).

The Observer

In this task we explore the detectability of signs of habitability and life for modeled observations from the previous tasks. We also observe and retrieve environmental properties of Solar System planets and exoplanets, and generate improved retrieval algorithms for exoplanet data.

This year Misra, with Crisp and Meadows, completed modification of the VPL’s line-by-line radiative transfer model (SMART) to generate transit transmission spectra (Misra et al., in prep). The model includes gas absorption, cloud and aerosol extinction, refraction, and the effects of stellar limb darkening. The model has been validated against ATMOS limb spectra of the Earth and lunar eclipse NAI Information Management System 6 of 91

spectra, which provide a proxy for Earth seen in transmission. We find that the inclusion of refraction decreases the detectability of spectral absorption features in transit transmission, and that this effect is dependent on atmospheric composition, the size of the star, and the planet-star distance (Misra et al., in prep). We have used the model to show that simultaneous measurements of the absorption features from the O2-O2 dimer molecule and molecular oxygen (O2) can be used as a new technique to probe planetary atmospheric pressure for oxygenated atmospheres and biosignatures (Misra et al., 2013, in press).

In other biosignature research, Domagal-Goldman, Segura, Claire and Meadows completed a study into the generation of abiotic false positives for ozone for early Earth-type planets in orbit around M dwarf stars (Domagal-Goldman, in prep). Segura and colleagues calculated a possible maximum methane production for the abiotic process of serpentinization and for a planet of Earth-like composition (Guzmán-Marmolejo et al., 2013). Schwieterman and Meadows collaborated with Cockell of the UK Center for Astrobiology to explore the detectability of non-photosynthetic pigments, especially for halophiles, in an Earth-like planet’s disk-averaged spectra (Schwieterman et al., in prep). Catling, with Robinson and Krissanson-Totton initiated a study of thermodynamic disequilibrium in planetary atmospheres. A first step in understanding physical entropy in planetary atmospheres resulted in a paper that improved our generalized understanding of the origin and maintenance of atmospheric vertical temperature and pressure structures (Robinson and Catling, 2013). This understanding can also be used as an initial framework to improve retrieval of exoplanetary atmospheric properties and structure.

In planetary observation research, observations of Venus, an analog for a hot, haze covered exoplanet, were obtained by Arney and Meadows at the Apache Point Observatory. These observations were used to produce the first simultaneous spatially-resolved maps of retrieved H2O, HCl, CO, OCS, and SO2 abundances in the Venusian lower atmosphere, revealing surprising hemispherical dichotomies that are still unexplained, but that may be due to cloud processes (Arney et al., in prep). Gao, Yung, Crisp and colleagues validated a generalized 1-D microphysical and vertical transport cloud model for use in the VPL 1-D Climate Model against Venus data and revealed an oscillatory “rain out” of the Venus clouds in the process (Gao et al., 2013, in press). Bailey, Agol, Barnes, Raymond and collaborators continued their work searching for extrasolar terrestrial planets using radial velocity surveys and Kepler data and discovered some of the most potentially habitable planets to date, as well as a unique system with definitely 3, and possibly 5 planets in the habitable zone of its parent M dwarf star (Agol et al. 2013; Anglada-Escude et al., 2013). Deming, Agol, Dobbs-Dixon and Wilkins used HST to observe transiting giant exoplanets with a new spatial scanning mode, greatly improving the observational sensitivity (Deming et al., 2013). They detected water vapor in several of these planets and are now extending the new technique to smaller planets. Deming and Sheets pioneered a new statistical technique to study the atmospheres of super-Earth planets discovered by Kepler, which would otherwise be inaccessible to observation. Line, Crisp and Yung developed, tested, and published the relative performance of three commonly used remote sensing retrieval algorithms (optimal estimation, Markov-Chain Monte Carlo, and Bootstrap Monte Carlo) for interpreting realistic, synthetic spectra of exoplanets. (Line et al., 2013). NAI Information Management System 7 of 91

Education and Public Outreach

We completed our interactives “Extreme Planet Makeover” where users get to change parameters to control the appearance and habitability of their world, and a second interactive Eyes on Exoplanets 3D to allow users to visualize the position of known planetary systems on the sky, and learn about their potential habitability. Looking forward to outreach products under the new award, we have organized two Science Café experiences where VPL scientists (Rory Barnes and Aomawa Shields) present to and mingle with the public in bars and cafes. The theme for our two Science on a Sphere shows at the Pacific Science Center in Seattle have been determined, they are The Earth Through Time and Signatures of Habitability and Life, and work is underway to develop these shows in collaboration with VPL scientists. The UW VPL contingent hosted the Lakewood High Astrobiology class again this year for a field trip to UW to learn about astrobiology research. Several of our scientists again gave public lectures this year, Lucianne Walkowicz pioneered Science Train, an interaction between scientists and the public in the New York subway, and VPL research was featured in numerous popular science magazines, newspapers and television documentaries.

Our Research: Year 2

Solar System Analogs for Extrasolar Planet Observations.

In this task we use observations of our home planet and other planets in our Solar System to explore the detectability of signs of habitability and life on terrestrial planets. In collaboration with LCROSS mission scientists, this year Robinson, Meadows and Sparks published a paper comparing predictions from the VPL 3-D spectral Earth model with UV to infrared spectra of the Earth obtained by the LCROSS mission (Robinson et al., 2014). This comparison was used to validate our predictions of the detectability and spectral dependence of glint from the Earth’s ocean, and it also revealed an error in the spectral calibration of data from the LCROSS mission, which we were able to help correct. We also discuss using the UV Hartley band of ozone as a biosignature. We completed models of the Earth as seen from the Moon through a lunar month. Schwieterman, Meadows, Misra and Crisp demonstrated the first detection of the nitrogen dimer (N2-N2) in the Earth’s disk-averaged near- infrared spectrum, and showed that it produced a 40% modification of the spectrum. This band may be the only way to complete the inventory of atmospheric bulk gases for extrasolar terrestrial planets. In another exoplanet observation analog, Robinson led a paper that used Cassini occultation observations of Titan to simulate a transit transmission observation of a haze-enshrouded world. The observations showed a strong slope in the spectrum due to the hydrocarbon haze, and these data were combined with our existing knowledge of the Titan atmosphere to quantify the atmospheric depths probed in transit transmission (Robinson et al., 2014). Arney, Meadows, Crisp and Bailey published work on the first simultaneous near-infrared spectral mapping observations of the Venus atmosphere, an analog for a hot, haze-covered planet (Arney et al., 2014). These observations were used to produce simultaneous spatially-resolved maps of H2O, HCl, CO, OCS, and SO2 abundances in the Venusian lower atmosphere, revealing unexpected dichotomies, and showing spatial NAI Information Management System 8 of 91

correlations that were indicative of chemical interactions between several atmospheric species. Gao, Yung, Crisp and colleagues validated a generalized 1-D microphysical and vertical transport cloud model for use in the VPL 1-D Climate Model against Venus data, and revealed an oscillatory “rain out” of the Venus clouds in the process (Gao et al., 2014).

Early Earth and Mars:

In this task we worked to understand the early Earth and Mars environments, both of which serve as potential analogs for habitable environments unlike those seen on Earth today. Atmospheric chemistry models were used to explore the formation of surface salts through the oxidation of volcanic gases, and the redox state of the early atmosphere due to volcanic outgassing. Assuming past volcanic outgassing in the expected range, we found that the early Martian atmosphere could have been anoxic, weakly reducing, and CO-rich. These conditions are favorable for an origin of life, and may leave geochemical traces in the soil (Sholes et al 2015). Work also tested the hypothesis that the perchlorate, sulfates and nitrates seen by the Phoenix Lander were produced from atmospheric deposition over the last 3 billion years (Smith et al 2014). Although atmospheric deposition works well for sulfates, we found that gas phase reactions for making perchlorates are insufficient to explain levels of ~0.5wt% in the soil sampled by Phoenix. Thus, unknown gas-solid reactions are likely required. Work was also published that places such results in the broader context of the evolution of Mars and the general behavior of planetary atmospheres (Catling 2014, Catling 2015, Haberle et al 2015). Conrad and Domagal- Goldman developed models that account for the evolution of the Martian interior and how the process of differentiation works with interior cooling to off-gas volatiles, and subsequent processes that may lead to atmospheric escape. This work was used to interpret results from MSL and in reviews of MSL science (Conrad 2014, Mahaffey et al., 2014, Webster et al., 2014). In laboratory work, Toner, Catling and colleague studied the properties of super-cooled brines, which may have existed on early Mars, to determine how much salt solutions will supercool and remain amorphous (Toner et al., 2014a,b). They found that perchlorate-rich (ClO4) salt solutions tending to supercool down to -120°C - much more than analogous chlorides and sulfates - and then transition into an amorphous glass. Such glasses could potentially preserve organic molecules and structures, such that organisms can remain viable upon rewarming. Improvements to models and additional laboratory data were also used to identify a suite of potentiall habitable brines (Toner et al., 2015a).

For studies of the early Earth’s environment, we made progress in the areas of Earth’s atmospheric and geochemical history and its implications for life, climate evolution, and the evolution of atmospheric oxygen levels. Stüeken and Buick conducted N, S and Se isotope analyses to investigate the oxygenation state of early Earth environments. They discovered extreme nitrogen isotope enrichments, that indicated that lakes in the Archean may have been significantly alkaline, and by comparison, the ocean was not as alkaline as previously thought (Stüeken et al., 2015). This high pH will affect the free energy for chemosynthetic energy pathways, and this new information on the Archean environment will be included in energy-based ecosystem models currently under development by our team. Claire, Buick, Domagal-Goldman, Kasting and Meadows increased the spectral resolution of our photochemical models to explore UV effects on S isotope fractionation to reassess hypotheses for the preservation of these signals in the rock record (Claire et al., 2014). NAI Information Management System 9 of 91

Updated k-coefficients were incorporated into our climate models to show that at low atmospheric pressures, the addition of H2 to the early Earth atmosphere has a negligible effect on warming, and to recalculate the limits of the radiative habitable zone. Byrne and Goldblatt (2014) developed a library of forcings for different greenhouse molecules applicable to studies of early Earth and Mars.

We reconsidered haze formation in the Archean, and computed the first self-consistent solutions for Archean Earth’s atmospheric chemistry and climate with a fractal hydrocarbon haze (Arney et al 2014a,b,c; Arney et al 2015). These models indicated that the fractal haze covered Earth could remain habitable, even under a fainter early Sun, and that the haze provided a significant surface UV shield in the pre-oxygenated atmosphere. Transit transmission models of these hazy Archean environments show a strongly sloped spectrum similar to that seen for Titan. We also explored haze formation for planets orbiting stars of different spectral type (Domagal-Goldman, 2013) and found that hazes did not form at very high or low UV fluxes. Charnay analyzed hot climates of the late Hadean/early Archean with high amounts of CO2 and explored the climate and carbon cycle for these hot early Earths.

Catling and Krissansen-Totton re-examined the multi-billion year record of marine carbon and sulfur isotopes to derive optimal statistical estimates of their implications for oxygen fluxes into the atmosphere and ocean (Krissansen-Totton and Catling, 2014). They show that fractional organic carbon burial has increased in the last 3.6 Gy by a factor from 1.2-2, with the larger value consistent with the rise of oxygen in the Proterozoic. Higher fractionation in the later Archean may be consistent with the advent of photosynthesis at 2.8 Gya or earlier. Zahnle and Catling (2014) considered the 0.5 Gy gap between the advent of oxygenic photosynthesis and the rise of O2, exploring possible explanations for the delay, including the rate of hydrogen escape to space, and the size of the reduced reservoir that needed to be oxidized before O2 became favored. They also discussed how hydrogen escape may have been linked to the history of continental growth. Domagal-Goldman wrote a pair of review papers exploring the geochemical constraints that make higher- and lower-than-accepted amounts of O2 in the Archean unlikely, and that argue that parts of the Earth system, including the ocean, may track redox states unrelated to atmospheric proxies (Domagal-Goldman, 2014).

The Habitable Planet

VPL’s core research is in the area of planetary habitability. This VPL task explores habitable planet formation, and the effect on planetary habitability of interactions between the potentially habitable planet, its star, other planets in the system, and the host galaxy.

(Jacobson et al. 2014), and the formation of known systems such as Kepler-186 (Bolmont et al. 2014), which houses the first Earth-sized planet found in the HZ.

To explore the star’s gravitational influence on planetary habitability, and in particular how planetary orbital, rotational and climate evolution are coupled, Armstrong, Barnes, Domagal-Goldman, Meadows and colleagues studied hypothetical systems with large relative inclinations between planetary orbital planes (Armstrong et al. 2014). They found that planets that experience strong orbital and rotational evolution could support habitable conditions at larger stellar distances than planets with more stable orbital and spin configurations. Dietrick, Barnes, Quinn, Luger and colleagues used orbital stability models to reveal the full 3-dimensional orbital architecture of the Upsilon Andromedae system, and showed that a potentially habitable planet in this system would have its orbit strongly perturbed by the mutual inclination of its companions (Deitrick et al. 2015). Kress and colleagues used stability arguments to investigate the plausibility of an Earth-sized planet existing in the habitable zone of the known multi-planet system Gl 581 (Joiner et al., 2014). Barnes and colleagues discovered that terrestrial exoplanets in mean motion resonances with non-planar orbits can evolve chaotically for at least 10 Gyr. In extreme cases, the simulated orbital eccentricities grow so large that planets would graze the host star, compromising habitability (Barnes et al. 2015). Barnes also showed that tidal theory applied to a large ensemble of transiting planets (e.g. Kepler data) could reveal the boundary between rocky and gaseous exoplanets based on the different tidal circularization timescales experienced by these different types of bodies (Barnes 2015).

Looking to the effects of companion planets and moons, Barnes and colleagues showed that a wide range of companion properties can sustain modest tidal heating in potentially habitable exoplanets for more than 50 billion years, offering a stable energy source that could sustain geochemical cycles conducive to life (van Laerhoven et al., 2014). Barnes and colleagues also explored the respective roles of tidal heating and planetary radiation on the habitability of exomoons (Heller & Barnes, 2014) and contributed to an invited review on exomoon habitability (Heller et al. 2014). Barnes, Dietrick, Luger and Quinn worked on the development of the VPLanet model architecture, which can calculate the coupled effects of orbital and spin/obliquity evolution of a planetary system much more quickly than standard N-body algorithms. This new model will allow for efficient calculation of constraints on planetary orbits for newly-discovered potentially-habitable planets.

VPL team members also studied the effects of stellar radiation on planetary habitability and evolution. Kopparapu, Domagal-Goldman, Kasting and colleagues used climate models to calculate the habitable zone boundaries for planets of different masses. They found that the habitable zone moves outward for smaller mass planets due to the enhanced greenhouse effect from an increased water column depth (Kopparapu et al., 2014). Kopparapu, Domagal-Goldman and collaborator determined the occurrence rate of potential Venus analogs from Kepler data, and showed that this type of planet is more likely to be common around more Sun-like - rather than smaller - stars (Kane et al., 2014). Shields, Bitz, Meadows and colleagues used a hierarchy of climate models to explore of the effect of a star’s spectrum on the rate at which a planet can exit a snowball state. They found that planets orbiting M-dwarf stars require a smaller stellar flux to initiate deglaciation and melt out of a snowball state than planets orbiting hotter, brighter stars (Shields et al. 2014). NAI Information Management System 11 of 91

Wordsworth and Pierrehumbert undertook a range of calculations to explore water photolysis and hydrogen loss for terrestrial exoplanets, and found that CO2 can only cause significant water loss by increasing surface temperatures over a narrow range of conditions. They concluded that many "Earth-like" exoplanets in the habitable zone may have ocean-covered surfaces, stable CO2/H2O-rich atmospheres, and high mean surface temperatures (Wordsworth and Pierrehumbert, 2013). Pierrehumbert and Ding continued to work on development of a generalized 3-D GCM model for exoplanet atmospheres. Charnay and Wordsworth explored possible solutions to the faint young Sun problem, and the possible climates of the Archean Earth with a 3-D GCM (Charnay et al., 2013). Work by Hawley and colleagues (Hawley et al., 2014; Davenport et al., 2014) characterized the frequency and characteristics of stellar flares on M dwarf stars from the Kepler data. These data will feed into future work on characterizing the effect of stellar flares on habitability.

In pioneering work on star-planet interactions and planetary evolution, Luger and Barnes (2015) showed that the extended pre-main sequence super-luminous phase of an M-dwarf can lead to strong atmospheric escape of water, and dessication of terrestrial planets that form within the star’s main sequence habitable zone. This result has important implications for the search for habitable worlds using transit transmission. Luger, Barnes, Meadows and colleagues also used coupled atmospheric escape and tidal orbital evolution models to illustrate how the pre-main sequence evolution of M- dwarfs could strip the gaseous envelopes from migrating mini-Neptunes, transforming them into potentially-habitable, Earth-sized rocky bodies (Luger et al., 2015). Together, the papers imply that habitability is more likely for planets that have migrated into the habitable zones of M dwarf stars, rather than on those that formed in the HZ.

VPL researchers also explored how internal planetary processes may impact habitability. Sleep and colleagues discussed the state of the mantle and crust soon after the moon-forming impact, the influence of lunar induced earth-tides on these zones, and the fate of CO2 in the mantle and atmosphere in the Hadean (Sleep et al., 2014). Sleep and Lowe (2014) examined the physics of crustal fracturing and dike formation triggered by a large meteorite impact in the Barberton greenstone belt, South Africa and speculated on the formation of habitable fracture in the Earth’s crust. Barnes and Kopparapu participated in the ASU NAI Stellar Stoichiometry Workshop Without Walls and contributed to the conference’s review paper (Young et al., 2014). Driscoll and Barnes explored thermal-orbital planetary evolution with the coupling of orbital tidal evolution and interior models, showing different rates of circularization for inner and outer edge HZ M dwarf planets that result in either an early burst of tidal heating or an extended influx tidal energy for much of the lifetime of the planet. Bolton continued work on weathering models to understand the effects of weathering in low oxygenation states that may have been present in the Proterozoic.

The Living Planet

In this research area, VPL team members use modeling, laboratory and field work to understand the co-evolution of the environment and biosphere, and aspects of life’s global impact that could potentially be detected remotely as biosignatures. Recent work has focused on understanding the long-wavelength limit for photosynthesis, which is relevant to haze enshrouded planets such as the NAI Information Management System 12 of 91

early Earth, and for planets orbiting red M dwarf stars. Kiang, Parenteau, Blankenship and Seifert continued field and laboratory experiments to isolate new strains of far-red utilizing oxygenic photosynthetic organisms, and to understand their natural light regime. Kiang undertook field work with the far-red light utilizing Acaryochloris marina in the Salton Sea. Kiang and Parenteau collected red algae samples at three sites on the California coast and isolated the longer-wavelength Chlorophyl d. Seifert analyzed samples taken from Cuatro Cienegas, where freshwater stromatolites are found, and found evidence for Acaryochloris. Additional samples for culture in Blankenship’s lab are being sought. Field measurements of the light environments for these organisms will also be used to drive kinetic models of photon energy use to ascertain light thresholds of survival. Kiang published a white paper exploring theoretical challenges in quantifying the long wavelength limit of oxygenic photoysynthesis in a special issue on Astrobiology in The Biochemist magazine (Kiang, 2014).

Parenteau, Hoehler, Kiang and Blankenship measured the reflectance spectra of pure cultures and field in situ samples of a variety of anoxygenic photosynthesizers, searching for spectral signatures. They found a rise in reflectivity just past absorption maxima for bacteriochlorophyll pigments. This is a near-infrared (NIR) bacterial analog to the vegetation red edge, that is comparable in magnitude to that found in vegetation (Parenteau et al., 2014). Some of these measurements were used as input to models of the Archean Earth, to test for the detectability of anoxygenic photosynthesizers on a global scale (Palle et al., 2014). Future work on these samples includes measurement of biogenic gases and volatile organics for mats housed under different environmental parameters such as starting atmosphere and radiation environment. Schwieterman and Meadows collaborated with Cockell of the UK Center for Astrobiology to conduct an interdisciplinary study of the diversity and detectability of non-photosynthetic pigments as biosignatures, especially for halophiles (Schwieterman et al., 2015). This study included reflectance spectra measurements of a diverse collection of pigmented non-photosynthetic organisms and an analysis of the remote detectability of analogs of these organisms in disk-averaged spectra, with a planetary environment that includes other surfaces, an atmosphere and clouds. These spectra will be made available through the existing VPL Pigment Database.

This year the VPL Team also explored the generation of false positives for the oxygen atmospheric biosignature, and ways in which such a false positive could be identified and discriminated from a true biosignature. Domagal-Goldman, Segura, Claire and Meadows published a study into the generation of abiotic false positives for ozone for early Earth-type planets in orbit around M dwarf stars (Domagal-Goldman et al., 2014). This work took a conservative approach to the conditions for false positive production, yet still produced some model atmospheres with detectable amounts of abiotically-generated O3. Wordsworth and Pierrehumbert (2014) explored the potential buildup of photolytically produced O2 in the upper atmosphere of planets with a low abundance of non- condensible atmospheric species, such as N2. These less-dense atmospheres are less likely to trap water vapor near the surface, and if water escapes to the stratosphere then it can be photolyzed to produce O2. Another possible mechanism to photolytically generate abundant oxygen could occur on planets orbiting M dwarfs due to the high FUV/NUV flux ratio of some M dwarfs in comparison to that of Sun-like stars. VPL team members Gao and Yung explored the stability of CO2 under this incident UV spectrum and showed that a CO2-dominated atmosphere can be converted into a CO2 NAI Information Management System 13 of 91

3 4 /CO/O2 -dominated atmosphere in 10 -10 years by CO2 photolysis. Our results indicate that it is unlikely that CO2 atmospheres can remain stable on terrestrial planets around M dwarfs with high FUV/NUV flux ratios. Luger and Barnes (2015) explored the pre-main-sequence evolution of terrestrial HZ planets under the influence of super-luminous young M-dwarf stars, and also discovered a potential abiotic oxygen source. Pre-main sequence M dwarfs are powered by nuclear reactions in the core as well as energy of contraction as they slowly collapse to their main-sequence radius. For these low-mass stars, the process can be extremely slow, lasting over a billion years, and subjecting planets that form in the main-sequence HZ to extended periods of high radiation. Luger and Barnes showed that this can result in the potential loss of several oceans of water, and the photolysis of this escaping water abiotically generates a dense, O2 rich atmosphere. Schwieterman, Meadows, Domagal-Goldman, Robinson, Misra, Crisp, Luger, Barnes and Arney are now working on possible spectral discriminators for these different scenarios, including looking for CO, CH4 and O4 in the planetary spectra.

