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Deep Atmosphere of Venus Probe As a Mission Priority for the Upcoming Decade Deep Atmosphere of Venus Probe as a Mission Priority for the Upcoming Decade AUTHOR:: James B. Garvin NASA’s Goddard Space Flight Center Email: [email protected] Phone (cell): 301-646-4369 CO-AUTHORS: Giada N. Arney (NASA GSFC) Martha Gilmore (Wesleyan) Stephen Kane (UC Riverside) Sushil Atreya (Univ. Michigan) David Grinspoon (PSI) Walter Kiefer (LPI) Stephanie Getty (NASA GSFC) Natasha Johnson (NASA GSFC) Ralph Lorenz (JHU/APL) CO-SIGNEES: Michael Amato (NASA GSFC) Noam Izenberg (JHU/APL) Mike Ravine (MSSS) Bruce Campbell Erika Kohler (NASA GSFC) Jacob Richardson (NASA GSFC) (NASM, Smithsonian) Paul Mahaffy (NASA GSFC) Melissa Trainer (NASA GSFC) Dave Crisp (NASA JPL) Charles Malespin (NASA GSFC) Chris Webster (NASA JPL) Scott Guzewich (NASA GSFC) Alex Pavlov (NASA GSFC) Mikhail Zolotov (ASU) Sarah Horst (JHU/APL) August 15, 2020 Submitted to the National Academies of Sciences as a Mission White Paper for the 2023-2032 Planetary Sciences & Astrobiology Decadal Survey Deep Atmosphere of Venus Probe as a Mission Priority for the Upcoming Decade EXECUTIVE SUMMARY The deep atmosphere of Venus is largely unexplored and yet may harbor clues to the evolutionary pathways for a major silicate planet with implications across the solar system and beyond. In situ data is needed to resolve significant open questions related to the evolution and present-state of Venus, including questions of Venus’ possibly early habitability and current volcanic outgassing. Deep atmosphere “probe-based” in situ missions carrying analytical suites of instruments are now implementable in the upcoming decade (before 2030), and will both reveal answers to fundamental questions on Venus and help connect Venus to exoplanet analogs to be observed in the JWST era of astrophysics. INTRODUCTION AND BACKGROUND The Challenge of Measuring Venus’ Massive Atmosphere Previous Venus exploration has led to significant advance- ments in our understanding of the geodynamics and bulk at- mospheric composition of the planet [Grinspoon & Bullock, 2007; Kane et al., 2019; Way & Del Genio, 2020; Lammer et al., 2020], even as profound questions remain such as those concerning atmospheric chemical stratification, possible sig- natures of present-day geologic and chemical activity, as well as Venus evolution. Compositional constraints from orbital near- IR night-side imaging have further produced new perspectives on the possible existence of “evolved” high-silica lithologies on Venus at scales of ~100 km [Hashimoto et al., 2008; Gilm- ore et al., 2015; Gilmore et al., 2017; Weller & Kiefer, 2020], and high priority ancient terrains are ready to be interrogated. Plans for next-generation radar and night-side near IR emis- sion spectrometers for mapping the surface at scales from tens of meters (SAR) to ~70 km (NIR) call for missions in the 2030s such as ESA’s EnVision that will determine composi- tional patterns at regional to global scale for advancing models of Venus’ crustal evolution [Ghail et al., 2018]. Absence of compositional and dynamical information for the deepest atmosphere including noble gas, water, and major atmospheric isotopes and the basic T-P profile places limits on present understanding of Venus, such as outlined in Weller DD001 and Kiefer (2020), with connections to the role of past oceans Figure 1: Conceptual Venus deep atmosphere [Donahue et al., 1982; Kasting 1988; Way et al., 2016; Way probe mission for the 2020’s decade. Such & Del Genio, 2020]. This fundamental knowledge gap can be a mission could employ parachutes within effectively treated in the next decade via a new class of in situ the cloud deck (50-70 km) to enable time for mission, embracing key aspects of the VISE concept described gas ingest and processing and then freely fall to the surface at 10-15 m/s as it images the in the 2011 NAS Decadal Survey [V&V, 2011] and recent surface in the Near Infrared (NIR) windows to VEXAG documents [VEXAG Goals, 2020]. Thanks to ad- permit compositional mapping while profiling vancements in compact analytical instrumentation, high sen- trace gases down to the surface (in their envi- sitivity descent imaging systems, and FPGA-enhanced probe ronmental context). flight systems, deep atmosphere “probes” are ready now to an- swer the top priority next questions posed by Venus in ways not possible for the past ~35 years. The case for such a “deep atmosphere probe with analytical chem- istry” has been articulated since the 1983 Solar System Exploration Committee analysis [Morrison & Hinners, 1983] as the mission to follow the radar mapper Magellan and in all intervening planetary decadal surveys [NFSS, 2002; V&V, 2011]. Without definitive compositional measurements of Venus atmosphere down to its presently uninhabitable surface, advancements in models of its thermal and climate evolution will be impossible, thereby limiting the impact of our sister planet on our knowledge of solar system evolution (Figure 1). This mission White Paper describes the case for this class of mission concept (DEep Atmosphere Probe: DEAP) to resolve these knowledge gaps for the upcoming decade. 1 Deep Atmosphere of Venus Probe as a Mission Priority for the Upcoming Decade Composition of the Venus Atmosphere: the Essential Next Step in Venus Exploration The Venus atmosphere holds clues to its origin, evolution, and dynamics and how it reflects the his- tory of putative past oceans and active volcanism [Baines et al., 2013; Bougher et al., 1989; Treiman 2007; Garvin et al., 2020]. The late-1970’s measurements from Pioneer Venus (PVLP) were incom- plete and did not offer the precision to measure the noble gases, especially Xenon and Helium [Lam- mer et al., 2020], leaving ambiguities in our understanding of the planet. The single mid-atmosphere D/H value (~150) was suggestive of a large water inventory that was lost [Donahue et al., 1982], but did not survey the variability in this key value from the top of the atmosphere to the near surface. No complete inventory of diagnostic trace gases was accomplished, especially for the deep atmosphere from ~16 km to the surface, where most (66%) of the atmosphere resides [Bougher et al., 1989]. The lapse rate (temperature as a function of altitude) is insufficiently constrained and represents a key vari- able for current models of the deep atmosphere, where dominant CO2 is super-critical [Lebonnois & Schubert, 2017]. No systematic compositional cross-section as a function of altitude from the mid- atmosphere clouds to the surface has ever been achieved. Without definitive compositional measure- ments of the bulk and lower-most Venus atmosphere, essential boundary conditions for evolutionary models that seek to explain Venus as a “system” cannot be developed [Kane et al., 2019, Figure 2]. The composition of the near-surface atmosphere is needed to constrain the chemical alteration of surface materials and exchange of volatiles in the coupled atmosphere-surface rocks system [Zolotov, 2018, 2019]. Venus stands out as the least well-measured large atmosphere in the solar system (Lammer and others 2020), further limiting what our nearest neighbor planet can tell us about habitability of Earth- like planets and the broader workings of our solar system and planetary systems beyond [Kane et al., 2019; NAS Exoplanets Strategy, 2018; Way et al., 2016]. Venus Transit Spectrum Venus Atmosphere 100 100 HO CO 80 CO 80 CO Typical altitudes sensed by transits 60 CO 60 HSO 40 40