We also worked on two projects this year to understand how to quantify disequilibrium biosignatures, and to develop a coupled environment-ecosystem model that can be used to predict the magnitude and type of biosignatures for a range of different planetary environments. Krissansen-Totton, with Catling and Robinson, developed a method to quantify the degree of thermodynamic disequilibrium in planetary atmospheres, as a means of detecting a potential biosignature. They found that the Earth’s atmosphere-ocean disequilibrium was maintained by biology, and was an order of magnitude larger than for any other Solar System atmosphere (Krissansen-Totton et al., 2015). Hoehler, Domagal-Goldman, Som, Kasting and Meadows have begun modeling chemosynthetic-based biospheres by completing the coupling and benchmarking of two models: one model codifies a thermodynamic energy balance concept for habitability (Hoehler, 2009); the other model predicts a planet’s atmospheric composition and climate while balancing the redox state of the surface environment (Domagal-Goldman et al., 2014). These models will allow an ecosystem to be coupled into an interactive planetary environment including atmospheric, oceanic and subsurface components. This will allow us to predict standing biomass size, net biological productivity, and resulting gas fluxes to and from a biosphere (and therefore the nature and magnitude of any potential biosignature) with a given set of volcanic and hydrothermal inputs to the planetary surface.

The Observer

potential planet variability, which are being interpreted with the help of 3D simulations (Dobbs- Dixon and Agol, 2013). They are now extending these new technique to smaller planets.

Misra, Meadows and Crisp completed modification of the VPL’s line-by-line radiative transfer model (SMART) to generate a state-of-the-art transit transmission model that includes the effects of gas absorption, cloud and aerosol extinction, refraction, and the effects of stellar limb darkening. The model has been validated against ATMOS limb spectra of the Earth and lunar eclipse spectra. We have used the model to show that simultaneous measurements of the absorption features from the O2-O2 dimer molecule and molecular oxygen (O2) can be used as a new technique to probe planetary atmospheric pressure for oxygenated terrestrial atmospheres and biosignatures (Misra et al., 2014a). We have shown that inclusion of refraction decreases the detectability of spectral absorption features in transit transmission, and that this effect is dependent on atmospheric composition, the size of the star, and the planet-star distance (Misra et al., 2014b), and we have postulated that refraction effects in transit transmission observations could be used to discriminate between planets with and without clouds (Misra et al., 2014c). Work is currently underway on a publication that describes the effects of scattering on terrestrial exoplanet spectra.

Deming and Sheets used the predictions from these models to begin searching for refracted light near transit, to estimate the degree of cloud coverage and scale height of super-Earth atmospheres as an important precursor to JWST spectroscopy during transit. Deming and Sheets also measured the albedos of small transiting exoplanets by coadding Kepler data for each planet class, to search for secondary eclipses. Observations for planets of similar radius were grouped and transformed to a common orbital phases scale so that data from multiple planets could be coadded (Sheets and Deming, 2014). They were able to detect reflected light from close-in super-Earth planets between one and two Earth radii and conclude that these objects have a relatively low albedo, indicating significant absorption in their atmospheres or surfaces. Kopparla and Yung have undertaken a modeling study using the vector radiative transfer model VLIDORT to study the phase space of expected atmospheric composition and the observable polarization signal for a range of potential exoplanets (Kopparla et al., 2014). Line, Crisp and Yung developed, tested, and published the relative performance of three commonly used remote sensing retrieval algorithms (optimal estimation, Markov-Chain Monte Carlo, and Bootstrap Monte Carlo) for interpreting realistic, synthetic spectra of exoplanets. (Line et al., 2013) and used the retrieval algorithms to search for chemical disequilibria in observations of exoplanetary atmospheres (Line &Yung, 2013) and to undertake a systematic retrieval of secondary eclipse spectra of nine planets to determine their C/O ratios. Lustig-Yaeger analyzed existing exoplanet spectra to determine that broadband observations were often inadequate for detection of molecular species, but that their accuracy could be enhanced with even a small section of spectroscopic data (Lustig-Yaeger et al., 2015). Lustig-Yaeger, Meadows and Crisp are currently developing VPL’s retrieval model, which is at the core of the proposed fifth task. This model will combine VPL’s existing forward models of planetary environments developed by Crisp, Robinson, Misra and Meadows, with instrument models for direct imaging and transit transmission missions, obtained or developed by Meadows and Deming, and the sophisticated retrieval models developed by Line. The resulting model will also use all known

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information about the planet and planetary system to more robustly determine the family of environmental conditions that best fit the observed planetary spectrum.

Education and Public Outreach

This year we completed the prototype for our first Science on a Sphere show in collaboration between the VPL science team and the Pacific Science Center in Seattle. This show uses the modern Earth as a setting for understanding Signatures of Habitability and Life, and Harnett, Robinson and Meadows served as the principal liaisons with the PSC for this EPO product. Barnes and Shields delivered Science Café experiences, presenting and mingling with the public in bars and cafes. Work was initiated on our database for educational materials for teaching undergraduates astrobiology, led by Harnett. The UW VPL contingent hosted the Lakewood High Astrobiology class again this year for a field trip to UW to learn about astrobiology research. Several of our scientists again gave public lectures this year, and we also hosted public lectures at UW on the latest Kepler results and the origin of life. VPL research was featured in numerous popular science magazines, newspapers and television documentaries, including a feature article in the Atlanitic, which highlighted work by Meadows, Barnes and Kiang.

Our Research: Year 3

Solar System Analogs for Extrasolar Planet Observations

In this task we use observations of Solar System planets and moons to explore the detectability of signs of habitability and life on terrestrial planets. In Schwieterman et al. (2015), VPL team members made the first-ever detection of an absorption feature in Earth’s whole-disk spectrum due to molecular nitrogen (N2 ). This feature—the 4.2 μm N2 -N2 collision-induced absorption band—was found using comparisons between observations of the distant Earth from NASA’s EPOXI mission and simulations from the VPL 3-D spectral Earth model (which is a tool for simulating the phase- and time-dependent spectrum of the Pale Blue Dot). Molecular nitrogen can be a major constituent in planetary atmospheres, but its abundance is notoriously difficult to constrain due to a lack of spectral features. Thus, this work provides a new means for determining N2 concentrations in exoplanet atmospheres, which could be used to constrain surface pressure (thus helping to indicate liquid water stability) and could also rule out certain mechanisms for producing abiotic atmospheric oxygen.

In other Solar System work, Glein et al. (2015) used observational data from NASA’s Cassini mission and chemical models to constrain the pH of a sub-surface ocean on Saturn’s moon Enceladus, which is a key target of astrobiological interest due to its potential for originating and/or harboring life. This work suggests that Enceladus’ ocean is a Na-Cl-CO3 solution with a very alkaline pH. Such a high pH is indicative of serpentinization of chondritic rock that, if still occurring on Enceladus at the present day, would provide an energy source for life through the production of molecular hydrogen. Thus, these results provide an example of how signatures of habitability and life from sub-surface environments could be remotely constrained. NAI Information Management System 16 of 91

Also, in a modeling study of another outer Solar System world—Titan—Charnay et al. (2015) explained the observed eastward propagation of dunes on this planet using a coupling between tropical methane storms and superrotation. This work provides insights into volatile cycles on dry planets, as might occur for habitable worlds that form dry or lose most of their water.

Early to Current Earth and Mars

In this task we performed research to understand the early Earth and Mars environments, both of which serve as potential analogs for habitable environments unlike those seen on Earth today. We are expanding this line of work from past reports to span the entire histories of both planets. On Mars, we have developed explanations of modern-day measurements of the Martian atmosphere from Curiosity (SAM in particular), as well as explanations for the presence of liquid water on the surface of Mars billions of years ago. On Earth, we have done work from the origins of life all the way through the effects of anthropogenic greenhouse gas emissions on modern-day climate cycles.

Our work this year stretches all the way back to hypotheses on the origin of life on Earth and the geological environment at life’s origin. Baross and colleague proposed a concept for a ribofilm in which RNA’s origin-of-life role would have been more akin to a slowly changing platform than a spontaneous moment in time when a self-replicator arose. This paper linked the RNA world to realistic early Earth settings for the origin of life. It also presented a testable benchmark for attaining the hallmark characteristic of all Earth life: “the unity of biochemistry”. Sleep also published a review of the tectonic history of the Earth including a discussion of the conditions for the origin and evolution of early life on Earth, biological effects on global geological processes, and other concepts of high relevance to astrobiology (Sleep, 2015b).

However, most of our work this year focused on the period after the origins of life, but prior to the rise of oxygen in Earth’s atmosphere. This included a paper by Smith, Catling and colleagues (Pecoits et al., 2015) that demonstrated that photosynthesis that does not produce oxygen was likely present to account for iron formations formed 3.8 Ga (Ga = billions of years ago). Iron formation deposition tells us about major changes in the biosphere and atmosphere, and this analysis suggests the presence of photosynthetic life at the time of the very earliest sedimentary rock record on Earth. A separate study by Stüeken, Buick and colleague (Stüeken et al., 2015a) showed that the process by which biology incorporated nitrogen had evolved by 3.2 Ga. This would imply that this process is ancient and potentially not very difficult to evolve, and could plausibly exist elsewhere. This work is synergistic with separate work by Stüeken, Buick and colleague (Stüeken et al., 2015b) on the likely alkalinity of Archean lakes, which may serve as good analogs for the alkaline lake environments Curiosity is discovering on Mars.

Our work on biological nitrogen incorporation also has significant implications for the amount of nitrogen in Earth’s atmosphere. We continued VPL’s work on understanding the amount of nitrogen - and the total pressure - of Earth’s ancient atmosphere. This included the development of a proxy for the planet’s total atmospheric pressure (Som et al., submitted), some reanalysis of previously used

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proxies for pressure (Kavenagh and Goldblatt, 2015), research on the planet’s nitrogen budget over time (Johnson and Goldblatt, 2015), and the development of a semi-analytic treatment of Earth’s ancient nitrogen cycle (Goldschmidt abstract: http://goldschmidtabstracts.info/2015/487.pdf ).

Given those constraints on pressure, we studied Earth’s ancient climate. This included studies of the maximum amount of warming that different greenhouse gases can deliver (Byrne and Goldblatt, 2015), and 1D simulations of the maximum temperatures that could have been achieved given geological constraints.

Prior to the rise of oxygen, Earth’s climate may have been driven by the thickness of an organic haze, which appears to have been intermittently thick prior to the rise of oxygen. Our work this year supported this hypothesis by expanding the global extent of the geological data sets that are consistent with such a haze (Izon, et al., 2015). In a separate study Arney, Domagal-Goldman, Meadows, Schwieterman, Charnay and colleague (Arney et al., submitted) examined the climatic effects of this haze, and studied its implications for future exoplanet observations.

A separate analysis by Krissansen-Totton, Buick and Catling of Earth’s carbon isotope record over the last 3.6 billion years shows that although this varied over time, it did not vary enough to explain the rise in atmospheric oxygen 2.4 Ga (Krissansen-Totton, et al., 2015). This was a rigorous assessment of one potential explanation for the magnitude and timing of the oxygenation of Earth’s atmosphere and oceans, one of the great unsolved problems in studies of Earth history.

Stüeken, Buick and colleague continued to look at geochemical proxies for and implications of the Earth’s rise of atmospheric oxygen. This includes work that suggests a spike in selenium weathering occurred during the brief 'whiff' of oxygen prior to its permanent appearance in the atmosphere (Stüeken et al., 2015c). This has implications not only for atmospheric chemistry, but also the rigorousness of weathering of the continents, and the chemistry of the oceans at the time. It also has implications for biological production of potential biosignature gases. This also led to work on the ability of one particular element - Se - both participate in novel biosignature species (Stüeken et al., 2015d) and to have revealed the redox state of the ocean during one of Earth’s past extinction events (Stüeken et al., 2015e).

Geologic records over the past million years indicate a 100,000 year cycle in the extent of Earth’s surface covered by ice. These ice age cycles are a result of variations in Earth’s orbital geometry, but it is unclear how these variations will continue in the presence of significant human emissions. Haqq- Misra used a simple climate model to demonstrate the potential for human-induced climate change to damp out these variations in ice coverage, which suggests that human actions today could have long- lasting impacts into the future (Haqq-Misra, 2015).

The Habitable Planet

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VPL has theoretically explored the role of water in the atmospheres as of habitable planets. Kasting, Kopparapu and colleague (Kasting et al. 2015) found that for planets near the inner edge of the habitable zone, water vapor can still penetrate into the stratosphere and escape, implying planets do not have to develop a full-blown runaway greenhouse to lose their water over geologic time. We also showed that multiple stable states of climate could exist for a water world, including both habitable and uninhabitable states, at the same level of incident radiation, suggesting that water-rich planets in the habitable zone are not necessarily habitable (Goldblatt 2015). Zahnle and colleagues explored conditions under which a wet, Earth-like habitable planet can evolve into a dry, Dune-like habitable desert planet. Desert planets have broader habitable zones than ocean planets like Earth, and hence their origins and evolutions are pertinent to characterizing habitable zones in general (Kodoma et al. 2015).

In planet formation, Quinn and Backus ran supercomputer simulations of planet formation and migration around M dwarf stars, where planet-forming material can be relatively scarce near the star. As large planets are unlikely to form under these conditions, this work explores mechanisms for super-Earth migration and “parking” in the habitable zone. Raymond and colleagues explored the late stages of terrestrial planet formation, and showed that the migration of super-Earths through the terrestrial planet-forming region is extremely destructive (Izidoro et al. 2014), that Jupiter and Saturn can migrate outward in multiple orbital configurations (Pierens et al. 2015), and that gas giants provide a barrier to the inward migration of more distant planetary cores. Raymond and colleagues proposed that Jupiter's presence prevented the ice giants from becoming super-Earths and identified observational tests of the model (Izidoro et al. 2015a), and showed that the ice giants' masses and orbits can be reproduced if they formed from a population of cores whose inward migration was blocked by Jupiter. This implies a common origin for ice giants and hot super-Earths (Izidoro et al. 2015b). We show that the asteroid belt's orbital structure is a key constraint on models of the formation of the terrestrial planets. The classical model appears to fail systematically in reproducing the inner Solar System (Izidoro et al. 2015c).

The VPL explored numerous aspects of the formation of the Moon and the subsequent coupled evolution of the Earth-Moon system. Raymond and colleagues showed that the compositional similarity between the Earth and Moon may have arisen naturally during the accretion process (Mastrobuono-Battisti et al. 2015). Zahnle, Sleep and colleagues showed that thermal blanketing by Earth’s water atmosphere limits the rate the Moon’s orbit can evolve in the first ten million years after the Moon-forming impact, provided the first good estimates of how quickly the surface of the Earth can freeze after the Moon-forming impact, and calculated the geothermal heat flow for the first hundred million years or so of the Hadean (Zahnle et al. 2015). We also extended satellite modeling to exomoons (natural satellites of exoplanets) to elucidate the role of planetary radiation on the exomoon’s habitability (Heller & Barnes, 2014).

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VPL explored the role of tidal phenomena in the Solar System and in the exoplanets. Driscoll & Barnes (2015) coupled a 1-D Earth interior model to standard models of tidal processes to produce the first self-consistent geophysical model of an Earth-like planet. Their simulations showed that, contrary to prevailing opinions, planets orbiting red dwarfs are likely to maintain strong magnetic fields that may shield the planet from the host star’s activity. Raymond and colleagues created a new code for calculating tidal and spin interactions in multiple planetary systems to show how planet- planet-tide interactions affect the long-term evolution of planetary systems (Bolmont et al. 2015). Barnes (2015) showed that tidal circularization of exoplanet orbits proceeds at different rates and demonstrated how large samples of high quality transit data, e.g. from TESS, can be used to identify the boundary between rocky and gaseous exoplanets. Finally, we examined the similarities and differences between tidally-induced cracks on Enceladus and the San Andreas fault, which can be used to infer processes on tidally evolving exoplanets (Sleep, 2015b).

To explore the star’s gravitational influence on planetary habitability, VPL scientists performed N- body simulations of real and hypothetical planetary systems. Deitrick, Barnes, Quinn, Luger and colleagues used orbital stability models to reveal the full 3-dimensional orbital architecture of the Upsilon Andromedae system (Deitrick et al. 2015), the first system discovered with misaligned orbital planes. Barnes, Deitrick, Quinn, Raymond and colleagues discovered that terrestrial exoplanets in mean motion resonances with non-planar orbits can evolve chaotically for at least 10 Gyr (Barnes et al., 2015).

VPL researchers also simulated the role of stellar radiation on atmospheric escape from potentially habitable worlds, and the impacts of stellar activity on planetary atmospheres. Luger, Barnes, Meadows and Fortney demonstrated that small gaseous worlds (mini-Neptunes) in the habitable zones of red dwarfs may have their envelopes blown away by their host star, revealing a “habitable evaporated core”. Luger and Barnes (2015) also showed that the high luminosity of young M dwarfs is likely to lead to a prolonged runaway greenhouse state for planets that are discovered in the habitable zone after several billion years, potentially dessicating the planet. Hawley and collaborators examined the flaring and activity of M dwarfs stars in the Kepler field, including the long-term tracking of starspots (Davenport et al. 2015), as well as quantifying the flare rates for M dwarfs as a function of rotation period (Lurie et al. 2015). Tilley, Meadows and Hawley are currently using observed flare sequences provided by Hawley and Davenport to explore the impact of multiple flares on the photochemistry and surface UV flux of an Earth-like planet orbiting an M dwarf.

To perform a comprehensive assessment of habitability and in particular to calculate constraints on planetary orbits for newly discovered exoplanets, Barnes, Dietrick, Luger and Quinn worked on the development of the VPLanet framework, which can calculate the coupled effects of orbital, rotational, stellar, geophysical, atmospheric and climate evolution of planetary systems with potentially habitable planets. Additionally, Barnes, Meadows and Evans (2015) defined a new method to assess and compare the potential habitability of transiting exoplanets. Their approach is markedly different from classic habitable zone calculations in two important ways. It is directly tied to transit observables, like transit duration, rather than just being a function of stellar mass and orbital

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distance. Second, it assigns a numerical likelihood of potentially habitable conditions to each planet, allowing ranking, as opposed to the more binary habitable zone concept.

The Living Planet

This past year, three separate modeling investigations found cases in which O2 and/or O3 could build up abiotically in an exoplanet’s atmosphere and create possible false positives for life. Gao, Robinson, Yung and colleague used VPL models to show that for a planet orbiting an M dwarf with a high FUV/NUV flux ratio - if the planetary atmosphere is dry and CO2-dominated - then recombination of the products of CO2 photolysis is inhibited. The atmosphere can then build up CO 3 4 and O2 in 10 -10 years, with abundances of abiotic O2 and O3 rivaling that of modern Earth (Gao et al. 2015). In this case, the abiotic source of the O2 is made more likely due to the lack of water in the planetary spectra. However, via the same mechanism of CO2 photolysis, Harman, Schwieterman, Kasting and colleague did a comparison of stellar types, and showed that for planets with liquid water, while abiotic O2 should not accumulate to detectable levels around F and G stars, with K and M stars, the low near-UV flux may allow build-up of O2 if the sinks for O2 are low; meanwhile, O3 could be detectable for a wider range of stars (Harman et al. 2015). Instead of CO2, Luger and Barnes (2015) looked at extreme atmospheric H2O loss during the early high luminosity phase of M dwarfs for planets. For planets that form at the position that will become the 5-Gyr habitable zone, atmospheric evolution is sufficiently severe that the planet can generate large amounts of atmospheric O2 while retaining liquid water (Luger and Barnes, 2015). Thus O2 and O3 detection alone are not robust biosignatures but must be accompanied by knowledge of stellar parameters and a more comprehensive census of atmospheric composition and conditions on terrestrial exoplanets to rule out false positives. As the direct detection of nitrogen would provide a means to characterize the bulk atmosphere of potentially habitable exoplanets and constrain the likelihood of oxygen production by abiotic processes, Schwieterman, Robinson, Meadows, Misra, and Domagal-Goldman explored a novel way to detect and quantify N2 in planetary atmospheres. Although the N2 molecule is extremely challenging to observe in exoplanet spectra, N2 has a collisional-induced absorption band near 4.2 µm, which is significant in Earth's spectrum and potentially in those of Earth-like exoplanets with similarly N2-dominated atmospheres. The VPL team quantified the potential magnitude of this spectral signature by producing synthetic transit transmission and radiance spectra using VPL radiative transfer models (Schwieterman et al. 2015b).

In other innovative work on biosignature definition, Krissansen-Totton, Catling and colleague (2015) tested the idea that atmospheric chemical disequilibrium can act as a biosignature. Performing the first, detailed quantification of thermodynamic disequilibrium in Solar System planetary atmospheres, they showed that the Earth has ~20 times the thermodynamic chemical disequilibrium of other planets because of the biosphere. They identifiy the high abundance N2, O2 and the presence of an ocean as the strongest disequilibrium state for our planet. Work is continuing on the topic of NAI Information Management System 21 of 91

biosignatures and disequilibrium with an effort to identify “anti-biosignatures”, which is where gases that should be consumed by microbes are present as a result of purely abiotic processes.

We have also made significant advances this year in the area of the nature and detection of photosynthetic biosignatures, (oxygenic and anoxygenic), as well as non-photosynthetic biosignatures. Kiang published a white paper exploring theoretical challenges in predicting photosynthetic spectral features in a special issue on Astrobiology in The Biochemist magazine (Kiang, 2014). Addressing the long wavelength limit of oxygenic photosynthesis, Kiang, Parenteau, Blankenship, and Siefert discovered another strain, possibly two, of a chlorophyll d-containing cyanobacterium collected from red algae at Moss Beach, California. Enrichments and isolations are currently being conducted and 16S rDNA sequences identify a new Acaryochloris strain most closely related to another found in Japan. Comparisons of its light use with other varieties of far-red photosynthesizers will help identify the mechanism that induces changes in the primary chlorophyll, and this knowledge can be used to better understand likely pigment biosignatures on planets orbiting different stellar types.

VPL research on biosignatures from anoxygenic phototrophs informs the search for life on exoplanets at a similar stage of evolution or biogeochemical state as the Archean Earth, as well as on planets orbiting M dwarfs. Adding to preliminary work on pure cultures and microbial mats of anoxygenic phototrophs, Parenteau measured more detailed reflectance spectra of field in situ samples of a variety of mat environments, uncovering striking spectral features that show that the full suite of complementary pigment niches of layered mat communities is visible at the surface. This is perhaps the first identification of an ecosystem community class of surface biosignature, as opposed to features from isolated species. Sparks and Parenteau have also measured the reflectance and transmission full Stokes polarization spectra of the same pure cultures and environmental mat samples, and found strong correlations between spectral and polarization features. To look for new gaseous biosignatures, Parenteau and Hoehler have constructed an anaerobic chamber and LED array for simulating Archean (and M-dwarf) radiation and atmospheric compositions. They are quantifying biogenic gas fluxes in anoxygenic microbial mats using a high resolution membrane inlet mass spectrometer.

Schwieterman and Meadows collaborated with Cockell of the UK Center for Astrobiology to conduct an interdisciplinary study of the diversity and detectability of non-photosynthetic pigments as biosignatures, especially for halophiles (Schwieterman et al., 2015). This study included reflectance spectra measurements of a diverse collection of pigmented non-photosynthetic organisms and an analysis of the remote detectability of analogs of these organisms in disk-averaged spectra, with a planetary environment that includes other surfaces, an atmosphere and clouds. These spectra are now available through the existing VPL Biological Pigments Database (http://vplapps.astro.washington.edu/pigments).

At the ocean depths, Anderson and Baross, with collaborator Mitch Sogin at the Marine Biological Laboratory in Woods Hole, MA, conducted a 16S rRNA tag sequencing survey to reveal a picture of microbial colonization and dispersal both within and between hydrothermal vent systems, with results NAI Information Management System 22 of 91

also indicating novel strains (Anderson et al. 2015). The deep, hot microbial biosphere harbors the most ancient of extant organisms and would help provide novel biosignatures and clues for the environmental distribution of life elsewhere in the universe.

Rounding together the atmosphere, surface, and subsurface interactions of life’s impacts, Hoehler, Domagal-Goldman, Som, Kasting and Meadows have modeled chemosynthetic-based biospheres by completing the coupling and benchmarking of two models: one model codifies a thermodynamic energy balance concept for habitability (Hoehler, 2009); the other model predicts a planet’s atmospheric composition and climate while balancing the redox state of the surface environment (Domagal-Goldman et al., 2014). Preliminary results on applying this to methanogenesis on early Earth were presented at AGU, and are being prepared for publication. These models will enable an ecosystem to be coupled into an interactive planetary environment including atmospheric, oceanic and subsurface components. This will allow us to predict standing biomass size, net biological productivity, and resulting gas fluxes to and from a biosphere (and therefore the nature and magnitude of any potential biosignature) with a given set of volcanic and hydrothermal inputs to the planetary surface.

The Observer

In this task we explore the detectability of signs of habitability and life for modeled observations from the previous tasks. We also observe and develop new observational, analysis and retrieval techniques to improve our understanding of the environmental properties of exoplanets for current and future observations. In exoplanet discovery and observations this year, Agol was part of the 'Citizen Scientists' team that discovered a new super-Neptune planet and characterized its multi- planet system (Schmitt et al., 2014). This demonstrated techniques to determine precision masses of exoplanets, which can be applied to habitable zone exoplanets in the future. Sheets and Deming (2015) measured and coadded reflected light from Kepler planets smaller than Saturn, finding that they have low (~20%) geometric albedos. This new technique probes the nature of small planet atmospheres and my eventually lead to an understanding of the atmospheres of super-Earths. In exoplanet spectroscopy, Deming participated in HST transit observations of HAT-P_11b, a Neptune- sized planet that showed clear skies with water vapor absorption clearly detected in the spectrum (Fraine et al., 2014). We hope to extend this measurement technique to transiting habitable super- Earth (to be discovered by TESS). Yung helped interpret pioneering ground-based polarimetry data on hot Jupiter HD189733b, and using a Rayleigh scattering model was able to constrain the geometric albedo of the planet to be < 0.36 (Wiktorowicz et al., 2015).

In addition to observations, we also developed several new detection techniques for planets and their moons. New, fast techniques to calculate the size of the perturbation expected to planetary orbits as planets pass by each other were developed that can potentially be used to measure the masses of Earth-sized exoplanets with JWST (Deck & Agol, 2015). Agol, Robinson and Meadows, along with undergraduates Jansen and Lacy developed new techniques to detect and characterize exomoons and their parent planets, using spectroastrometry, the measurement of the center of light in a planet/moon system at different wavelengths (Agol et al., 2015). This technique may allow for detection of NAI Information Management System 23 of 91

potentially habitable exomoons, as well as mass measurements and disentangling of the spectra for the exoplanet/exomoon system. Misra and Meadows (2014) developed a new means of rapidly discriminating between cloudy, hazy and clear sky exoplanets by searching for refraction signals prior to the planet transiting the stellar disk. This will allow efficient sorting of potential targets for more time-intensive follow-up observations to search for water and biosignatures. Schwieterman, Robinson, Meadows, Misra and Domagal-Goldman collaborated on a paper that uses collisional pairs of N2-N2 molecules to understand planetary atmospheric bulk composition and that may also help discriminate whether the source of oxygen in an atmosphere is biotic or abiotic. Robinson additionally co-wrote a review on techniques for 1-D thermal structure modeling for planetary and brown dwarf atmospheres (Marley & Robinson, 2015), drawing on results from many of astrobiology’s sub-fields.

To model mini-Neptunes and ultimately learn how these - likely uninhabitable - worlds can be discriminated from habitable super-Earths, Charnay, Meadows, Misra and Arney developed 3D models of mini-Neptune GJ1214b’s atmosphere using the Laboratoire Meteorologie Dynamique’s LMDZ. Charnay, Meadows & Leconte, (2015a), described the new LMDZ mini-Neptune model and analyzed the atmospheric circulation and the transport of tracers in GJ1214b's atmosphere, which are important to understand the photochemistry and cloud formation on mini-Neptunes. In Charnay et al., (2015b) we performed the first 3D simulations of realistic clouds on a gaseous exoplanet, and validated the model by reproducing the observed HST transit spectrum of GJ1214b. We then predicted what information could be obtained with future telescopes and in particular showed that mini-Neptunes should show strong features from molecules longward of 3um in JWST transit spectra, even if haze precludes deeper observations at visible wavelengths. This work provides insight into the best observational techniques to decipher cloudy atmospheres, and how to distinguish mini-Neptunes from potentially habitable ocean exoplanets.

In further developments in instrument models and retrieval for potentially habitable exoplanets, Robinson and colleagues developed an instrument noise mode suitable for studying the spectral characterization potential of a coronagraph-equipped, space-based telescope and applied it to a broad set of rocky and gaseous exoplanet types (Robinson, Stapelfeldt & Marley, 2015). This is being used to explore the capability of near-future coronagraphic missions (like WFIRST-AFTA) to detect biosignatures gases in the atmospheres of nearby Earths and super-Earths. In ongoing work for VPL’s terrestrial exoplanet spectral retrieval suite, Lustig-Yaeger has completed an end-to-end retrieval suite, which is the core of the Observer task. The current version uses optimal estimation, but we are implementing MCMC and Multinest algorithms to replace OE. Luger and Lustig-Yaeger are using Gaussian processes to develop a cost function that penalizes unphysical atmospheres so that the retrieval will be constrained not just by the limitations in the spectral data, but by known characteristics of the planet and planetary system.

These tools, and the science and small exoplanet observing expertise developed by the VPL have played an integral role in the development and delivery of final reports for two NASA spacecraft concepts for exoplanet observations, Exo-Coronagraph and Exo-Starshade. Meadows is a Science and Technology Definition Team member for Exo-C (Stapelfeldt et al., 2015), Domagal-Goldman

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and Bill Sparks are Science and Technology Definition Team members for Exo-S (Seager et al., 2015). Both teams are continuing in an extended phase to develop these concepts beyond the original baseline. Robinson contributed modeling and predicted spectra for both final reports. Additionally, Meadows, Schwieterman, Arney, Deming and Lustig-Yaeger are working on end-to-end simulations of self-consistent Earth-like planets orbiting M dwarfs, as viewed by JWST. These simulations are being used to calculate the exposure time needed to observe diagnostic features in the exoplanet spectra, and to test the robustness of the retrieval techniques on simulated data for JWST. These results were presented by Meadows at the international conference on JWST at ESTEC in October 2015. Many of these tools and techniques also formed the basis for a WFIRST Science Investigation Team proposal that was recently submitted by Meadows (PI), Domagal-Goldman, Barnes, Robinson, Lincowski, Lustig-Yaeger and colleagues, to simulate forward and instrument modeling, and data analysis and retrieval, for observations of small planets taken with a WFIRST-AFTA coronagraphic mission.

Education and Public Outreach

During the past year the EPO team finalized the script and images for the first Science on Sphere show "Earth - the Pale Blue Dot" describing exoplanet detection and what Earth would look like as an exoplanet. A training manual has also been created to facilitate adoption at other institutions. The show has been put into regular rotation at the Pacific Science Center and evaluation is ongoing. The second Science On Sphere show (title TBD) describing the Earth through time and how life on Earth has changed during different epochs is currently under development with a scheduled completion date of Spring 2016. This year 2 addition Astrobiology Science Communication fellows have received training, bringing the total to 4.

Our Research: Year 4

A significant research highlight this year spanning several tasks was the submission of team papers assessing the likely evolutionary history, current state and possible observable characteristics of Proxima Centauri b, a potentially Earth mass planet orbiting in the habitable zone of the Sun’s nearest neighbor (Barnes et al., 2016; Meadows et al., 2016; Luger et al., 2016). VPL team members were also instrumental in leading an extremely productive community workshop on exoplanet biosignatures.

Solar System Analogs for Extrasolar Planetary Processes and Observations

In this task we use observations of Solar System planets and moons to explore the detectability of signs of habitability and life in planetary environments, and to constrain planet formation processes. This year we used observations of Europa, models of Venus, and the properties of Mercury, Mars and the asteroid belt to learn about potentially habitable current and past environments in the Solar System and to constrain mechanisms for terrestrial planet formation and evolution.

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In work to understand possible habitable environments in the Solar System, VPL team member Bill Sparks led research on the exciting evidence for plumes on Europa via the detection of water vapor (Sparks et al., 2016). The HST/STIS measurements were made by observing Europa in transit across Jupiter, and detecting patches of UV absorption from water that were backlit by Jupiter off the limb of Europa. This is a remote-sensing opportunity to study the ocean below the ice, which is one of the most plausible sites for extant life beyond Earth. Gao and Yung performed work to understand ice particle size in the plumes of Enceladus (Gao et al., 2016). In NExSS-initiated work that sought to understand Venus as an analog for highly-irradiated terrestrial planets, VPL team member Nancy Kiang contributed to GCM model simulation of early Venus. This work indicated that the rotation period of the planet was crucial in understanding its planetary climate, and that even with its current rotation period, Venus could have been habitable as recently as ~0.7 Gya (Way et al., 2016).

In work on terrestrial planet formation VPL Team member Raymond and colleagues demonstrated that the size of Mars and the dynamical properties of the asteroid belt offer a key, quantifiable constraint on models of terrestrial planet formation (Izidoro et al. 2015). They conclude that these bodies could not have been formed by a simple gradient in planet forming material, and that a more complicated process, such as the Grand Tack migration of Jupiter and Saturn to the inner Solar System, would be needed to explain their properties. In work that could impact our understanding of terrestrial planet formation, Raymond and colleagues postulate that Jupiter’s core could have plausibly formed due to a buildup of pebble accretion material at 0.1 AU and migrated outward through the terrestrial planet forming region (Raymond et al., 2016). If the migration was fast, terrestrial planet formation would be only minimally disrupted, but if slow, material in the terrestrial planet forming region interior to 0.5-1.0 AU could be cleaned out, possibly explaining the lack of material closer-in than Mercury. In a paper entitled “Is there an exoplanet in the Solar System?” Raymond and colleagues ran N-body simulations that showed that it was possible that Planet 9 – the putative super-Earth planet perturbing trans-Neptunian objects in our Solar System – may have been captured from another star in the Sun’s birth cloud. A capture occurred in a few percent of their simulations, and they proposed observations of the structure and distribution of the TNOs that might help to distinguish capture from other formation processes for Planet 9 (Mustill et al., 2016).

Early to Current Earth and Mars

Research in this task focuses on studying early Earth and Mars as examples of habitable planetary environments that are very different to our modern Earth. Understanding the many faces of Earth through geological time, and a possibly habitable early Mars, can provide a diversity of habitable environment examples that may also be encountered in the exoplanet population. Our work this year further constrained the properties of the atmosphere, interior and biosphere of the early Earth, and explored factors affecting the climate of early Mars.

Exploring the very early Earth VPL Team member Sleep considered how bombardment by the core of Theia – the moon-forming impactor – rather than by asteroids, may have supplied much of the platinum group veneer component to the silicate Earth and the lunar core, and also provided the

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excess FeO in the silicate moon (Sleep, 2016). In this hypothesis, the later bombardment by other asteroids would have been relatively benign and may not have rendered the planet uninhabitable.

Several VPL team members focused on aspects of nitrogen and nitrogen cycling on the early Earth. Studies on ancient nitrogen fixation were performed by Stüeken, Buick, and coauthors (Stüeken et al. 2016; Stüeken 2016). The first study used nitrogen isotope measurements to show that biogeochemical nitrogen cycling rates varied markedly over the Earth’s history. These variations should be expected on other inhabited planets, and thus nitrogen speciation and distribution on the modern Earth is not a robust biosignature over a planet’s habitable life-span. They also concluded that a large biosphere was unlikely to be sustainable through abiotic nitrogen sources, and this would have favored the early evolution of biological nitrogen fixation. In the second study, Stüeken argues that nitrogen in 3.7 billion year old metasediments is so abundant that it could only have been produced by biological, as opposed to abiotic, nitrogen fixation. VPL team member Wordsworth also wrote a comprehensive paper on nitrogen evolution on Earth and Venus, identifying a range of biotic and abiotic mechanisms governing nitrogen exchange between a terrestrial planet’s surface and interior over time (Wordsworth, 2016). He found that the biological influence on nitrogen on Earth was likely limited in the early Archean, due in part to limitations in nutrient availability. He also found that planetary water loss due to photolysis could ultimately oxidize a planet’s mantle, enhancing the release of mantle N2 into the planetary atmosphere, as may have happened on Venus.

We continued our work on constraining the composition and total pressure of the early Earth’s atmosphere, which also ties into the history of N2 on the early Earth. Research by Som, Buick, Catling, and coauthors (Som et al. 2016) analyzed the volume of bubbles trapped in lava flows and concluded that the atmosphere 2.7 billion years ago was less massive than today’s by a factor of two or more. An incomplete nitrogen cycle in the Archean – whereby the biologically fixed nitrogen was not eventually released back to the atmosphere as N2 the way it is today – could have caused this low atmospheric pressure. VPL team members Zahnle and Buick were invited to write a Nature News and Views article (Zahnle and Buick, 2016) commenting on the significance of the discovery of oxidized micrometeorites in Archean limestones. This work proposed that the ancient upper atmosphere may have become oxidizing due to lower atmosphere pressure, which allowed water vapor to leak into the stratosphere. Photolysis of this water leaves O2 behind, as the hydrogen escapes to space. Therefore oxygen in a planet’s upper atmosphere is not necessarily a biosignature.

VPL team members also explored the impact of a hydrocarbon haze in the early Earth’s atmosphere on habitability and planetary spectral appearance. Arney, Domagal-Goldman, Meadows, Schwieterman, Charnay and colleagues (Arney et al., 2016) found that habitable conditions could be maintained in the presence of an organic haze even with the fainter young sun and geochemical constraints on the atmospheric CO2. This haze can also provide a UV shield, reducing the UV surface flux by up to two orders of magnitude. The strong spectral features produced by organic haze may also be detectable across interstellar distances. In ongoing work, the authors also explored the probability of formation, and the environmental impact, of hydrocarbon hazes on early Earth analog planets orbiting stars of different spectral type.

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Work on the Phanerozoic Earth focused on climatic impacts of the rise of O2 and catastrophic volcanic release of CO2. Kasting and Catling contributed to a study that showed how lower O2 levels occurring in the last few hundred million years actually resulted in a cooler climate – rather than the warmer climate that was believed to occur due to decreased Rayleigh scattering from the decreased O2 (Payne et al. 2016). This work suggests high climate sensitivity to CO2, which is important for modeling and understanding the habitability of Earth-like planets. Goldblatt (2016) also discusses how other existing studies on O2 impacts on the Phanerozoic climate have incomplete physics and may not be robust. VPL team member Bitz and coauthors (Tobin et al 2016) modeled the climate effects of CO2 emissions from Deccan Trap flood volcanism at the Cretaceous-Paleogene boundary. This mechanism has been implicated as a primary or aggravating contributor to the mass extinction that defines the Cretaceous-Paleogene boundary. The CO2 released by the Deccan Trap volcanism is capable of generating warming events seen in the geological record, but drawing CO2 back down requires rates of silicate weathering higher than the modern level.

This year we also published several papers focused on understanding the conditions, climate and possible habitability of ancient Mars. Wordsworth presented a review of the climate of early Mars, concluding that it was likely cold with transient warming events (caused by a combination of impacts, volcanism, and orbital forcing), but may have still been hospitable to microbial life (Wordsworth, 2016). Armstrong and collaborators (Kite et al., 2015) discuss the connection between stratigraphic deposits on Mars and orbital dynamics, including changes in the precession and obliquity. Similar orbital forcing may contribute to climate variations on exoplanets. In a theoretical study on Mars climate, Kasting and colleague (Ramirez and Kasting, 2016) considered whether cirrus clouds could have warmed ancient Mars, and conclude that this is unlikely to have been important. Early Mars therefore must have been warmed by other mechanisms, possibly by a H2-CO2 greenhouse. In another study by Batalha, Kopparapu, Haqq-Misra, and Kasting, the authors describe a geophysical mechanism that could have induced dramatic climate cycles on Mars with extended periods of glaciation punctuated by warm epochs lasting for up to 10 million years. On modern Mars, Conrad and colleagues (Conrad et al., 2016) used the Curiosity Rover to perform in situ measurements of all Xe and Kr isotopes in the Mars atmosphere, providing a new benchmark for disentangling the sources of Kr and Xe in SNC meteorites. Gao and Yung contributed to a hypothesis for near-surface exchange of methane on Mars (Hu et al., 2016).

The Habitable Planet

This VPL task explores habitable planet formation, and the effect on planetary habitability of interactions between the potentially habitable planet, its star, other planets in the system, and the host galaxy. Significant research this year includes modeling of planet formation, internal, orbital and atmospheric evolution, and feedbacks between the relevant processes.

In the area of planet formation and migration, VPL team members Quinn and Backus explored the evolution of protoplanetary disks around M dwarf stars with supercomputer simulations (Backus & Quinn, 2016) and found that gravitational collapse of the outer gaseous disk to form giant planets is more likely for disks of M dwarfs. Hence M dwarf habitable zone planets may have their formation NAI Information Management System 28 of 91

and orbits influenced by gas giants in the outer planetary system. Fleming and Quinn simulated planets embedded in circumbinary protoplanetary disks and found that circular binary stars weakly couple to the disk, remain circular, and excite disk eccentricity, while eccentric binaries strongly couple to the disk – causing eccentricity growth for both disk and binary (Fleming & Quinn 2016), potentially affecting both terrestrial planet formation and habitability.

VPL scientists also continued to explore the role of orbital evolution in extrasolar planetary systems and the implications for habitability. Team members J.W. Barnes and R. Barnes participated in a study of a unique Kepler system that may have planets in significantly mutually inclined orbits (Ahlers et al. 2015), pioneering a technique to use transit data alone to identify the mutual inclination of planets. This technique could be used on upcoming TESS data to constrain the orbital evolution of habitable zone exoplanets. Team member Raymond contributed to a study of the coupling between N-body and tidal methods to look at the effects of planet-planet interactions in densely packed M dwarf planetary systems (Bolmont et al. 2015). They applied this new code to the Kepler-62 system and showed that the two habitable zone planets, Kepler-62 e and f, may have very different obliquities and spin rates, affecting possible climates and volatile retention for these two planets.

We also used climate models to understand how atmospheric composition and dynamics – along with orbital parameters – can strongly affect planetary habitability and the limits of the habitable zone. Team members Pierrehumbert and Ding developed a new climate convection scheme for atmospheres with high fractions of condensable gases, as may be the case in water-rich and CO2-rich atmospheres near the inner and outer edges of the habitable zone (Ding & Pierrehumbert, 2016). They explored climate near the runaway greenhouse limit for planets on circular and eccentric orbits and showed that the latent heat of the condensable component could damp out the effects of even extreme seasonal forcing. They also explored the role of significant condensable gas fractions on dynamics, discovering unusual convection features and a tendency towards 100% humidity for the condensable gas, which has significant implications for runaway greenhouse thresholds (Pierrehumbert & Ding, 2016). Team members Kasting and Kopparapu modeled atmospheres in a moist greenhouse state and found that significant water loss can occur, explaining why a strongly heated terrestrial planet can become desiccated and uninhabitable (Kasting et al., 2015). Abbot developed 1D radiative convective climate models and used 3D GCMs to understand fundamental atmospheric processes for tidally locked Earth-like planets (Koll & Abbot, 2016). Their models indicate that thermal phase curves observed with JWST can constrain atmospheric scenarios for terrestrial exoplanets like GJ1132b. Team member Raymond also contributed to a study of water-loss from the TRAPPIST-1 M dwarf planets (Bolmont et al. 2016). Team members Goldblatt, Ding and Abbot contributed to a study that compared different 1D radiative transfer models, and showed that uncertainties in water vapor absorption used by the models could result in significant uncertainties in habitable zone inner edge calculations (Yang et al., 2016). Further exploring the inner edge for tidally-locked M dwarf planets, team members Kopparapu, Haqq-Misra, Kasting, and Meadows found that when planetary rotation rates are treated self-consistently, the habitable zone for M stars may not be as far in or as wide as previously calculated. This is because synchronous rotation for closer-in orbits produces faster atmospheric rotation rates that result in zonal, less-continuous, cloud patterns. This drops the planetary albedo and increases the surface temperature – when compared to NAI Information Management System 29 of 91

continuous cloud over the illuminated hemisphere for more slowly rotating planets (Kopparapu et al., 2016).

Several VPL researchers also studied processes that could impact the outer edge of the habitable zone. Abbot considered the possibility of limited CO2 outgassing rates for outer habitable zone planets and derived analytical formulas to approximate the onset and period of climate limit cycles between a warm state and a globally glaciated state under these conditions (Abbot, 2016). Haqq- Misra, Kopparapu, Batalha, Harman and Kasting also explored the limit cycle concept for planets with volcanic outgassing rates lower than that of modern Earth, showing that the habitable zone may be narrowed for planets around hotter stars like our Sun (Haqq-Misra et al., 2016). Lincowski, Meadows, Robinson and Crisp used a new 1D generalized terrestrial RCE model to explore the effects of CO2 condensation on the outer limits of the habitable zone. They found that condensation and removal of CO2 from the atmosphere produced one of the biggest climatic impacts, as this process precluded the build-up of the 8-10 bar CO2 atmosphere required to reach the postulated maximum greenhouse limit. This work was presented at the Astrophysics of Habitability, Exoclimes and DPS conferences. Beyond the classical habitable zone outer boundary, team member Abbot studied possible biological feedback mechanisms that could regulate the climate on distant planets warmed by the greenhouse effect of thick hydrogen atmospheres (Abbot, 2015). He found that the action of life could potentially increase the time period over which this type of planet could be habitable. VPL team members Shields, Barnes, Agol, Bitz, Charnay and Meadows combined orbital evolution and climate models to identify multiple plausible combinations of orbital and atmospheric properties that permit surface liquid water on the potentially habitable planet Kepler-62 f, which sits near the outer edge of its habitable zone (Shields et al., 2016).

Finally, VPL team members Barnes, Meadows, Arney, Schwieterman, Deitrick, Luger, Lustig- Yaeger, Fleming, Lincowski, Domagal-Goldman, Driscoll, Robinson, Quinn and Crisp studied the potential habitability of the M Dwarf habitable zone terrestrial Proxima b from an interdisciplinary perspective (Barnes et al., 2016). They examined the evolution of the planet’s interior, atmosphere, water inventory, and orbit under the influence of the host star, stellar system, and the Milky Way galaxy. They found a diversity of plausible scenarios under which this HZ planet could support or have lost liquid water on its surface. Team member Raymond contributed to another study of Proxima Cen b focused on the irradiation, rotation and volatile inventory from formation to the present (Ribas et al. 2016). This study concluded that Proxima Cen b may have lost less than an ocean of water, despite the early high radiation levels from its star, and so could still be habitable. Shields also led a recently accepted review paper on the habitability and occurrence rate of M dwarf planets.

The Living Planet

early life, measurements of surface photosynthetic biosignatures, and quantifying disequilibrium biosignatures in a planetary environment. Team members also contributed to research and policy on planetary protection for Mars.

Black and colleague proposed that a self-assembled aggregate composed of a fatty acid membrane and the building blocks of biological polymers provides a first step in the emergence of protocells (Black & Blosser, 2016). Buick and colleagues reviewed current thinking on the origins of eukaryotic organisms using fossil, phylogenetic and cell biological evidence. They argued that the 3 domain model of the Tree of Life is outdated, and that complex cells arose from simple prokaryotic precursors within the Archaea (Dacks et al., 2016). This work suggests that complex life may be less specialized than previously thought. Baross contributed to interdisciplinary work studying the nature and limits of life in hydrothermal vent chimney environments, using analog environments in the Endeavour Segment of Juan de Fuca Ridge (Lin et al., 2016). This work demonstrated the importance of an interdisciplinary approach in understanding the potential for subsurface life on planets and moons. Siefert contributed a chapter to a book – Future of Life in the Desert – that reviews our understanding of how microbial life tolerates the extreme conditions in the Chihuahuan desert, an analog for desiccated environments. Siefert and colleagues also submitted work that the nutrient environment can shape microbial development in shallow ponds.

Alternative surface biosignatures relevant to the early Earth before the rise of oxygen are being studied by VPL in the lab, field, and via modeling. Parenteau, Kiang, Blankenship, Hoehler, Meadows, and colleagues continued work measuring the reflectance spectra of environmental samples of anoxygenic phototrophs from continental and marine intertidal areas. Photosynthetic pigments from all layers of a microbial mat were detected in reflected light measurements at the top of the mat (Parenteau et al., 2015), suggesting that the layering order of microbial mats does not affect the ability of microbes to thrive or alter the detectable spectral features of "community biosignatures." This may strengthen support for the detection of surface biosignatures on exoplanets at a similar stage of evolution as the pre-oxic Earth. In ongoing work, significant circular polarization measured by Sparks, Parenteau, Blankenship, Meadows and colleagues in the spectra of anoxygenic phototrophs is indicative of the presence of a chiral phototrophic apparatus. This finding has the potential to lead to a pure surface biosignature, applicable over an extremely broad span of time, including pre-oxic early Earth. Kiang, Parenteau, Blankenship and colleagues continue field sampling and lab isolations in work on expanding the spectral range of oxygenic photosynthesis through far- red light-harvesting antennas, with discoveries of such features in new organisms as well as in organisms in which such capabilities had not been previously suspected. These longer wavelength pigments may serve as analogs for pigments on the early, haze-covered Earth, or on planets orbiting M dwarfs, where photosynthetically-active radiation may be dominated by longer wavelengths. Kiang and colleagues, in collaboration with the NExSS GISS group, continue work on modeling vegetation for GISS 3D GCM models of hypothetical extrasolar planets.

In innovative work on the definition of new biosignatures, Krissansen-Totton, Catling and colleague tested the idea that atmospheric chemical disequilibrium can act as a biosignature. Performing the first, detailed quantification of thermodynamic disequilibrium in Solar System planetary NAI Information Management System 31 of 91

atmospheres, they showed that the Earth has ~20 times the thermodynamic chemical disequilibrium of other planets because of the biosphere (Krissansen-Totton et al., 2016). They identified the high abundance of atmospheric N2, O2 and the presence of an ocean, as the strongest disequilibrium state for our planet. Work is continuing on the topic of biosignatures and disequilibrium with an effort to identify “anti-biosignatures”, gases present as a result of purely abiotic processes that should be consumed by microbes.

VPL team members also contributed to understanding planetary protection issues for Mars exploration. Baross participated in a MEPAG report focused on identifying and assessing the planetary protection issues with “special regions” on Mars – where Earth life might survive or replicate (Rettberg et al., 2015). Baross and colleagues also presented the case for updating the current planetary protection policy applied to these special regions, based on new information about Earth organisms (Rettberg et al., 2016).

The Observer

In this task we explore the detectability of signs of habitability and life for modeled observations from the previous tasks, and address target selection for exoplanet observations. We also observe and develop new observational, analysis and retrieval techniques to improve the detection of terrestrial sized exoplanets, and enhance our understanding of the environmental properties of exoplanets for current and future observations.

This year, we developed new concepts and methods for planet detection and explored criteria to prioritize target selection of terrestrial planets for more detailed observational follow-up. VPL Team members Luger, Agol, Barnes and Deming developed the EVEREST method to de-trend K2 light curves and overcome the jitter in K2 observations. This pipeline technique can now recover the photometric precision of the original Kepler mission for bright stars (Luger et al., 2016). In a follow- up activity EVEREST is being used to discover previously unknown small HZ planets in the K2 data, which may be amenable to follow-up with JWST. Kasting and Kopparapu contributed to a new catalog of Kepler habitable zone exoplanet candidates (Kane et al., 2016). These are now high priority targets for follow-up studies to first prove that they are planets, and to then characterize them further. Agol and colleagues used the transit timing technique to measure masses for 10 Kepler exoplanets between 3-8 times the mass of the Earth (Jontof-Hutter et al., 2016). These data were used to further support the density/size relation for exoplanets, which will support selection of terrestrial planet targets for follow-up. Barnes, Meadows and Evans developed a new comparative habitability assessment framework to convert observed transit parameters into a single number that represents the likelihood that a planet is rocky and receives the insolation required to permit habitable surface conditions (Barnes et al., 2015)

VPL team members also contributed to research on how we might characterize terrestrial habitable zone planets. Agol and colleague derived expressions for transit timing variations near first and second order orbital resonances, and these can be used to estimate densities for small planets, including, eventually, those in the habitable zone (Agol & Deck, 2016; Deck & Agol, 2016). Dobbs- NAI Information Management System 32 of 91

Dixon, Agol and Deming showed than an offset in the timing of secondary eclipse could be due to a displaced hot spot in a planet’s atmosphere. They showed that observations at multiple wavelengths could allow this effect to be discriminated from a similar offset in timing due to an eccentric orbit (Dobbs-Dixon et al., 2015). This observational technique could potentially be applied to exoplanet terrestrials to help constrain atmospheric circulation without having to measure the entire phase function. Krissansen-Totton, Schwieterman, Charnay, Arney, Robinson, Meadows and Catling used reflectance spectra of planets in our Solar System and VPL simulated exoplanet environments to identify the wavelength regions needed so that broadband photometry can best discriminate Earth twins from other planets. Such a technique could be used to screen newly discovered exoplanets to find targets that have a spectral color that appears Earth-like (Krissansen-Totton et al., 2016). Robinson and colleagues also published a recent survey of the capabilities of small, space-based coronagraphs (such as WFIRST-AFTA) to characterize the atmospheres of our nearest exoplanet neighbors, and showed that larger planets such as Jupiter and Neptune were prime targets for these smaller telescopes. Potentially habitable terrestrial planets were shown to be accessible to larger telescopes, or smaller telescopes with major advancements in throughputs, as may be obtained by pairing a starshade with a 2 m class telescope (Robinson et al., 2016). Kopparla, Crisp and Yung developed a new PCA-based radiative transfer model for planetary atmospheres that is faster than our current workhorse radiative transfer model, SMART (Kopparla et al., 2016a). Kopparla and Yung also developed a new multiple-scattering polarization radiative transfer model and applied it to HD189733b as an initial test (Kopparla et al., 2016b). This model will ultimately be used to understand polarization signals from terrestrial planets. VPL Team member Raymond contributed to a paper on the specification and capabilities of the Planet Formation Imager, an instrument to be deployed on future extremely large telescopes (Kraus et al., 2016). Deming and colleague submitted an invited review on observations of exoplanet atmospheres for the 25th anniversary of JGR-Planets.

In work on how to detect planetary biosignatures and observationally discriminate them from their false positives, this year we explored spectral discriminants for false positives for oxygen, and submitted a comprehensive review of oxygen as a biosignature. Schwieterman, Meadows, Domagal- Goldman, Deming, Arney, Luger, Harman, Misra and Barnes identified spectral features that could be used to discriminate abiotic sources of O2 in a planetary atmosphere. They also calculated the detectability of CO and O4 from abiotic processes in JWST spectra for exoplanets orbiting M stars (Schwieterman et al., 2016). This work strengthens our confidence in identifying true oxygen biosignatures in advance of the first spectroscopic studies of rocky habitable zone planets. Meadows wrote a comprehensive and detailed review of O2 as a biosignature in exoplanetary atmospheres, also covering potential false positives for O2, and the observations required to detect O2 and discriminate whether it has a biological or abiological source. This review emphasizes that O2 and O3 detection alone are not robust biosignatures, and that false positives must also be ruled out to increase confidence in the biosignature interpretation. This can be done using knowledge of stellar parameters and a more comprehensive observational census of atmospheric composition and conditions on terrestrial exoplanets.

In ongoing work for VPL’s terrestrial exoplanet spectral retrieval suite, Lustig-Yaeger has completed an end-to-end retrieval suite and Luger and Lustig-Yaeger are developing a cost function that NAI Information Management System 33 of 91 penalizes unphysical atmospheres – so that the retrieval will be constrained not just by the limitations in the spectral data, but by known characteristics of the planet and planetary system. Lustig-Yaeger, Schwieterman, Robinson, Meadows and colleagues have developed a new synthetic dataset of the Earth and are using it to investigate how future time-dependent observations of terrestrial exoplanets can be used to map heterogeneous planetary surfaces and discriminate between different surface types (e.g. liquid water, exposed rock, and vegetation). The goal of this work is to identify the key telescope parameters or observing sequences that will optimize the exoplanet mapping capability of next generation space-based telescopes like WFIRST, HabEx, and LUVOIR. Lustig-Yaeger presented this work at the Exoplanets I and DPS conferences, while Schwieterman presented the new synthetic 3D Earth model dataset at DPS.

In addition to the work on the evolutionary states of the potentially habitable, potentially Earth-mass planet Proxima Cen b (c.f. Habitable Planet task), VPL team members also simulated current environmental states and observational discriminants for these states. Meadows, Arney, Schwieterman, Lustig-Yaeger, Lincowski, Robinson, Domagal-Goldman, Barnes, Fleming, Deitrick, Luger, Driscoll, Quinn and Crisp examined the current climatic states that may be possible for Proxima Centauri b, given the plausible evolutionary scenarios described in Barnes et al., (2106). Several of these were found to be habitable at Proxima Cen b’s position in the star’s habitable zone – but several were not habitable. Synthetic spectra of these states were generated and used to identify potential observational spectral discriminants for the Proxima Cen b planetary environment, including the presence of O2 and O4 due to ocean loss (Meadows et al., submitted). Luger, Lustig- Yaeger, Fleming, Tilley, Agol, Meadows, Deitrick, and Barnes also submitted a paper exploring the feasibility of detecting the auroral oxygen green line from Proxima Centauri b with a coronagraph- equipped next generation space-based or ground-based telescope. Detection of this O line would confirm the existence of the planet, pin down its mass and eccentricity, and establish that it is a terrestrial planet with an atmosphere – all of which are critical to understanding its habitability (Luger et al., submitted). Raymond participated in another paper analyzing the habitability of Proxima Centauri b (Turbet et al., 2016).

Community and Mission Support Activities

working on end-to-end simulations of self-consistent Earth-like planets orbiting M dwarfs, as viewed by JWST. These simulations are being used to calculate the exposure time needed to observe diagnostic features in the exoplanet spectra, and to test the robustness of the retrieval techniques on simulated data for JWST. These results were presented by Meadows at the conference on Transiting Exoplanets at the Space Telescope Science Institute in November 2015. Many of these tools and techniques also formed the basis for a WFIRST Science Investigation Team proposal that was submitted late last year by Meadows (PI), Domagal-Goldman, Barnes, Robinson, Lincowski, Lustig- Yaeger and colleagues, to simulate forward and instrument modeling, and data analysis and retrieval, for observations of small planets taken with a WFIRST-AFTA coronagraphic mission. Although this proposal was not successful, Lustig-Yaeger and Domagal-Goldman are participating in the WFIRST Community Exoplanet Data Challenge to assess the potential for the upcoming NASA mission to characterize the atmospheres of terrestrial exoplanets. This year, Robinson became a new member of the 10 member Executive Council of the NASA Exoplanet Exploration Program Analysis Group (ExoPAG), joining VPL Team members Domagal-Goldman and Walkowicz on the EC.

The VPL also became a member of the NASA Nexus for Exoplanet System Science research network this year, and as part of the NExSS consortium, Fortney, Robinson, Domagal-Goldman, Claire, Crisp, Kopparapu, Meadows and Pierrehumbert contributed to a white paper on the need for laboratory work in understanding exoplanet atmospheres (Fortney et al., 2016). VPL team members Domagal- Goldman, Parenteau and Kiang spearheaded organization of the NExSS/NAI Biosignatures Workshop in July 2016, and Domagal-Goldman, Barnes and Meadows participated as Science Organizing Committee members on the Upstairs Downstairs Workshop on planetary interiors and atmospheres held in February 2016. In other community activities, Domagal-Goldman was the chief editor and Anderson, Arney, Goldblatt and Stüeken contributed to the Astrobiology Primer 2.0 (Domagal-Goldman et al., 2016). VPL team members Barnes, Baross, Des Marais, Domagal- Goldman, Meadows, Robinson and Wordsworth comprised seven of the 33 Lead Authors of NASA’s Astrobiology Strategy Document. Blankenship, Driscoll, Kiang and Som served as authors, and Meadows was also a reviewer. Buick, Hoehler, Meadows, Pierrehumbert, Som and Zahnle were contributors to the document. VPL Team member John Baross wrote the Introduction to the Strategy (Baross, 2015).

Education and Public Outreach

We now have both of the Science on a Sphere shows deployed at the Pacific Science Center: “Earth – The Pale Blue Dot” describing exoplanet detection and what Earth would look like as an exoplanet, and “Alien Earths” a description of life’s coevolution with its environment and how that changed the Earth throughout its history. This year, two additional Astrobiology Science Communication fellows have received training. Work is now well underway for a hands-on temporary exhibit on Biosignatures at Pacific Science Center.

Our Research: Year 5

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This year the VPL was at the forefront of terrestrial habitable zone detection and characterization with the publication of papers discovering the TRAPPIST-1 planetary system, which contains 3 potentially habitable planets (Gillon et al., 2017; Luger et al., 2017a), and assessing the current state and possible observable characteristics of Proxima Centauri b, the closest habitable zone exoplanet (Luger et al., 2017b).

Solar System Analogs for Extrasolar Planetary Processes and Observations

In this task we use observations of Solar System planets and moons to explore the detectability of signs of habitability and life in planetary environments, and to constrain planetary processes. This year we used observations of Enceladus and modeling and observations of Mars to learn about potentially habitable current and past environments in the Solar System and to constrain mechanisms for terrestrial planet formation and evolution.

We studied the climate of early Mars, exploring mechanisms that may have allowed it to be habitable. In, Batalha et al (2016), VPL members Batalha, Kopparapu, Haqq-Misra, and Kasting explored a geophysical mechanism that could have induced cycles of glaciation and deglaciation on early Mars. Their model produces dramatic climate cycles with extended periods of glaciation punctuated by warm periods lasting up to 10 Myr. This hypothesis can be tested by future Mars exploration that better establishes the time scale for valley formation. In another study, Sholes, Claire, Zahnle, and Catling used a photochemical model to study the reducing power of volcanic gases and the environment of ancient Mars (Sholes et al 2017), showing that under conditions of extensive volcanism, the Martian atmosphere would have become anoxic, similar to early Earth’s, with implications for the early climate, microbiology, and the modern environment. We also reviewed the constraints and paradoxes set by climate and volatile budgets of ancient Mars, which impact its potential as an abode for life. (Haberle et al 2017).

In addition to these studies of early Mars, we considered present Mars and Mars in the context of exoplanets. Co-I Claire contributed to a study considering the Tindouf Basin region in Southern Morocco as an analog for Noachian Mars soils (Oberlin et al 2017). Additionally, Co-I Parenteau contributed to a review summary for the Conference on Biosignature Preservation and Detection in Mars Analog Environments (Hays et al 2017). In Ehlmann et al. (2016), co-I Zahnle contributed to a study aimed at understanding the sustainability of habitability of terrestrial planets in the context of needed measurements from Mars for understanding the evolution of Earth-like worlds.

To understand other possible Solar System habitable environments, Zahnle contributed to a study examining the ballistics of impact ejecta launched by impact cratering events on Enceladus (Alvarellos et al 2017). This is of general relevance to characterizing Enceladus, a potentially habitable ocean world with geysers erupting from its south polar “Tiger Stripes” region. In Del Genio et al (2017), Domagal-Goldman, Kiang, and Kopparapu leveraged VPL models to discuss how recent trends and advances in Earth weather and climate modeling can be used to guide our modeling of Solar System and exoplanet climates in the coming decades NAI Information Management System 36 of 91

Early to Current Earth

Research in this task focuses on studying early Earth to provide examples of habitable planetary environments that are very different to our modern Earth and that may also be encountered in the exoplanet population. Our work this year further constrained the properties of the atmosphere, interior and biosphere of the early Earth.

For constraining the characteristics of early Earth environments Sleep (2016) provided constraints on the Hadean (before 4 billion years ago) showing that most of the “late veneer” of Pt-group elements in the silicate earth may have arrived during the moon-forming impact from the core of the impactor Theia. This suggests that the subsequent late heavy bombardment may have been relatively benign. The Archean Earth (4-2.5 billion years ago) may have been intermittendly cloaked by a global organic haze and Arney, Meadows, Domagal-Goldman, Deming, Robinson, and Schwieterman (Arney et al., 2016) used photochemical climate models to show that similar environment on planets orbiting stars that produce higher UV levels than the sun do not form organic haze as readily as lower UV stars. This organic haze produces a spectral signature that can be detected by future space telescopes including JWST and potential future direct imaging missions such as LUVOIR. In a follow-on study, Arney, Domagal-Goldman, and Meadows (Arney et al. (in press)), explored the idea that haze formation may be a biosignature, showing that organic haze formation requires high methane fluxes consistent with Earthlike biological methane production in the presence of Earthlike CO2 levels, and organic haze forms at lower CH4/CO2 levels in the presence of biological organic sulfur gases. Krissansen-Totton and Catling (2017) used an inverse geological carbon cycle model to deduce that continental weathering is a weaker planetary thermostat than what was previously assumed, and that seafloor weathering is an important complimentary thermostat. This has implications for the climate evolution of ocean exoplanets, where seafloor weathering is the dominant thermostat. VPL team members Charnay and Catling (Charnay et al. 2017) used a 3-D climate- carbon model to show that the carbon cycle was efficient for producing moderate climate conditions on the early Earth and that similar processes could moderate climate on Earth-like exoplanets. Bolton contributed to a study on reactive transport models (Li et al 2017) which can be used to explore multiple subsurface processes relevant to habitability. Stüeken et al (2016) compiled a box model of the Earth’s planetary nitrogen cycle to determine the biogeochemical conditions that would be necessary to induce large mass changes in the atmospheric nitrogen reservoir. They found that subsets of the model architectures could produce large, biologically-mediated changes in atmospheric nitrogen abundance, implying that the abundance of N2 in exoplanetary atmospheres may be useful as an ancillary biosignature. Importantly, organic burial is the most likely way the account for low atmospheric pressure. In Oze et al (2017), Co-I Sleep contributed to a study that determined that Cr(III) is oxidized to Cr(VI) during serpentinization, providing a possible path to a strong oxidant on + the early Earth. Johnson et al (2017) adapted a fluorometric technique for measuring NH4 in water + to use in geologic samples. This technique could be used to measure silicate bound NH4 in extraterrestrial materials, and N is an important component of life.

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In addition to modeling and laboratory work, team members undertook field work to collect geological samples that probe the environmental conditions and evolution of life on early Earth. Stüeken and Buick (Stüeken et al 2017a) determined that bulk rock samples yield more reliable nitrogen isotope measurements at low metamorphic grades, and using nitrogen isotopic ratios as a biosignature on other planets will probably require bulk rock analysis. Johnson and Goldblatt (Johnson et al in press) measured N-isotopes, redox sensitive trace elements, and Fe-speciation in Marinoan aged Snowball Earth samples that indicate there was oxygen production, active nitrogen cycling, and open water during Snowball Earth, with several microbial processes persisting during this intense glacial period. Johnson et al (2017) also measured N-isotopes and concentration in glacial tills (0-2.9 Ga) to reveal a secular increase in N in upper continental crust through time, due to biologic N-fixing. The sequestration of N into rocks and minerals by life could result in less bioavailable N in the biosphere, but produce a long-term storage vessel for future use. Stüeken et al (2017b) explored how carbon and nitrogen isotopic biosignatures indicate Archean lakes with differing environmental conditions supported different microbial metabolisms, with ramifications for early Mars, where lakes may have been prevalent, and if inhabited, similar metabolic diversity may be expected. Stüeken (2017) reviewed the biogeochemical selenium isotope proxy, estimating average crustal selenium levels and deriving a first marine selenium isotope mass balance. Selenium can trace relatively subtle redox changes over time, and thereby constraint the evolution of complex life. Kipp, Stüeken, and Buick used heavy selenium isotopes, which are a good redox proxy in deep time, to characterize the mid-Paleoproterozoic Lomagundi carbon isotope excursion, supporting the "oxygen overshoot" hypothesis (Kipp et al 2017). In Izon et al (2017), VPL team member Claire performed the highest resolution geochemical study of the Archean, and found a direct correlation between robust proxies for methanogenesis and for changes in atmospheric chemistry. They argue that biology was in control of atmospheric chemistry, and temporarily drove organic hazes on the Early Earth. Additionally, in Zerkle et al (2017), Claire contributed to a high resolution study over the interval where the atmosphere became oxic, and they showed that the microbial nitrogen cycle responded immediately, implying rapid evolutionary to environmental change. Koehler, Stüeken, Kipp, and Buick (Koehler et al 2017) found that nitrate levels in the Mesoproterozoic ocean (~1700- 800 Ma) were lower than in preceding and following eras. This would have potentially limited bioavailable N, and consequently the evolution and proliferation of more complex organisms, until ocean nitrate increased in the Neoproterozoic. Kipp and Stüeken (2017) demonstrated that recycling of organic matter was muted in the anoxic Precambrian oceans, lowering available phosphate, and identifying a new throttle on net biological productivity on dominantly anoxic planets. Stüeken et al (2017c) revisited rocks from the Mesoproterozoic Stoer Group in Scotland and showed that these purported ancient lake settings had seawater input, calling into question interpretations that lakes were favorable habitats for complex organisms in the mid-Proterozoic. Consequently more rigorous geochemical tests are required to accurately reconstruct under which conditions early eukaryotes were able to thrive. VPL member Claire also helped find rapidly fluctuating geochemical signatures at a Northern ocean site during the late Permian extinction, that implied there was a rapidly shifting redox-cline at the site, with temporary refuges for animals requiring oxygen (Mettem et al., 2017).

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Figure 1: Professor Roger Buick with 2.65 billion year old Aeolian cross-bedded sandstones that are relics of dunes formed from wind-blown sand near Koegas, South Africa. This year VPL geologists obtained samples from these sites to improve atmospheric pressure determinations for early Earth.

The Habitable Planet

This VPL task explores habitable planet formation, and the effect on planetary habitability of interactions between the potentially habitable planet, its star, other planets in the system, and the host galaxy. Significant research this year includes modeling of planetary disks, internal, orbital and atmospheric evolution, feedbacks between the relevant processes, and considerations of habitability from a variety of perspectives.

In the area of planet formation and migration, VPL team members Backus and Quinn explored dust migration in gravitationally active protoplanetary disks and found that gravitational “turbulence” can significantly slow the settling of dust, potentially slowing the formation of Earth-like planets, which may require dense regions of dust to form. Fleming and Quinn (2017) showed that the interaction of binary stars with their protoplanetary disk could results in the stars moving closer together, which may ultimately affect the formation and habitability of terrestrial planets forming in binary systems. Quinn and colleagues (Szulágyi, Mayer & Quinn, 2017) determined observational criteria to distinguish between the core accretion and gravitational instability planet formation scenarios in disks around young, giant planets. Deitrick, Quinn, Barnes, and co-author investigated the evolution of

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the Proxima Centauri system in its galactic context, finding that close encounters with Alpha Centauri and other stars have likely significantly altered the orbital evolution of the Proxima Centauri planetary system, and may have stripped outer planets.

VPL team members continue to study the impact of gravitational and radiative interactions between star and planet, on planetary habitability. Armstrong and colleagues (Zollinger, Armstrong & Heller, 2017) performed a numerical investigation into exomoon evolution for systems in the habitable zone

of low-mass stars ( 0.6 M☉) and showed that exomoons in the habitable zone of dwarf stars with masses 0.2 M would be uninhabitable due to extreme tidal heating. Barnes (2017) explored how ☉ ≲ tidal torques affect the spin evolution of habitable exoplanets and found that tidal locking may be ≲ prevalent, with even Earth-Sun analogs in danger of synchronous rotation, implying that most of the potentially habitable planets that will be discovered by TESS will be tidally locked. We used climate models to understand how atmospheric composition and dynamics – along with orbital parameters – can strongly affect planetary habitability and the limits of the habitable zone. VPL members Wolf, Shields, Kopparapu, Haqq-Misra, and co-author used the NCAR CAM GCM to identify four stable climate states for planets around F-, G-, K-, and M-dwarfs with constant CO2, which are defined by mutually exclusive global mean surface temperatures (Wolf et al., 2017). Kopparapu, Wolf, Arney, Batalha, Haqq-Misra and co-authors also used CAM for evaluating the inner edge of the habitable zone around M-dwarf stars, finding that planets in synchronous rotation can undergo water loss in a “moist greenhouse” state while maintaining a habitable surface temperature (Kopparapu et al., 2017). In a study of the lifetimes of habitable hydrospheres of hypothetical water-rich satellites of warm giant exoplanets, Lehmer, Catling and Zahnle (2017) found that water may survive indefinitely on the surface of a giant moon, provided it is in the habitable zone and the moon is sufficiently large. Checlair, Abbot and colleague (Checlair, Menou, & Abbot 2017) used GCM simulations and theory to show that potentially habitable, tidally locked planets are unlikely to be found in a snowball (globally glaciated) state due to the insolation geometry. This may be detrimental, as Snowball states in Earth history are correlated with increases in atmospheric oxygen and the complexity of life. Bitz and colleagues (Rose, Cronin, & Bitz 2017) determined that multiple equilibria and unstable transitions between climate states (ice-free, Snowball, or ice cap/belt) are found over wide swaths of parameter space. At high obliquity, these include a “Large Ice Belt Instability” and “Small Ice Belt Instability”.

We also focused this year on reviewing and probing the conditions for planetary habitability and how it fits within the broader context of the search for habitable exoplanets. Catling and Kasting (2017) published a research-level book surveying atmospheric evolution and climate on Earth, in the Solar System, and elsewhere. Shields and coauthors wrote a review of M dwarf habitability based on work done in this area over the past decade, and summarize future directions planned in this quickly evolving field (Shields et al., 2016). Zahnle and Catling (2017) published their work on the “cosmic shoreline”, the evidence that atmospheric escape determines which planets have atmospheres, and applied this to Proxima Centauri b and the TRAPPIST-1 system. VPL team member Abbot and colleague (Komacek & Abbot, 2016) calculated surface-mantle water exchange under a variety of parameterizations and found that surface water content is highly dependent on which

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parameterization is used. This suggests further work is needed to understand whether a planet has a dry surface, a surface like Earth with water and land, or is a water world. Abbot and co-authors (Bean, Abbot & Kempton, 2017) argued that the best scientific returns may come from using new telescopes to determine statistically whether scientific theories related to planetary habitability work, by studying large numbers of Earth-like planets rather than by focusing on how we may be able to tell whether a specific extrasolar planet is inhabited by life. Considering statistical versus specific- target studies could change the planning and design of future telescopes.

VPL member Haqq-Misra led two papers on understanding the search for life in the universe. The first, with Wolf and Kopparapu, examined the calculated width of the habitable zone using Bayesian statistics to argue that our existence around a G-dwarf, instead of a K- or M-dwarf, is not necessarily a statistical fluke, but suggests that the search for intelligent life should expand being G-dwarf stars (Haqq-Misra, Wolf, and Kopparapu, 2017). As a chapter in a book on habitability of the universe, Haqq-Misra and Kopparapu (2017) considered the Drake equation as a function of spectral type and time, suggesting that the maximum lifetime of communicative civilizations depends on the spectral type of the host star, and implying that F- and G-dwarf stars are the best places to search for signs of technological intelligence today.

The Living Planet

In this research area, VPL team members use modeling, laboratory and field work to understand the co-evolution of the environment and biosphere, aspects of life’s global impact that could be detected remotely as biosignatures, and the potential for planetary environments to generate false positives for life. In this past year, we considered the history and ecologies of microbial communities, including those in hydrothermal vent systems, we advanced the field’s understanding of photosynthetic processes, and we strengthened the study exoplanet biosignatures by highlighting the importance of environmental context for their interpretation.

In Souza et al (2017), Co-I Siefert contributed to a review of how humans impact the microbiota of desert oases. This research impacts our understanding of how severe environmental changes affect the biogeochemistry of prokaryotic dominated ecologies, which in turn informs the astrobiology community on the community dynamics that might exist on exoplanets or during past phases of Earth history. In Zarraz et al (2017), Co-I Siefert helped examine how stoichiometric imbalances change the metabolic capacity of microbial communities. This provides data for models that might be used to evaluate whether life can exist on other planets when the stoichiometry of primary elements for life are different from earth life. We also examined microbial populations at hydrothermal vents with different geochemistry in Anderson et al (2017), finding that they exhibit different evolutionary histories. Hydrothermal vents are thought to have been important settings for the origin and early evolution of life, so understanding how microbial populations evolve there can give us insight into selection pressures that molded life's earliest stages of evolution.

To improve our understanding of photosynthesis, we examined surface reflectance and polarization biosignatures of primitive anoxygenic phototrophs, which are relevant to the detection of life on NAI Information Management System 41 of 91

planets before the evolution of oxygenic photosynthesis (Parenteau et al 2017). In Wolf et al (2017), Parenteau and Blankenship helped examine the isolation of a variety of Chlorophyll d- and f- containing oxygenic phototrophs, which are adapted to preferentially harvest near-infrared light, and may inform us of the operation and detection of oxygenic photosynthesis on M dwarf exoplanets. VPL members Kiang and Parenteau isolated the original cyanobacterium in which the far-red pigment chlorophyll d was first discovered in 1943 and found that the organism's wavelength of peak absorbance corresponds to a peak in solar radiation transmittance by red algae, and the trap wavelengths of Photosystem I and II at 740 and 723 nm correspond, respectively, to a minimum and maximum in red algae spectral transmittance (Kiang et al 2017). These adaptations of far-red oxygenic photosynthesis to spectral light environment may provide insights into photosynthetic processes for planets orbiting red M dwarf stars. Blankenship reviewed the diversity of photosynthetic pigments that function as light absorbing antennae and as photochemical sensitizers (Blankenship 2017a), and provided a commentary on new findings on the origin and evolution of cyanobacteria, and their impact on the Great Oxidation Event (Blankenship 2017b).

Figure 2.Comparison of irradiance from the Sun at the Earth’s surface (Lean & Rind, 1998), from the red dwarf star Proxima Centauri at a habitable zone distance, and from the Sun transmitted through the algae E. delesseroides (red) showing similarity between M dwarf incident radiation and transmission through red algae. In vivo absorption spectra of Acaryochloris marina MBIC11017 and a Chl a cyanobacterium, Synechococcus are shown for comparison.

To support an improved understanding of O2 as a biosignatures Meadows (2017) provided a comprehensive review of O2 detectability and false positive work to date, arguing that studying a biosignature in the context of its environment strengthened biosignature interpretation and increased

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the chance of success for exoplanet characterization missions. In a companion News and Views article, Domagal-Goldman (2017) emphasizes the need for “self-skepticism” in the context of future biosignature detections that will be scrutinized heavily by the scientific community. In this mindset, scientific skepticism can be viewed as a proactive method to strengthen our case for biosignature interpretation in the face of provocative data. VPL Team Member Schwieterman worked with fellow UCR Team Members Reinhard, Lyons and others to use ocean biogeochemistry and atmospheric models (Reinhard et al, 2017) to suggests that the O2-CH4 disequilibrium, traditionally considered the most convincing biosignature for any Earth-like atmosphere, would have been challenging to detect remotely throughout most of Earth’s 4.5-billlion-year history. This work further supported spectral capabilities for exoplanet characterization missions from the UV through the near- IR to enhance the chance of success in searching for life beyond the solar system.

VPL Team Members also contributed substantially to a series of biosignature review papers that summarized the NExSS/NAI joint ExoPAG SAG 16 community biosignatures workshop held in August 2016 (Kiang et al 2017). These papers are all currently in review in Astrobiology, and aim to reflect the “state of the art” of the field in topics including a biosignature review, oxygen as a biosignature, a framework for biosignature assessment, future directions for biosignature research, and observational prospects for exoplanet biosignatures. To ensure broad interdisciplinary input, the international scientific community was widely solicited to participate in the writing of these manuscripts. VPL members serving as lead or co-authors on these papers include: Meadows, Arney, Catling, Deitrick, Domagal-Goldman, Harman, Kiang, Krissansen-Totton, Lincowski, Parenteau, Schwieterman, Lincowski, Lustig-Yaeger and Robinson.

The Observer

This year, VPL contributed to the discovery and characterization of exoplanets, including key terrestrial-sized planets in the habitable-zone that are likely targets for JWST characterization. Agol provided transit-timing analysis for an intensive photometric monitoring campaign of TRAPPIST-1 from the ground and with the Spitzer Space Telescope, which discovered and initially characterized the TRAPPIST-1 planetary system, with seven planets with sizes, masses and densities similar to the Earth (Gillon et al., 2017). Luger and Agol, in a campaign lead by graduate student Rodrigo Luger, then used data from the K2 telescope to confirm the orbit of TRAPPIST-1 h, the smallest and coldest planet in the TRAPPIST-1 exoplanet system, and found that all seven planets in the system are linked in an intricate chain of three-body resonances (Luger et al., 2017b). Agol contributed to an automated search for long-period exoplanets in archival Kepler light curves (Foreman-Mackey et al., 2016), finding 16 long-period exoplanet candidates and extending Kepler’s reach to planets with only one transit. Sheets and Deming (2017) also used co-added Kepler data to calculate average secondary NAI Information Management System 43 of 91

eclipse depths and infer planetary geometric albedos as a function of planetary radius. They found that, as a class, close-in super-Earth and Neptune-like planets have low geometric albedos, suggesting that they are typically not cloudy.

To support the imminent launch of the James Webb Space Telescope (JWST), VPL continued to model physical processes and observational phenomena that could impact transit transmission spectroscopy of terrestrial planets. Robinson (2017) demonstrated that aerosol forward scattering can impact exoplanet transit spectra, and showed under which conditions forward scattering will be an important process for understanding transit transmission observations. Deming and co-author report model calculations that elucidate an effect operating during exoplanet transit spectroscopy wherein stellar flux leaks from neighboring spectral regions into the exoplanetary absorption lines, creating errors in the spectra of transiting exoplanets (Deming and Sheppard, 2017).

Looking toward direct imaging observations, Gao, Zahnle, Robinson, and co-authors explored how thick sulfur hazes, like those that enshroud Venus might form in the atmospheres of some warm giant planets and influence the reflected-light spectra of directly imaged exoplanets (Gao et al., 2017). As Venus and its sulfuric-acid clouds are a model for the end-stage of a once-habitable world, we anticipate that future exoplanet direct imaging missions must be able to identify and understand the signatures of sulfur hazes. Thinking forward to the needs of future observers and observing missions, VPL members also contributed to the SAG 15 report (Apai et al 2017), which collected and organized top-level questions that can be studied with future direct imaging technology.

VPL continues to advance data retrieval methods, especially for mission concepts like HabEx and LUVOIR, which aim to characterize rocky planets in the habitable zones of nearby stars using direct imaging. Lustig-Yaeger and co-authors (Fujii, Lustig-Yaeger, and Cowan, 2017) identified a mathematical degeneracy when fitting multi-wavelength, time-series observations of directly-imaged terrestrial exoplanets, that makes it impossible to simultaneously extract the longitudinal map and reflectance spectrum for each surface detected on a planet. This will limit our ability to uniquely constrain the color and composition of individual planetary surfaces when searching for signatures of habitability and life on exoplanets. VPL team members Zahnle (Lupu et al., 2016), and Fortney and Robinson (Nayak et al., 2017), contributed to a pair of papers that developed atmospheric retrieval methods for reflected light imaging of gas giants to evaluate the capabilities of future coronagraphic space telescopes.

We continue to explore methods to characterize potentially habitable planets. In comprehensive, interdisciplinary work last year, Meadows, Arney, Schwieterman, Lustig-Yaeger, Lincowski, Robinson, Domagal-Goldman, Barnes, Fleming, Deitrick, Luger, Driscoll, Quinn and Crisp examined the current climatic states that may be possible for Proxima Centauri b, which has now been accepted for publication in Astrobiology (Meadows et al., 2017). Several of the modeled states were found to be habitable at Proxima Cen b’s position in the star’s habitable zone – but several were not habitable. Synthetic spectra of these states were generated and used to identify potential observational spectral discriminants for the Proxima Cen b planetary environment, including the presence of O2 and O4 due to ocean loss. We also calculated that phase curves with JWST/MIRI photometry can shed light on NAI Information Management System 44 of 91

whether or not Proxima b possesses an atmosphere, how efficiently heat redistributes from the dayside to the nightside, and whether or not the atmosphere contains CO2. Earlier this year, Luger, Lustig-Yaeger, Fleming, Tilley, Agol, Meadows, Deitrick, and Barnes explored the feasibility of detecting the auroral oxygen green line from Proxima Centauri b with a coronagraph-equipped next generation space-based or ground-based telescope. Detection of this O line would confirm the existence of the planet, pin down its mass and eccentricity, and establish that it is a terrestrial planet with an atmosphere – all of which are critical to understanding its habitability (Luger et al., 2017a). We also use models and observations to study using polarimetry to characterize exoplanets. VPL members Bailey and Bott contributed in a study mapping the polarization of the interstellar medium and characterizing the polarization of sun-like stars (Cotton et al., 2017a). This aids characterizing the polarized light contributions from noise sources that will affect the detection of polarized habitability signatures. Bailey and Bott also contributed to the first detection of polarized light from a rapidly rotating star (Cotton et al, 2017b), which was predicted theoretically 50 years ago. This not only provides a testament to the power of the new generations of polarimeters, but has also aided the development of tidal models of stellar atmospheres. This, in turn, has propelled the development of tidal models for exoplanet atmospheres and their effects on their host stars.

Community and Mission Support Activities

The tools, science and small exoplanet observing expertise developed by the VPL are playing an integral role in NASA exoplanet mission and mission concept development. The new pipeline developed by Luger, Agol and Barnes will enhance the yield of small planets from K2 and Kepler. Development of target selection procedures will benefit JWST observations. Meadows and Domagal-Goldman are Science and Technology Definition Team members for the Large UV Optical Infrared Survey Telescope (LUVOIR) and Domagal-Goldman and Robinson are Science and Technology Definition Team members for the Habitable Exoplanet Explorer mission (HabEx). Arney was recently promoted to lead of the LUVOIR Science Support and Analysis Team, which is the group that leads the simulations of LUVOIR performance. VPL Team Members Robinson, Arney, Lustig-Yaeger, Schwieterman, Domagal-Goldman and Meadows contributed to modeling and simulated spectra for these mission efforts, and led the development and writing of the biosignature and habitability science cases for the LUVOIR mission, and are now all contributing to the LUVOIR Interim Report to NASA HQ. Deming led a JWST Early Release Science proposal, which included Meadows and and Lustig-Yaeger, for observations of HZ terrestrials with JWST. This year, Meadows became Chair of the Execurive Council of the NASA Exoplanet Exploration Program Analysis Group (ExoPAG), joining VPL Team members Robinson, and Domagal-Goldman and Walkowicz, who rotate off the EC next year. This year, VPL team members contributed substantially to the review papers drafted as the output from the Domagal-Goldman, Parenteau and Kiang organized NExSS/NAI Biosignatures Workshop, which was held in July 2016.

Education, Public Outreach and Training

REU program - For 9 weeks, from the end of June to mid August, 2017, two students (Renae Stanley and Melvin (Sunny) Miles) from Northwest Indian College participated in a pilot NAI Information Management System 45 of 91

Astrobiology REU Program for students from Minority Serving Institutions at the University of Washington. The first week of the program, Co-I Harnett provided the students with an introduction to the University of Washington, taking them on a tour, and arranging for them to meet with groups on campus that support students of color and first generation college students. Dr. Harnett also provided them with foundation training for working in a research environment, including competency in working in a Linux computing environment, using Matlab to analyze data, using Python to conduct data analysis, and the basics of the Virtual Planet Laboratory modeling suite. The students then worked with PI Meadows, and two graduate students Lincowski and Lustig-Yaeger to study the habitability of Proxima b. R. Stanley presented a poster with the results at the annual Washington NASA Space Grant Reception in September.

Figure 3: Renae Stanley in front of here and Sunny’s poster at the annual Washington NASA Space Grant Reception in September.

Portal to Current Research – A six month exhibit at the Pacific Science Center, regarding the search for biosignatures went live March 17, 2017 and closed September 15, 2017. The exhibit, titled “Mission: Find Life!” explored three themes: “

• Are We Alone? – This area explored who are astrobiologists and what techniques do they use to conduct their research, such as how are extra solar planets found? Also included was information about Kepler and an interactive that allowed visitors to explore how different sized planets will cause different transit light curves. • Narrowing the search for life – This are explored how important water is for supporting and the ubiquity of microbial life on Earth • Searching for Life – This area explored the idea of biosignatures and the methods that could be used to find them.

A touch table also enabled visitors to use the NASA Eyes on Exoplanets program to investigate some of the strange exoplanets in our galaxy.

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Science on Sphere shows – The second SOS show,” Ancient Earth, Alien Earth”, was finalized and both shows were in regular rotation at the Pacific Science Center. Dave Cuomo, from PSC, presented both shows at a national SOS meeting in April in Chicago.

Fellows – The final cohort of Astrobiology Science Fellows received training their training. There are now 8 active Fellows (Erik Goosmann, Joshua Krissansen-Totton, Brett Morris, Steven Sholes, Max Showalter, Anna Simpson, Marshall (Moosh) Styczinski, Matt Tilley). A 9th Fellow, Megan Smith, left the area after completing her PhD. To remain active, the Fellows participate in at least 3 PSC sponsored events a year, such as the upcoming event in which B. Morris, M. Styczinski and E. Harnett will support a special showing of the new 3D IMAX film “The search for life in space” on Dec. 6, which will include short presentations by the Fellows prior to the movie and a Q&A session afterwards.

Supplemental Funds – Supplemental EPO funds were applied for from NAI Central, and awarded. These funds are being used to develop an Astrobiology curriculum for Middle and High school students regarding ecosystems and habitability. The curriculum was piloted at a teacher professional development workshop in Bremerton, WA, on October 7, 2017. Co-I Harnett co-taught the workshop with Dr. Kareen Borders, the Director of the Washington West Sound STEM Network. 35 teachers from elementary to high school learned about NASA search for exoplanets, the concept of biosignatures and habitability. They constructed self-contained eco-systems and used IR cameras to learn how remote sensing can indicate the nature of a planetary surface. A finalized curriculum is now being completed, incorporating feedback from the participants in the workshop.

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Our Research: Year 6 (NCE)

This final NCE year saw VPL publish several critical projects from the previous 5 years, including a series of six seminal papers on exoplanet biosignatures, as part of a broader community collaboration. We also published significant results on the nature, potential habitability and observational discriminants for the newly discovered Proxima Centauri and TRAPPIST-1 planetary systems.

Solar System Analogs for Extrasolar Planetary Systems

In this task we use observations of Solar System bodies to explore processes related to the detection and interpretation of habitability and life in other planetary systems. Key work this year was driven by a collaboration in keeping with one of the original themes of this task (“Earth as an exoplanet”), where team members used data from NASA’s Deep Space Climate Observatory (DSCOVR) to study techniques for characterizing a distant exo-Earth.

NASA’s DSCOVR satellite produces a wealth of visible photometric data for our planet from the Earth-Sun L1 point. While these data were primarily envisioned as critical for climate-related studies, VPL team members have instead taken an innovative look at the observations, choosing to view Earth as a Pale Blue Dot. In Jiang et al. (2018), team members Yung, Kopparla and Natraj explored how rotation rate, cloud coverage, and surface types could be extracted from unresolved DSCOVR data. Also, in Li et al. (2019), DSCOVR data were used to understand how both ice clouds and the ocean can produce glints in Earth observations.

Tools developed from VPL “Earth as an exoplanet” studies were adapted for a wide range of projects within this last year. In a VPL-led study, Lustig-Yaeger et al. (2018) explored how crescent- and non-crescent-phase maps of a distant Earth, produced from rotationally-resolved photometric observations, could reveal the presence of a glinting ocean surface. Also, in a cross-divisional application of VPL tools, team members Schwieterman and Robinson participated in a study of how radiative energy from the distant Earth impacts the thermal balance of certain Lunar regions (Glenar et al. 2019).

The Earth Through Time

Research in this task focuses on studying early Earth to provide examples of habitable planetary environments that are very different to our modern Earth and that may also be encountered in the exoplanet population, and to make testable predictions about remotely-detectable signatures of terrestrial exoplanets at different phases of evolution. Our work this year further constrained the properties of the atmosphere, interior and biosphere of the early Earth, and how those relate to observable properties on analog exoplanets.

Several papers and a book chapter resulted from our NAI cross-team collaboration with the NAI Alternative Earths team. One study including VPL Team member Schwieterman, with UCR colleagues Reinhard, Lyons, Olson and Ridgewell examines the challenges and opportunities of using NAI Information Management System 49 of 91 seasonally variable spectral features as a biosignature: ozone at Proterozoic-like levels, for instance, might be detectable as a seasonally-variable biosphere driven by variations in O2 production by photosynthetic life (Olson et al. 2018a). Additionally, in an effort to better understand the early nitrogen cycle VPL Team members Kipp, Stüeken, and Buick collaborated with Bekkar from the Alternative Earths team to measure nitrogen and organic carbon isotope ratios in ancient marine sediments deposited during and before the Great Oxidation Event (GOE) (Kipp et al. 2018). They found that aerobic nitrogen cycling (similar to modern Earth) lasted several hundred million years following the GOE, and they used a simple isotope mass-balance model to quantify the relationship between ocean oxygenation and sedimentary nitrogen isotope ratios. This study found persistently positive N isotope ratios, indicating that ocean water was continually oxygenated across this time interval, supporting the proposal that there was an “oxygen overshoot” to near modern levels after initial atmospheric oxygenation. Their work also implies fixed nitrogen was bioavailable prior to the earliest fossil record evidence of eukaryotic life. Olsen, Schwieterman, Reinhard and Lyons also contributed a chapter in the Handbook for Exoplanets on the atmospheric evolution of Earth through time (Olson et al. 2018b).

This year, VPL explored a new technique for determining atmospheric pressure on the early Earth that may also have relevance to Mars. VPL Team members Catling, Som, Buick, and colleagues (Goosmann et al. 2018) examined wind-blown sand dune deposits as a potential record of early atmospheric pressure. They found that this parameter is not very sensitive to small (less than an order of magnitude) pressure changes. However, it may be a useful tool for investigating the evolution of atmospheric pressure on Mars, where pressure changes of such scales may have occurred, and where wind-blown sand is abundant in the geological record.

On the nature of early life on Earth and its interaction with the atmosphere, VPL highlights this year include the discovery of multiple transient early oxygenation events, and research that showed microbes may have had a significant impact on atmospheric mass. Buick, Stüeken, and colleagues examined nitrogen and selenium abundance excursions indicating an earlier oxygen “whiff” prior to the GOE (Koehler et al. 2018). This suggests multiple transient oxygenation events occurred in the lead-up to permanent planetary oxygen of Earth (and possibly elsewhere). Additionally, Buick and colleagues reported nitrogen isotope ratios in a Mesoarchean marine drill-core from the NASA- funded Astrobiology Drilling program (Koehler et al. 2019). The drill cores fall near to the atmospheric nitrogen values, indicating the Mesoarchean ocean was anoxic, and nitrogen cycling was dominated by microbial fixation, which may have acted to draw down atmospheric nitrogen levels, lowering atmospheric pressure and thus acting as a possible biosignature. Finally, to better understand early methane production, Stüeken and Buick found that 3 billion year old lakes were sources of microbial methane to the ancient atmosphere whereas marine environments were not (Stüeken and Buick 2018). This shows that environmentally heterogeneous planets such as early Mars, or some exoplanets, may exhibit localized sources of greenhouse gases. When analyzing analogous carbon isotopes from rocks with total low organic carbon contents, organic contamination can be a significant issue as shown by a study by Stüeken, Buick, and collaborators (Muller et al. 2018). This contamination may be a major issue for analyzing samples from Mars.

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A series of modeling studies helped illuminate how habitable planets stay habitable, and identified potential novel biosignatures for early-Earth-like worlds. Krissansen-Totton, Arney, and Catling (Krissansen-Totton et al. 2018a) used a geological carbon cycle model to examine environmental conditions for the past 4 billion years. This modeling work predicts a temperate (0 – 50 °C) climate and circumneutral ocean pH throughout the Precambrian due to stabilizing feedbacks from continental and sea-floor weathering. Similar stabilizing feedbacks on climate and ocean pH may operate on earthlike exoplanets, implying life elsewhere could emerge in comparable environments. Krissansen-Totton, Catling (Krissansen-Totton et al. 2018c) and colleague examined disequilibrium biosignatures over Earth history, and showed that the atmosphere of a planet like the early Earth would have a chemical disequilibrium characteristic of the early biosphere between nitrogen, carbon dioxide, methane and water, and that carbon at each end of the redox spectrum (as CO2 and CH4) forms a detectable combinational biosignature for anoxic atmospheres. Finally, Arney, Domagal- Goldman, and Meadows (Arney et al. 2018) published a study on organic haze as a biosignature for anoxic planets analogous to Archean Earth, showing that for atmospheres with CO2-levels similar to early Earth, haze formation requires biological CH4 production rates.

The Habitable Planet

This VPL task explores habitable planet formation and the effect on planetary habitability of interactions between the potentially habitable planet, its star, and other planets in the system. Significant progress was made this year on understanding multiple factors that affect habitability. Highlights include 3-D modeling of planetary atmospheres, predictions for the observable features of the Proxima Centauri and TRAPPIST-1 planets, the effect of multiple, realistic stellar flare sequences on the atmospheric composition of a habitable planet, and the coupling between orbital dynamics and habitability.

The VPL performed several simulations of habitable planet formation in the Solar System and beyond, with multiple impacts on our understanding of how terrestrial planets may or may not acquire water. Raymond and collaborators developed new models of the formation of the Solar System that explore the possibility that the giant planets became unstable after the dispersal of the primordial protoplanetary disk (Clement et al., 2018, 2019a, 2019b), and examined how 1I/’Oumuamua can be used to constrain planet formation processes (Raymond et al., 2018a, 2018b). Raymond and colleagues also performed simulations that showed that super-Earth planets may be mostly rocky, even if they form from ice-rich particles (Raymond et al., 2018c).

This past year we also explored how orbital dynamics impacts a planet’s habitability, including a review on this topic by Barnes & Deitrick (2018). Fleming, Barnes, Luger, Quinn and colleague (Fleming et al. 2018) demonstrated how tidal interactions in young stellar binaries with orbital periods less than 10 days undergo orbital expansion that can eject planets, suggesting surveys for habitable exoplanets should avoid tight binaries. Deitrick et al. (2018a) showed the wide range of rotational behavior that may occur on a habitable planet due to perturbations from other worlds. Then in Deitrick et al. (2018b) a simple climate model was used to predict that the coupled rotational- orbital evolution incited by perturbations from other planets can often lead to a global ice sheet.

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Extensive VPL simulations of planetary atmospheres were used to understand impacts on habitability and observable characteristics of terrestrial exoplanet environments, for both the Proxima Centauri and TRAPPIST-1 systems. Meadows, Arney and Schwieterman led an interdisciplinary publication with 11 other VPL team members to model the self-consistent atmospheres for Proxima Centauri b, assuming early evolution for ocean and atmospheric loss (Meadows et al., 2018). This paper was the first attempt to identify observational discriminants for habitable and uninhabitable planets, and provided simulated observing requirements for observations with JWST and future coronagraphic telescopes like LUVOIR. Later in the year, Lincowski, Meadows, Crisp, Robinson, Luger, Lustig- Yaeger and Arney used a significantly upgraded 1-D climate-chemistry model to generate habitable and Venus-like post-ocean and atmospheric loss environments for the TRAPPIST-1 planets, and generated transit and thermal emission spectra to help JWST identify uninhabitable planets (Lincowski et al., 2018).

Significant advances were also made in improving the accuracy of terrestrial exoplanet models, to better understand the effect of various stellar and planetary characteristics on habitability. Arney, Kopparapu, Domagal-Goldman and colleagues updated a 1-D climate model (ATMOS) to include a realistic treatment of clouds, which was validated against observational data, and expands the versatility and accuracy of this model for terrestrial exoplanets (Fauchez et al., 2018). Abbot and student Komacek (Komacek & Abbot, 2018) used 3-D GCMs to model and better understand the effect of a planetary parameters on atmospheric circulation and the climate of terrestrial planets orbiting Sun-like and M-dwarf stars. Haqq-Misra, Wolf and Kopparapu (Haqq-Misra et al. 2018) investigated the atmospheric dynamics of terrestrial planets in synchronous rotation within the habitable zone of low-mass stars using the ExoCAM GCM, and determined how to infer the dynamical state of the atmosphere using thermal emission phase curve observations. VPL Team members Wolf and Abbot, with colleagues (Yang et al. 2018) reported on a comparison of habitable planet GCMs and found substantial differences in their atmospheric dynamics and cloud schemes. Shields and colleague showed that hydrohalite, a possible surface component of M-dwarf planets, could lower global mean surface temperature (Shields & Carns, 2018). VPL Team members Tilley, Segura, Meadows, Hawley and colleague also published work on repeated M dwarf flaring on an unmagnetized Earth-like planet in the habitable zone using an upgraded version of the Atmos model that handles time-variable insolation (Tilley et al., 2018). They found that repeated high-energy flares and proton events could sufficiently deplete O3 that it was unlikely to recover. Haqq-Misra and colleague performed the first GCM simulations of a terrestrial exomoon orbiting a planet in the habitable zone, taking into account incident radiation from both planetary and stellar illumination, and constraining the plausible climate configurations that would allow an exomoon to remain habitable (Haqq-Misra & Heller, 2018).

VPL team members also contributed to multiple chapters from the recently released Handbook of exoplanets, including a comprehensive review of our improved understanding of the factors affecting habitability by VPL and other researchers (Meadows & Barnes, 2018), and a review of the Habitable Zone concept (Kopparapu, 2018).

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Task D: The Living Planet

In this research area, VPL team members use laboratory and field work, and modeling, to understand the co-evolution of the environment and biosphere, aspects of life’s global impact that could be detected remotely as biosignatures, and the potential for planetary environments to generate false positives for life. In this past year, we re-examined the fundamental concept of chemical disequilibrium, and quantified the potential of several different kinds of metabolisms to produce planetary biosignatures, including CO and H2 metabolism. We also explored the role of a biosphere- coupled carbonate-silicate cycle on ocean pH and oxygenic photosynthesis. In addition to this focused research, VPL members produced major review papers on exoplanet biosignatures. The major theme has been life’s interactions and measurable impacts on a diversity of environmental contexts.

In new reference textbooks on exoplanets and astrobiology, in Hoehler et al. (2018b) and (Hoehler et al. 2018a), Co-Is Hoehler, Som, Kiang, and Parenteau lay out the fundamentals physical and chemical properties of life, and discuss energy use by organisms of a variety of metabolisms in different environments. The life-hosting potential of these environments – not just whether, but how abundantly and robustly they could support life – is expectedly as diverse as the conditions and processes that prevail there. These chapters review our current understanding of life’s requirements and provide a means of assessing whether alternative solutions might also be viable.

Several VPL modeling studies then explored constraints on the limits of life’s productivity. Sholes, Catling, Lustig-Yaeger (in press, 2019 ) show that the levels of CO and H2 in the martian atmosphere, which could be metabolized by microbes using O2, set an upper limit on the biomass that could be below Mars' surface and connected to the atmosphere through porous rock and soil, which is ≤10-4-10-5 of Earth’s biomass. Lehmer, Catling, Parenteau, Hoehler (Lehmer et al. 2018) showed that photosynthesis on inhabited planets around small red dwarf stars would be light-limited and so may not be sufficient to build up detectable levels of oxygen, even accounting for possible biological adaptation to near-infrared-shifted stellar spectra.

VPL members also explored a variety of novel candidate biosignatures, and much of that work used life’s impact on the Earth through time, and so is reported in Task B. Additionally DasSarma and Schwieterman (2018) (Co-Is Schwieterman) examined the possible role of retinal phototrophy in the early history of life on Earth to capitalize on the peak of energy from the Sun, and the implication for exoplanet biosignatures.

To quantify detectability of signs of life in an upcoming mission, Krissansen-Totton et al. (2018b) examine the detectability of the CH4+CO2 biosignature combination (developed under work in Task B) with the James Webb Space Telescope. They find that approximately 10 transits may be sufficient to detect such disequilibrium biosignatures on nearby transiting planets such as TRAPPIST-1e if it is an Archean Earth analog.

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This year VPL members led the astrobiology community in taking stock of the science to date and directions for exoplanet biosignatures, producing a Special Issue on Exoplanet Biosignatures for the journal Astrobiology (Kiang et al. 2018), with support from the Nexus for Exoplanet System Science (NExSS) Exoplanet Biosignatures Workshop Without Walls. Schwieterman et al. (2018) (with VPL team members Schwieterman, Kiang, Parenteau, Harman, Arney, and Meadows) compiled an encyclopedic review paper on the state of the science of remotely detectable exoplanet biosignatures, covering atmospheric and surface candidates, the life processes from which they originate, fundamentals about photosynthetic light harvesting as not previously summarized with regard to spectral limits, the spectral features of the numerous candidate biosignature targets, and the types of models that have been used to identify them. In a major review focusing on oxygenic photosynthesis, Meadows et al. (2018) (including VPL team members Meadows, Arney, Parenteau, Schwieterman, Domagal-Goldman, and Lustig-Yaeger) interpret perhaps the deepest body of literature for a biosignature to emphasize the importance of environmental context to interpreting a signal. This review covers the origins of oxygenic photosynthesis on Earth, its expression on Earth through time, the challenges for its detection, and especially the variety of potential false negative and false positive cases for oxygen as a biosignature on exoplanets. Catling et al. (2018) (Catling, Kiang, Crisp, Robinson, Domagal-Goldman, and Krissanssen-Totton) lay out an overarching framework for assessing the detection of the biosignature candidates summarized by Schwieterman et al. (2018), taking into account probabilities within the environmental context by drawing upon the Bayesian approach broached by Walker et al. (2018) (Domagal-Goldman, Kiang, and Schwieterman), who explore novel concepts of life, and how to best interpret biosignatures in the widely sampled, sparse data for the future. In Fujii et al. (2018), Domagal-Goldman provided vital input with regard to the technological capabilities of near-future missions to detect Earth-size exoplanets and biosignatures.

Task E: The Observer

With the discovery in 2017 of the TRAPPIST-1 system by Michael Gillon and collaborators, including several VPL team members, many of our efforts have pivoted to characterizing and modeling future observations of this touchstone M-dwarf system. Team members Bott, Crisp and Yung have assessed the detectability of oceans on terrestrial planets in crowded planetary systems like TRAPPIST-1 using the polarization signals in the light that they reflect (Kopparla et al. 2018). This paper examines what water worlds with and without clouds would look like in TRAPPIST-1 planetary orbits, and it shows that ocean glint is a strong signal. While this scenario is simplified, determining the strength of a polarized light signal from hypothetical aqua planets is central to determining polarimetry's utility for habitability studies. Team member Bott has also investigated polarization methods applied to other larger, hotter worlds, such as WASP-18b (Bott et al. 2018) and HD 189733b (Bailey et al. 2018). The methodologies in these papers, modeling the polarization NAI Information Management System 54 of 91

signal with the code VSTAR and examining noise sources for polarimetry, will be valuable for guiding future polarimetric observations of terrestrial planets.

VPL team members Agol and Raymond have investigated the dynamical characterization of the TRAPPIST-1 system (Grimm et al. 2018). This paper presented the most precise constraints for habitable-zone, Earth-sized exoplanets outside the Solar System using a transit-timing analysis enabled by Spitzer and Kepler observations, which required an improved detrending analysis developed by VPL team members Luger, Agol and colleagues (Luger et al. 2018).

Several members of the VPL team began preparations for the first “Cycle” of JWST observations (before it was found that JWST would be delayed until 2021 at the earliest). Agol and Morris investigated the impact of the inhomogeneity and variability of the star on the inference of the planet transmission spectra (Ducrot et al. 2018; Morris et al. 2018a,b). As discussed in Task C one of the main foci for JWST has been modeling transit transmission spectroscopy of the TRAPPIST-1 system, which is an observational technique reviewed by VPL team member Drake Deming (Deming et al. 2018). One of the main goals is to search for biosignatures in the atmosphere of planet e, which is closest in its current properties to Earth (Lincowski et al. 2018; Krissansen-Totton 2018). Submitted work by Lustig-Yaeger, Meadows and Lincowski follows on from Lincowski et al., (2018) to calculate the detectability of these features with JWST, and support observation planning for HZ terrestrial exoplanet characterization. This paper shows that TRAPPIST-1 planetary atmospheres could be detected in as little as two transits, and specific molecules detected in longer, but feasible, integration times. VPL Team work has also shown that future JWST observations may also allow for measurements of planet maps (Luger et al. 2019), atmospheric refraction (Sheets et al. 2018), and secondary eclipse measurements of TRAPPIST-1, as VPL team member Deming and colleagues have accomplished for 55 Cnc e (Tamburo et al. 2018).

Further in the future, we expect to characterize habitable-zone exoplanets around stars which are more Sun-like than the M dwarf TRAPPIST-1. Many VPL Team members have contributed to telescope concepts to accomplish these studies from both ground (Berdyugina et al. 2018), and from space with LUVOIR or HabEx, two missions under consideration by NASA for the 2020 decadal survey study. Several possibilities exist for characterization of these planets, and VPL Team members have been instrumental in determining the feasibility of many of these techniques: planets may be counted and categorized with a direct-imaging survey (Kopparapu et al. 2018); oceans may be detected from time-variable mapping to isolate the phase-dependent behavior of ocean glint (Lustig-Yaeger et al. 2018); the compositions of atmospheres might be inferred (Feng et al. 2018); and moons might be searched for given the commonality of habitable-zone giant planets (Hill et al. 2018). In complementary work, the lack of planets around short-period binary stars may help guide where to look for planets with direct imaging (Fleming et al. 2018a). The analysis of these datasets could be aided by a novel atmospheric computations (Robinson & Crisp 2018), as well as a novel machine-learning method adapted into Python by team member David Fleming (Fleming et al. 2018b).

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The VPL team continued its extensive presence on key NASA activities to support the exoplanet community and design missions capable of exoplanet astrobiology. This year, Meadows continued as the Chair of the Executive Council of the NASA Exoplanet Exploration Program Analysis Group (ExoPAG), and Robinson served on the Executive Committee. Meadows served on two National Academy of Sciences Committees on Astrobiology and Exoplanet Strategies, which published comprehensive reports on a very short time scale, as input to the upcoming Astro2020 decadal survey. Meadows and Domagal-Goldman are Science and Technology Definition Team members for the Large UV Optical Infrared Survey Telescope (LUVOIR) and Domagal-Goldman and Robinson are Science and Technology Definition Team members for the Habitable Exoplanet Explorer mission (HabEx). Arney continues as the lead of the LUVOIR Science Support and Analysis Team, which is the group that leads the simulations of LUVOIR performance. VPL Team Members Robinson, Arney, Lustig-Yaeger, Lincowski, Schwieterman, Domagal-Goldman and Meadows contributed to modeling and simulated spectra for these mission concept efforts, as well as for the Origins Space Telescope mission concept. VPL team members also led the development and writing of the biosignature and habitability science cases for the LUVOIR mission, and are now contributing to the LUVOIR Final Report to NASA HQ. Most notably, Lustig-Yaeger and Arney led the LUVOIR Design Reference Mission activity for science yield for habitable planets with LUVOIR. VPL team members also contributed substantially to numerous white papers submitted to the NAS committees on Astrobiology and Exoplanets, and to the recent call for science white papers for the Astro 2020 Decadal Survey. These efforts include: Haqq-Misra (2018) (with Co-Is Schwieterman and Som) who submitted a white paper in response to the National Academies of Sciences Astrobiology Science Strategy on The Astrobiology of the Anthropocene, proposing research on technologically derived biosignatures. White papers stemming from the six exoplanet biosignatures review papers were submitted to the National Academies of Science for the Astrobiology Science Strategy and Exoplanet Science Strategy (Domagal-Goldman et al. 2018a; Domagal-Goldman et al. 2018b), and a report was provided to the NASA Exoplanet Exploration Program as an ExoPAG activity.

New Technology

As part of our research, the VPL developed a new software tool with innovative features that may be broadly applicable. The software package, called VPLanet, links together multiple scientific models in a modular and expandable way such that an arbitrary planet can be simulated. Specifically, the code takes advantage of the C language's ability to build and define matrices of function pointers, which allows the software to collect models at run time and call only the subroutines relevant to the selected models. To our knowledge, this approach has not been implemented in scientific software previously, and possibly not as a commercial or open source project. VPLanet is open source and available at https://github.com/VirtualPlanetaryLaboratory/vplanet.

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Publications (398 Total)

Abbot, D.S. (2015) A Proposal For Climate Stability On H2-Greenhouse Planets, Astrophysical Journal Letters, 815, L3. (Doi: 10.1088/2041-8205/815/1/L3)

Abbot, D.S. (2016) Analytical Investigation Of The Decrease In The Size Of The Habitable Zone Due To Limited Co2 Outgassing Rate, The Astrophysical Journal, 827, 117 (Doi: 10.3847/0004- 637x/827/2/117) I

Agol, E., And Deck, K. (2016) Transit Timing To First Order In Eccentricity, The Astrophysical Journal, 818(2), 1-17 (Doi: 10.3847/0004-637x/818/2/177)

Agol, E., Jansen, T., Lacy, B., Robinson, T. D., Meadows, V. (2015) The Center Of Light: Spectroastrometric Detection Of Exomoons, The Astrophysical Journal, 812, 5. (Doi: 10.1088/0004- 637x/812/1/5)

Ahlers, J.P., Barnes, J.W. & Barnes, R. (2015) Spin-Orbit Misalignment Of Two-Planet-System Koi- 89 Via Gravity Darkening, The Astrophysical Journal, 814(1) (Doi: 10.1088/0004-637x/814/1/67)

Alvarello, J. L., Dobrovolskis, A.R., Hamill, P., Zahnle, K.J., Dones, L., & Robbins, S. (2017). Fates Of Satellite Ejecta In The Saturn System, Ii. Icarus, 284, 70-89. (Doi: Https://Doi.Org/10.1016/J.Icarus.2016.10.028)

Anderson, R. E., M. L. Sogin And J. A. Baross. (2015) Biogeography And Ecology Of The Rare And Abundant Biosphere In Deep-Sea Hydrothermal Vents. Fems Microbiology Ecology, 91, 1-11 (Doi: 10.1093/Femsec/Fiu016)

Anderson, R. E., Reveillaud, J., Reddington, E., Delmont, T. O., Eren, A. M., Mcdermott, J. M., ... & Huber, J. A. (2017). Genomic Variation In Microbial Populations Inhabiting The Marine Subseafloor At Deep-Sea Hydrothermal Vents. Nature Communications, 8(1), 1114.

Anglada-Escudé, G., Tuomi, M., Gerlach, E., Barnes, R., Heller, R., Jenkins, J. S., … Jones, H. R. A. (2013). A Dynamically-Packed Planetary System Around Gj 667c With Three Super-Earths In Its Habitable Zone. A&A, 556, A126. Doi:10.1051/0004-6361/201321331

Apai, Daniel, Nicolas Cowan, Ravikumar Kopparapu, Markus Kasper, Renyu Hu, Caroline Morley, Yuka Fujii Et Al. "Exploring Other Worlds: Science Questions For Future Direct Imaging Missions (Exopag Sag15 Report)." Arxiv Preprint Arxiv:1708.02821 (2017).

Armstrong, J. C.; Barnes, R.; Domagal-Goldman, S., Breiner, J.; Quinn, T. R.; Meadows, V. S., (2014) Effects Of Extreme Obliquity Variations On The Habitability Of Exoplanets, Astrobiology, Vol. 14, Issue 4, Pp. 277-291. Doi: 10.1089/Ast.2013.1129

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Arney, G., Domagal-Goldman, S. D., & Meadows, V. S. (2018). Organic Haze As A Biosignature In Anoxic Earth-Like Atmospheres. Astrobiology, 18(3), 311–329. Https://Doi.Org/10.1089/Ast.2017.1666

Arney, G., Meadows, V., Crisp, D., Schmidt, S. J., Bailey, J. And Robinson, T. (2014), Spatially Resolved Measurements Of H2o, Hcl, Co, Ocs, So2, Cloud Opacity, And Acid Concentration In The Venus Near-Infrared Spectral Windows, Jgr: Planets 119(8) 1860–1891. Doi: 10.1002/2014je004662

Arney, G., V. S. Meadows, S. D. Domagal-Goldman, D. Deming, T. Robinson, G. Tovar, E. T. Wolf, E. Schwieterman. (2017) Pale Orange Dots: The Impact Of Organic Haze On The Habitability And Detectability Of Earthlike Exoplanets. The Astrophysical Journal, 846(49).

Atreya, S. K., Trainer, M. G., Franz, H. B., Wong, M. H., Manning, H. L. K., Malespin, C. A., … Navarro González, R. (2013). Primordial Argon Isotope Fractionation In The Atmosphere Of Mars Measured By The Sam Instrument On Curiosity And Implications For Atmospheric Loss. Geophysical Research Letters, 40(21), 5605–5609. Doi:10.1002/2013gl057763

Backus, I., & Quinn, T. (2017). Dust Migration In Gravitationally Active Protoplanetary Disks. Lpi Contributions, 1975.

Backus, I., Quinn, T. (2016) Fragmentation Of Protoplanetary Discs Around M-Dwarfs, Monthly Notices Of The Royal Astronomical Society, 463(3): 2480-2493 (Doi: 10.1093/Mnras/Stw1825)

Bailey, J., Kedziora-Chudczer, L., & Bott, K. (2018). Polarized Radiative Transfer In Planetary Atmospheres And The Polarization Of Exoplanets. Monthly Notices Of The Royal Astronomical Society, 480(2), 1613–1625. Https://Doi.Org/10.1093/Mnras/Sty1892

Barclay, T., Rowe, J. F., Lissauer, J. J., Huber, D., Fressin, F., Howell, S. B., … Thompson, S. E. (2013). A Sub-Mercury-Sized Exoplanet. Nature, 494(7438), 452–454. Doi:10.1038/Nature11914

Barnes, R. (2015) A Method To Identify The Boundary Between Rocky And Gaseous Exoplanets From Tidal Theory And Transit Durations. International Journal Of Astrobiology, 14, 321-333. (Doi: 10.1017/S1473550413000499)

Barnes, R. (2017). Tidal Locking Of Habitable Exoplanets. Cel. Mech. Dyn. Astron., In Press. Online: Https://Link.Springer.Com/Article/10.1007/S10569-017-9783-7 (Doi: 10.1007/S10569-017- 9783-7)

Barnes, R., & Heller, R. (2013). Habitable Planets Around White And Brown Dwarfs: The Perils Of A Cooling Primary. Astrobiology, 13(3), 279–291. Doi:10.1089/Ast.2012.0867

Barnes, R., Deitrick, R. Greenberg, R. Quinn, T.R. & Raymond, S.N. (2015) Long-Lived Chaotic Orbital Evolution Of Exoplanets In Mean Motion Resonances With Mutual Inclinations. The Astrophysical Journal, 801, 101. (Doi: 10.1088/0004-637x/801/2/101)

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Barnes, R., Meadows, V. S., Evans, N. (2015) Comparative Habitability Of Transiting Exoplanets. The Astrophysical Journal, 814, 91. (Doi: 10.1088/0004-637x/814/2/91)

Barnes, R., Meadows, V.S., & Evans, N. (2015) Comparative Habitability Of Transiting Exoplanets, The Astrophysical Journal, 814(2) (Doi: 10.1088/0004-637x/814/2/91)

Barnes, R., Mullins, K., Goldblatt, C., Meadows, V. S., Kasting, J. F., & Heller, R. (2013). Tidal Venuses: Triggering A Climate Catastrophe Via Tidal Heating. Astrobiology, 13(3), 225–250. Doi:10.1089/Ast.2012.0851

Baross, J. A., And Martin, W. F. (2015) The Ribofilm As A Concept For Life’s Origin, Cell, 82:13- 15 (Doi:10.1016/J.Cell.2015.06.038)

Batalha, N. E., Kopparapu, R. K., Haqq-Misra, J., & Kasting, J. F. (2016). Climate Cycling On Early Mars Caused By The Carbonate–Silicate Cycle. Earth And Planetary Science Letters, 455, 7-13. (Doi: 10.1016/J.Epsl.2016.08.044)

Batalha, N. E., Kopparapu, R. K., Haqq-Misra, J., & Kasting, J. F. (2018). Reply To Shaw. Earth And Planetary Science Letters, 484, 415–417. Https://Doi.Org/10.1016/J.Epsl.2017.12.018

Baum, S. D., Denkenberger, D. C., & Haqq-Misra, J. (2015). Isolated Refuges For Surviving Global Catastrophes. Futures, 72, 45–56. (Doi: 10.1016/J.Futures.2015.03.009)

Bean, J.L., D.S. Abbot And E. M.-R. Kempton (2017), A Statistical Comparative Planetology Approach To The Hunt For Habitable Exoplanets And Life Beyond The Solar System, The Astrophysical Journal, 841:L24. (Doi: 10.3847/2041-8213/Aa738a)

Becker, A. C., Kundurthy, P., Agol, E., Barnes, R., Williams, B. F., & Rose, A. E. (2013). Observations Of The Wasp-2 System By The Apostle Program . The Astrophysical Journal, 764(1), L17. Doi:10.1088/2041-8205/764/1/L17

Berdyugina, S. V., Kuhn, J. R., Langlois, M., Moretto, G., Krissansen-Totton, J., Grenfell, J. L., … Apai, D. (2018). The Exo-Life Finder (Elf) Telescope: New Strategies For Direct Detection Of Exoplanet Biosignatures And Technosignatures. In R. Gilmozzi, H. K. Marshall, & J. Spyromilio (Eds.), Ground-Based And Airborne Telescopes Vii. Spie. Https://Doi.Org/10.1117/12.2313781

Black, R. A., And Blosser, M. C. (2016) A Self-Assembled Aggregate Composed Of A Fatty Acid Membrane And The Building Blocks Of Biological Polymers Provides A First Step In The Emergence Of Protocells, Life, 6(3), 33 (Doi: 10.3390/Life6030033)

Black, R. A., Blosser, M. C., Stottrup, B. L., Tavakley, R., Deamer, D. W., & Keller, S. L. (2013). Nucleobases Bind To And Stabilize Aggregates Of A Prebiotic Amphiphile, Providing A Viable Mechanism For The Emergence Of Protocells. Proceedings Of The National Academy Of Sciences, 110(33), 13272–13276. Doi:10.1073/Pnas.1300963110

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Blankenship, R. E. (2017) How Cyanobacteria Went Green. Science, 355: 1372-1373.

Blankenship, R. E. (2017). The Diversity Of Photosynthetic Pigments. Astrobiology Science Conference (Abscicon) Abstract #3288.

Bolmont, E., Raymond, S.N., Leconte, J., Correia, A. & Hersant, F. (2015) Mercury-T: A New Code To Study Tidally Evolving Multi-Planet Systems. Applications To Kepler-62, Astronomy & Astrophysics, 583, A116 (Doi: 10.1051/0004-6361/201525909)

Bolmont, E., Raymond, S.N., Leconte, J., Hersant, F., And Correia, A. C. M. (2015) Mercury-T: A New Code To Study Tidally Evolving Multi-Planet Systems. Applications To Kepler-62. Astronomy And Astrophysics, 583, A116. (Doi: 10.1051/0004-6361/201525909)

Bolmont, Emeline, Raymond, Sean N., Von Paris, Philip, Selsis, Franck, Hersant, Franck, Quintana, Elisa V., And Barclay, Thomas, 2014, Formation, Tidal Evolution, And Habitability Of The Kepler-186 System, The Astrophysical Journal, 793. Doi: 10.1088/0004-637x/793/1/3

Bonsor, A., Raymond, S. N., & Augereau, J-C. (2013). The Short-Lived Production Of Exozodiacal Dust In The Aftermath Of A Dynamical Instability In Planetary Systems. Monthly Notices Of The Royal Astronomical Society, 433(4), 2938–2945. Doi:10.1093/Mnras/Stt933

Borucki, W. J., Agol, E., Fressin, F., Kaltenegger, L., Rowe, J., Isaacson, H., … Winn, J. N. (2013). Kepler-62: A Five-Planet System With Planets Of 1.4 And 1.6 Earth Radii In The Habitable Zone. Science, 340(6132), 587–590. Doi:10.1126/Science.1234702

Bott, K., Bailey, J., Cotton, D. V., Kedziora-Chudczer, L., Marshall, J. P., & Meadows, V. S. (2018). The Polarization Of The Planet-Hosting Wasp-18 System. The Astronomical Journal, 156(6), 293. Https://Doi.Org/10.3847/1538-3881/Aaed20

Bradley, K., Weiss, B.P. & Buick, R. (2015) Records Of Geomagnetism, Climate And Tectonics Across A Paleoarchean Erosion Surface. Earth & Planetary Science Letters, 419, 1-13. (Doi: 10.1016/J.Epsl.2015.03.008)

Byrne, B. And Goldblatt, C. (2014) Radiative Forcings For 28 Potential Archean Greenhouse Gases, Clim. Past, 10, 1779-1801. Doi: 10.5194/Cp-10-1779-2014

Byrne, B. And Goldblatt, C. (2015) Diminished Greenhouse Warming From Archean Methane Due To Solar Absorption Lines, Climate Of The Past Discussions, 11, 559-570. (Doi:10.5194/Cp-11-559- 2015)

Cady Lawrence P., Brack André, Bueno Prieto Jorge E., Cockell Charles, Horneck Gerda, Kasting James F., Lineweaver Charles H., Raulin François, Schopf J. William, Sleep Norman, Von Bloh Werner, Westall Frances, Deamer David, Lehman Niles, And Pérez-Mercader Juan. Astrobiology. August 2014, 14(8): 629-644. Doi:10.1089/Ast.2014.1405.

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Carter, J. A., & Agol, E. (2013). The Quasiperiodic Automated Transit Search Algorithm . The Astrophysical Journal, 765(2), 132. Doi:10.1088/0004-637x/765/2/132

Catling Dc. 2014. Mars Atmosphere: History And Surface Interactions. In Encyclopedia Of The Solar System (2nd Ed.), Ed. T Spohn, Tv Johnson, D Breuer, Pp. 343-57. New York: Elsevier

Catling, D. Astrobiology : A Very Short Introduction. Very Short Introductions.

Catling, D. C. (2015) Planetary Atmospheres In Treatise On Geophysics, Vol. 10, 429-472 (Doi:10.1016/B978-0-444-53802-4.00185-8)

Catling, D. C., And Kasting, J. F. (2017) Atmospheric Evolution On Inhabited And Lifeless Worlds, Cambridge University Press, 592 Pages.

Catling, D. C., Krissansen-Totton, J., Kiang, N. Y., Crisp, D., Robinson, T. D., Dassarma, S., Rushby, A. J., Del Genio, A., Bains, W., Domagal-Goldman, S.: Exoplanet Biosignatures: A Framework For Their Assessment. Astrobiology 18, 2018. Doi: 10.1089/Ast.2017.1737.

Catling, D.C. (2014). 6.7 – The Great Oxidation Event Transition. In: Holland, H.D. & Turekian, K.K. (Eds.). Treatise On Geochemistry (Second Edition). Oxford: Elsevier.

Charnay, B., Barth, E., Rafkin, S., Narteau, C., Lebonnois, S., Rodriguez, S., Courrech Du Pont, S., And Lucas, A. (2015) Methane Storms As A Driver Of Titan/'S Dune Orientation. Nature Geoscience, 8, 362. (Doi: 10.1038/Ngeo2406)

Charnay, B., Le Hir, G., Fluteau, F., Forget, F., And Catling, D. C. (2017) A Warm Or Cold Early Earth? New Insights From A 3-D Climate-Carbon Model, Earth & Planetary Science Letters.

Charnay, B., Meadows, V., Leconte, J. (2015) 3d Modeling Of Gj1214b's Atmosphere: Vertical Mixing Driven By An Anti-Hadley Circulation. The Astrophysical Journal, 813, 15. (Doi: 10.1088/0004-637x/813/1/15)

Charnay, B., Meadows, V., Misra, A., Leconte, J., Arney, G. (2015) 3d Modeling Of Gj1214b's Atmosphere: Formation Of Inhomogeneous High Clouds And Observational Implications. The Astrophysical Journal, 813, L1. (Doi: 10.1088/2041-8205/813/1/L1)

Charnay, B.; Forget, F.; Wordsworth, R.; Leconte, J.;Millour, E.; Codron, F.; Spiga, A., (2013) Exploring The Faint Young Sun Problem And The Possible Climates Of The Archean Earth With A 3-D Gcm, Jgr – Atmospheres, 118 (18), 10,414-10,431. Doi: 10.1002/Jgrd.50808

Checlair, J., K. Menou, And D.S. Abbot (2017), No Snowball On Habitable Tidally Locked Planets, The Astrophysical Journal, 845:132. (Doi: 10.3847/1538-4357/Aa80e1)

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Claire, M.W., Kasting, J.F., Domagal-Goldman, S.D.,Stüeken, E. E., Buick, R. & Meadows, V.S. (2014) Modeling The Signature Of Sulfur Mass-Independent Fractionation Produced In The Archean Atmosphere. Geochimica Et Cosmochimica Acta,141, 365-380. Doi: 10.1016/J.Gca.2014.06.032

Clement, M. S., Kaib, N. A., Raymond, S. N., & Walsh, K. J. (2018). Mars’ Growth Stunted By An Early Giant Planet Instability. Icarus, 311, 340–356. Https://Doi.Org/10.1016/J.Icarus.2018.04.008

Clement, M. S., Kaib, N. A., Raymond, S. N., Chambers, J. E., & Walsh, K. J. (2019). The Early Instability Scenario: Terrestrial Planet Formation During The Giant Planet Instability, And The Effect Of Collisional Fragmentation. Icarus, 321, 778–790. Https://Doi.Org/10.1016/J.Icarus.2018.12.033

Clement, M. S., Raymond, S. N., & Kaib, N. A. (2019). Excitation And Depletion Of The Asteroid Belt In The Early Instability Scenario. The Astronomical Journal, 157(1), 38. Https://Doi.Org/10.3847/1538-3881/Aaf21e

Conrad, P. G. (2014). Scratching The Surface Of Martian Habitability. Science, 346(6215), 1288- 1289. Doi: 10.1126/Science.1259943

Conrad, P. G., Malespin, C. A., Franz, H. B., Pepin, R. O., Trainer, M. G., Schwenzer, S. P., ... & Owen, T. (2016). In Situ Measurement Of Atmospheric Krypton And Xenon On Mars With Mars Science Laboratory. Earth And Planetary Science Letters, 454, 1-9.

Cossou, C., Raymond, S. N., & Pierens, A. (2013). Convergence Zones For Type I Migration: An Inward Shift For Multiple Planet Systems. A&A, 553, L2. Doi:10.1051/0004-6361/201220853

Cotton, D. V., Evensberget, D, Marsden, S. C., Bailey, J., Kedziora-Chudczer, L., Carter, B. D., Bott, K., Zhao, J. The Rotationally Modulated Polarization Of Ksi Boo A; The Bcool Collaboration. Accepted To Mnras

Cotton, D.V., Bailey, J., Howorth, I.D., Bott, K., Kedziora-Chudczer, L., Lucas, P.W., Hough, J., 2017, Polarization Due To Rotational Distortion In The Bright Star Regulus, Nature Astronomy, 1, 690-696

Cotton, D.V., Marshall, J.P., Bailey, J., Kedziora-Chudczer, L., Bott, K., Marsden, S.C., Carter, B.D., 2017, The Intrinsic And Interstellar Broad-Band Linear Polarization Of Nearby Fgk Dwarfs, Monthly Notices Of The Royal Astronomical Society, 467, 1

Dacks, J. B., Field M. C., Buick R., Esme, L., Gribaldo, S., Roger, A., Brochier-Armanet, C., And Devos, D. (2016) The Changing View Of Eukaryogenesis - Fossils, Cells, Lineages And How They All Come Together. J Cell Sci; 129(20), 3695-3703 (Doi: 10.1242/Jcs.178566)

Dassarma, S.D. & Schwieterman, E. W. 2018. Early Evolution Of Purple Retinal Pigments On Earth And Implications For Exoplanet Biosignatures. International Journal Of Astrobiology, 1-10, Doi: 10.1017/S1473550418000423

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Davenport, J.R.A., And 14 Colleagues (2014) Kepler Flares. Ii. The Temporal Morphology Of White-Light Flares On Gj 1243. The Astrophysical Journal 797, 122. Doi: 10.1088/0004- 637x/797/2/122

Davenport, James R. A.; Hebb, Leslie; Hawley, Suzanne L. (2015) Detecting Differential Rotation And Starspot Evolution On The M Dwarf Gj 1243 With Kepler. The Astrophysical Journal, 806, 212 (Doi: 10.1088/0004-637x/806/2/212)

De Lee, N., Ge, J., Crepp, J. R., Eastman, J., Esposito, M., Femenía, B., … Zhao, B. (2013). Very Low Mass Stellar And Substellar Companions To Solar-Like Stars From Marvels. V. A Low Eccentricity Brown Dwarf From The Driest Part Of The Desert, Marvels-6b. The Astronomical Journal, 145(6), 155. Doi:10.1088/0004-6256/145/6/155

Deck, K. M. And Agol, E., (2015), Measurement Of Planet Masses With Transit Timing Variations Due To Synodic “Chopping” Effects. The Astrophysical Journal, 802:116 (Doi: 10.1088/0004- 637x/802/2/116)

Deck, K. M., E. Agol, Et Al. (2014). "Ttvfast: An Efficient And Accurate Code For Transit Timing Inversion Problems." The Astrophysical Journal 787(2): 132. Doi: 10.1088/0004-637x/787/2/132

Deck, K., And Agol, E. (2016) Transit Timing Variations For Planets Near Eccentricity-Type Mean Motion Resonances, The Astrophysical Journal, 821(2), 1-13 (Doi: 10.3847/0004-637x/821/2/96)

Deienno, R., Et Al., Excitation Of A Primordial Cold Asteroid Belt As An Outcome Of Planetary Instability, 2018, The Astrophysical Journal, 864, 50

Deitrick, R. Et Al. (2015) The Three-Dimensional Architecture Of The Υ Andromedae Planetary System. Astrophys. J., 798, 46. Doi: 10.1088/0004-637x/798/1/46

Deitrick, R. Et Al. (2015) The Three-Dimensional Architecture Of The Υ Andromedae Planetary System. The Astrophysical Journal, 798, 46. (Doi: 10.1088/0004-637x/798/1/46)

Deitrick, R., Hilton, E.J. (2014). Kepler Flares. I. Active And Inactive M Dwarfs. The Astrophysical Journal 797, 121. Doi: 10.1088/0004-637x/797/2/121

Del Genio, A. D., S. D. Domagal-Goldman, N. Y. Kiang, R. K. Kopparapu, G. A. Schmidt, And L. E. Sohl. "The Future Of Planetary Climate Modeling And Weather Prediction." (2017).

Deming, D., A. Wilkins, Et Al. (2013). "Infrared Transmission Spectroscopy Of The Exoplanets Hd 209458b And Xo-1b Using The Wide Field Camera-3 On The Hubble Space Telescope." The Astrophysical Journal 774(2): 95. Doi: 10.1088/0004-637x/774/2/95

Deming, D., And Sheppard, K. (2017). Spectral Resolution-Linked Bias In Transit Spectroscopy Of Extrasolar Planets, The Astrophysical Journal Letters, 841, L3.

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Deming, D., Louie, D., And Sheets, H. 2018, "How To Characterize The Atmosphere Of A Transiting Exoplanet", Pasp, 131, 013001.

Deming, D., Wilkins, A., Mccullough, P., Burrows, A., Fortney, J. J., Agol, E., … Showman, A. P. (2013). Infrared Transmission Spectroscopy Of The Exoplanets Hd 209458b And Xo-1b Using The Wide Field Camera-3 On The Hubble Space Telescope . The Astrophysical Journal, 774(2), 95. Doi:10.1088/0004-637x/774/2/95

Ding, F., And Pierrehumbert, R. T. (2016) Convection In Condensable-Rich Atmospheres, The Astrophysical Journal, 822(24), 16pp (Doi: 10.3847/0004-637x/822/1/24)

Dobbs-Dixon, I. And E. Agol (2013). "Three-Dimensional Radiative-Hydrodynamical Simulations Of The Highly Irradiated Short-Period Exoplanet Hd 189733b." Monthly Notices Of The Royal Astronomical Society 435(4): 3159-3168. Doi: 10.1093/Mnras/Stt1509

Dobbs-Dixon, I., & Agol, E. (2013). Three-Dimensional Radiative-Hydrodynamical Simulations Of The Highly Irradiated Short-Period Exoplanet Hd 189733b. Monthly Notices Of The Royal Astronomical Society, 435(4), 3159–3168. Doi:10.1093/Mnras/Stt1509

Dobbs-Dixon, I., Agol, E., And Deming, D. (2015) Spectral Eclipse Timing, The Astrophysical Journal, 815, 1- 7 (Doi: 10.1088/0004-637x/815/1/60)

Domagal-Goldman Shawn D., Wright Katherine E., Adamala Katarzyna, Arina De La Rubia Leigh, Bond Jade, Dartnell Lewis R., Goldman Aaron D., Lynch Kennda, Naud Marie-Eve, Paulino-Lima Ivan G., Singer Kelsi, Walter-Antonio Marina, Abrevaya Ximena C., Anderson Rika, Arney Giada, Atri Dimitra, Azúa-Bustos Armando, Bowman Jeff S., Brazelton William J., Brennecka Gregory A., Carns Regina, Chopra Aditya, Colangelo-Lillis Jesse, Crockett Christopher J., Demarines Julia, Frank Elizabeth A., Frantz Carie, De La Fuente Eduardo, Galante Douglas, Glass Jennifer, Gleeson Damhnait, Glein Christopher R., Goldblatt Colin, Horak Rachel, Horodyskyj Lev, Kaçar Betül, Kereszturi Akos, Knowles Emily, Mayeur Paul, Mcglynn Shawn, Miguel Yamila, Montgomery Michelle, Neish Catherine, Noack Lena, Rugheimer Sarah, Stüeken Eva E., Tamez-Hidalgo Paulina, Walker Sara Imari, And Wong Teresa. (2016) Astrobiology Primer V 2.0, Astrobiology, 16(8): 561- 653. (Doi:10.1089/Ast.2015.1460)

Domagal-Goldman, S. (2014). How Low Can You Go? Maximum Constraints On Hydrogen Concentrations Prior To The Great Oxidation Event. Geological Society Of America Special Papers, 504, 11-13. Doi: 10.1130/2014.2504(02)

Domagal-Goldman, S. (2014a). The Upside-Down Biosphere:“Evidence For The Partially Oxygenated Oceans During The Archean Eon”. Geological Society Of America Special Papers, 504, 97-99. Doi: 10.1130/2014.2504(10)

Domagal-Goldman, S. D. "The Power Of Self-Skepticism In Astrobiology." (2017): 956-957.

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Domagal-Goldman, Shawn D., Segura, Antígona, Claire, Mark W.; Robinson, Tyler D., Meadows, Victoria S. (2014) Abiotic Ozone And Oxygen In Atmospheres Similar To Prebiotic Earth, The Astrophysical Journal, Volume 792(2). Doi: 10.1088/0004-637x/792/2/90

Driscoll, P. & Barnes, R. (2015) Tidal Heating Of Earth-Like Exoplanets Around M Stars: Thermal, Magnetic, And Orbital Evolutions. Astrobiology, 15, 739-760. (Doi: 10.1089/Ast.2015.1325)

Ducrot, E., Sestovic, M., Morris, B. M., Gillon, M., Triaud, A. H., Wit, J. D., . . . Grootel, V. V. (2018). The 0.8–4.5 Μm Broadband Transmission Spectra Of Trappist-1 Planets. The Astronomical Journal, 156(5), 218. Doi:10.3847/1538-3881/Aade94

Eastman, J., Gaudi, B. S., & Agol, E. (2013). Exofast: A Fast Exoplanetary Fitting Suite In Idl . Publications Of The Astronomical Society Of The Pacific, 125(923), 83–112. Doi:10.1086/669497

Ehlmann, B. L., 44 Others, And K.J. Zahnle (2016). The Sustainability Of Habitability Of Terrestrial Planets: Insights, Questions, And Needed Measurements From Mars For Understanding The Evolution Of Earth-Like Worlds. Journal Of Geophysical Research: Planets 121, 1927-1961.

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Kane, S. R., Hill, M., Kasting, J. F., Kopparapu, R. K., Quintana, E. V., Barclay, T., Batalha, N., Borucki, W. J., Ciardi, D. R., Haghighipour, N., Hinkel, N. R., Kaltenegger, L., Selsis, F., And Torres, G. (2016) A Catalog Of Kepler Habitable Zone Exoplanet Candidates, The Astrophysical Journal. 830:1 (Doi: 10.3847/0004-637x/830/1/1)

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315-337. This Study Followed Up On Previous Work That Argued For Spatial Gradients In Fixed Nitrogen Availability During Earth's Middle Age. By Finding A Similar Nitrogen Isotopic Trend In Two Other Mid-Proterozoic Basins, We Were Able To Demonstrate That Eukaryotes May Have Been Ecologically Relegated To Near-Shore Environments At This Time Due To A Scarcity Of Nitrate In Deeper Waters.

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Koehler, M.C., Buick, R., Kipp, M.A., Stüeken, E.E. & Zaloumis, J. Transient Surface Ocean Oxygenation Recorded In The ~2.66-Ga Jeerinah Formation, Australia. Proceedings National Academy Of Science Usa, 115, 7711-7716.

Koll, D.D.B., And Abbot, D.S. (2016) Temperature Structure And Atmospheric Circulation Strength Of Tidally Locked Rocky Exoplanets, The Astrophysical Journal, 825, 99 (Doi: 10.3847/0004- 637x/825/2/99)

Kollmeier, J. A., Raymond, S. N., Can Moons Have Moons?, 2019, Monthly Notices Of The Royal Astronomical Society, 483, L80

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Kopparapu, R. K., Ramirez, R., Kasting, J. F., Eymet, V., Robinson, T. D., Mahadevan, S., … Deshpande, R. (2013). Habitable Zones Around Main-Sequence Stars: New Estimates . The Astrophysical Journal, 765(2), 131. Doi:10.1088/0004-637x/765/2/131

Kopparapu, R. K., Wolf, E. T., Arney, G., Batalha, N., Haqq-Misra, J., Grimm, S. L., & Heng, K. (2017). Habitable Moist Atmospheres On Terrestrial Planets Near The Inner Edge Of The Habitable Zone Around M-Dwarfs. The Astrophysical Journal, 845(1), 5. (Doi: 10.3847/1538-4357/Aa7cf9)

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Kopparapu, R. K.., Wolf, E. T., Haqq-Misra, J., Yang, J., Kasting, J. F., Meadows, V. S., Terrien, R., And Mahadevan, S. (2016) The Inner Edge Of The Habitable Zone For Synchronously Rotating Planets Around Low-Mass Stars Using General Circulation Models, The Astrophysical Journal. 819, 84, 14 Pp (Doi: 10.3847/0004-637x/819/1/84)

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Leshin, L. A., Mahaffy, P. R., Webster, C. R., Cabane, M., Coll, P., Conrad, P. G., … Moores, J. E. (2013). Volatile, Isotope, And Organic Analysis Of Martian Fines With The Mars Curiosity Rover. Science, 341(6153), 1238937–1238937. Doi:10.1126/Science.1238937

Lewis, A. R., Quinn, T., & Kaib, N. A. (2013). The Influence Of Outer Solar System Architecture On The Structure And Evolution Of The Oort Cloud . The Astronomical Journal, 146(1), 16. Doi:10.1088/0004-6256/146/1/16

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Luger, R., Barnes, R., Lopez, E., Fortney, J., Jackson, B., & Meadows, V. (2015) Habitable Evaporated Cores: Transforming Mini-Neptunes Into Super-Earths In The Habitable Zones Of M Dwarfs. Astrobiology, 15, 57-88. (Doi: 10.1089/Ast.2014.1215)

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Luger, R., Lustig-Yaeger, J., Fleming, D., Tilley, M., Et Al., 2017a, The Pale Green Dot: A Method To Characterize Proxima Centauri B Using Exo-Aurorae, The Astrophysical Journal, 837, 63

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Lustig-Yaeger, J., Tovar, G., Fujii, Y., Schwieterman, E.W., And Meadows, V.S. (2017) Mapping Surfaces And Clouds On Terrestrial Exoplanets Observed With Next-Generation Coronagraph- Equipped Telescopes. Astrobiology Science Conference 2017, Lpi Contrib. No. 1965, 3558

Ma, B., Ge, J., Barnes, R., Crepp, J. R., De Lee, N., Dutra-Ferreira, L., … Zhao, B. (2012). Very- Low-Mass Stellar And Substella Companions To Solar-Like Stars From Marvels. I I I . A Short- Period Brown Dwarf Candidate Around An Active G0iv Subgiant . The Astronomical Journal, 145(1), 20. Doi:10.1088/0004-6256/145/1/20

Mahaffy, P. R., Webster, C. R., Atreya, S. K., Franz, H., Wong, M., Conrad, P. G., … Moores, J. E. (2013). Abundance And Isotopic Composition Of Gases In The Martian Atmosphere From The Curiosity Rover. Science, 341(6143), 263–266. Doi:10.1126/Science.1237966

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Marty, B., Alexander, C. M. O., & Raymond, S. N. (2013). Primordial Origins Of Earth’s Carbon. Reviews In Mineralogy And Geochemistry, 75(1), 149–181. Doi:10.2138/Rmg.2013.75.6

Mastrobuono-Battisti, Alessandra, Perets, Hagai B., And Raymond, Sean N (2015) A Primordial Origin For The Compositional Similarity Between The Earth And The Moon, Nature, 520, 212. (Doi: 10.1038/Nature14333)

Mazeh, T., Nachmani, G., Holczer, T., Fabrycky, D. C., Ford, E. B., Sanchis-Ojeda, R., … Welsh, W. (2013). Transit Timing Observations From Kepler . Viii. Catalog Of Transit Timing Measurements Of The First Twelve Quarters . The Astrophysical Journal Supplement Series, 208(2), 16. Doi:10.1088/0067-0049/208/2/16

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Meadows, V. S., Arney, G. N., Schwieterman, E. W., Lustig-Yaeger, J., Lincowski, A. P., Robinson, T., Domagal-Goldman, S. D., Deitrick, D., Barnes, R. K., Fleming, D. P., Luger, R., Driscoll, P. E., Quinn, T. R., And Crisp, D.: The Habitability Of Proxima Centauri B: Environmental States And Observational Discriminants, Astrobiology 18, 2018. Doi: 10.1089/Ast.2016.1589.

Meadows, V.S. (2017) Reflections On O2 As A Biosignature In Exoplanetary Atmospheres, Astrobiology, 17(10): 1022-1052. Https://Doi.Org/10.1089/Ast.2016.1578

Meadows, V.S., Reinhard, C.T., Arney, G.N., Parenteau, M.N., Schwieterman, E.W., Domagal- Goldman, S.D., Et Al. (2018) Exoplanet Biosignatures: Understanding Oxygen As A Biosignature In The Context Of Its Environment. Astrobiology 18(6).

Meadows, V.S., Reinhard, C.T., Arney, G.N., Parenteau, M.N., Schwieterman, E.W., Domagal- Goldman, S.D., Lincowski, A.P., Stapelfeldt, K.R., Rauer, H., Dassarma, S., Hegde, S., Narita, N., Deitrick, R., Lyons, T.W., Siegler, N., Lustig-Yaeger, J. (2017). "Exoplanet Biosignatures: Understanding Oxygen As A Biosignature In The Context Of Its Environment," Astrobiology (In Revision). Arxiv Preprint 1705.07560. Https://Arxiv.Org/Abs/1705.07560

Mettam, C., Zerkle, A.L., Claire, M.W., Izon, G., Junium, C.J., Twitchett, R.J. High-Frequency Fluctuations In Redox Conditions During The Late Permian Extinction Event Palaeogeography, Palaeoclimatology, Palaeoecology, 485, 210-223.

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Misra, A., Meadows, V., Claire, M., & Crisp, D. (2014). Using Dimers To Measure Biosignatures And Atmospheric Pressure For Terrestrial Exoplanets. Astrobiology, 14(2), 67–86. Doi:10.1089/Ast.2013.0990

Misra, Amit K.; Meadows, Victoria S., (2014) Discriminating Between Cloudy, Hazy, And Clear Sky Exoplanets Using Refraction, The Astrophysical Journal Letters, Volume 795, Issue1, Article Id. L14. Doi: 10.1088/2041-8205/795/1/L14

Misra, Amit; Meadows, Victoria; Crisp, Dave, (2014), The Effects Of Refraction On Transit Transmission Spectroscopy: Application To Earth-Like Exoplanets, The Astrophysical Journal, Volume 792, Issue 1, 61-72. Doi: 10.1088/0004-637x/792/1/61

Morris, B. M., Agol, E., Davenport, J. R., & Hawley, S. L. (2018). Possible Bright Starspots On Trappist-1. The Astrophysical Journal, 857(1), 39. Doi:10.3847/1538-4357/Aab6a5

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Morris, B. M., Agol, E., Hebb, L., Hawley, S. L., Gillon, M., Ducrot, E., . . . Demory, B. (2018). Non-Detection Of Contamination By Stellar Activity In The Spitzer Transit Light Curves Of Trappist-1. The Astrophysical Journal, 863(2). Doi:10.3847/2041-8213/Aad8aa

Muller, E., Thomazo, C., Stüeken, E., Hallmann, C., Leider, A., Chaduteau, C., Buick, R., Baton, F., Philippot, P., Ader, M. Bias In Carbon Concentration And Δ13c Measurements Of Organic Matter Due To Cleaning Treatments With Organic Solvents. Chemical Geology, 495, 405-412.

Mustill, A. J., Raymond, S. N., And Davies, M. B. (2016) Is There An Exoplanet In The Solar System?, Monthly Notices Letters Of The Royal Astronomical Society, 460(1), L109-L113 (Doi: 10.1093/Mnrasl/Slw075)

Nayak, M., Lupu, R., Marley, M. S., Fortney, J. J., Robinson, T., & Lewis, N. (2017). Atmospheric Retrieval For Direct Imaging Spectroscopy Of Gas Giants In Reflected Light. Ii. Orbital Phase And Planetary Radius. Publications Of The Astronomical Society Of The Pacific, 129(973), 034401.

Oberlin, E., Claire, M., And Kounaves, S. (2017) Evaluation Of The Tindouf Basin Region In Southern Morocco As An Analog Site For Soil Geochemistry On Noachian Mars, Astrobiology, In Press.

O'brien, D. P., Et Al., The Delivery Of Water During Terrestrial Planet Formation, 2018, Space Science Reviews, 214, 47

O'brien, David P., Walsh, Kevin J., Morbidelli, Alessandro, Raymond, Sean N., And Mandell, Avi M., 2014, Water Delivery And Giant Impacts In The Grand Tack Scenario, Icarus, 239, 84. Doi: 10.1016/J.Icarus.2014.05.009

Olson, S.L., Schwieterman, E.W., Reinhard, C.T., Ridgwell, A., Kane, S.R., Meadows, V.S., And Lyons, T.W., 2018, Atmospheric Seasonality As An Exoplanet Biosignature: Astrophysical Journal Letters, 858, L14. Doi.Org/10.3847/2041-8213/Aac171.

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Parenteau M.N., Sparks W.B., Blankenship R.E., Germer T.A., Telesco C.M. Kiang N.Y., Hoehler T., Pallé E., Robb F.T., Meadows V.S. (2017) Global Surface Photosynthetic Biosignatures Of Anoxic Biospheres. Astrobiology Science Conference (Abscicon) Abstract 3668.

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Payne, R. C., A. V. Britt, H. Chen, J. F. Kasting, And D. C. Catling (2016) The Response Of Phanerozoic Surface Temperature To Variations In Atmospheric Oxygen Concentration, Journal Of Geophysical Research: Atmospheres, 121(17), 10,089–10,096. (Doi:10.1002/2016jd025459)

Pecoits, E., Smith, M.L., Catling, D. C., Philippot, P., Kappler, A., Konhauser, K. O. (2015) Atmospheric Hydrogen Peroxide And Eoarchean Iron Formations. Geobiology, 13, 1-14. (Doi: 10.1111/Gbi.12116)

Pierens, A., And Raymond, S. N. (2016) Migration Of Accreting Planets In Radiative Discs From Dynamical Torques, Monthly Notices Of The Royal Astronomical Society, 462(4), 4130-4140 (Doi: 10.1093/Mnras/Stw1904)

Pierens, A., Cossou, C., & Raymond, S. N. (2013). Making Giant Planet Cores: Convergent Migration And Growth Of Planetary Embryos In Non-Isothermal Discs. A&A, 558, A105. Doi:10.1051/0004-6361/201322123

Pierens, A., Raymond, S.N., Nesvorny, D., And Morbidelli, A. (2014) Outward Migration Of Jupiter And Saturn In 3:2 Or 2:1 Resonance In Radiative Disks: Implications For The Grand Tack And Nice Models. Apj, 795, L11.

Pierrehumbert, R. T., And Ding, F. (2016) Dynamics Of Atmospheres With A Non-Dilute Condensable Component, Proceedings Of The Royal Society A, 2015(June) (Doi: 10.1098/Rspa.2016.0107)

Quintana, Elisa V., Barclay, Thomas, Raymond, Sean N., Rowe, Jason F., Bolmont, Emeline, Caldwell, Douglas A., Howell, Steve B., Kane, Stephen R., Huber, Daniel, Crepp, Justin R., Lissauer, Jack J., Ciardi, David R., Coughlin, Jeffrey L., Everett, Mark E., Henze, Christopher E., Horch, Elliott, Isaacson, Howard, Ford, Eric B., Adams, Fred C., Still, Martin, Hunter, Roger C., Quarles, Billy, And Selsis, Franck, 2014, An Earth-Sized Planet In The Habitable Zone Of A Cool Star, Science, 344, 280. Doi: 10.1126/Science.1249403

Ramirez, R. M. And Kasting, J. F. (2016) Could Cirrus Clouds Have Warmed Early Mars? Icarus, 281, 248-261 (Doi: 10.1016/J.Icarus.2016.08.016)

Ramirez, R. M., Kopparapu, R., Zugger, M. E., Robinson, T. D., Freedman, R., & Kasting, J. F. (2013). Warming Early Mars With Co2 And H2. Nature Geosci, 7(1), 59–63. Doi:10.1038/Ngeo2000

Raymond, S. N., & Armitage, P. J. (2012). Mini-Oort Clouds: Compact Isotropic Planetesimal Clouds From Planet-Planet Scattering. Monthly Notices Of The Royal Astronomical Society: Letters, 429(1), L99–L103. Doi:10.1093/Mnrasl/Sls033

Raymond, S. N., Armitage, P. J., Veras, D., Interstellar Object ‘Oumuamua As An Extinct Fragment Of An Ejected Cometary Planetesimal, 2018, The Astrophysical Journal, 856, L7

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Raymond, S. N., Et Al., Implications Of The Interstellar Object 1i/'Oumuamua For Planetary Dynamics And Planetesimal Formation, 2018, Monthly Notices Of The Royal Astronomical Society, 476, 3031

Raymond, S. N., Et Al., Migration-Driven Diversity Of Super-Earth Compositions, 2018, Monthly Notices Of The Royal Astronomical Society, 479, L81

Raymond, S. N., Izidoro, A., Bitsch, B., And Jacobson, S. A. (2016) Did Jupiter's Core Form In The Innermost Parts Of The Sun's Protoplanetary Disc?, Monthly Notices Of The Royal Astronomical Society, 458(3), 2962-2972 (Doi: 10.1093/Mnras/Stw431)

Raymond, S. N., Izidoro, A., Morbidelli, A., Solar System Formation In The Context Of Extra-Solar Planets, 2018, Arxiv E-Prints, Arxiv:1812.01033

Raymond, S. N., Kokubo, E., Morbidelli, A., Morishima, R., And Walsh, K. J., 2014, Terrestrial Planet Formation At Home And Abroad, Protostars And Planets Vi, 595. Doi: 10.2458/Azu_Uapress_9780816531240-Ch026

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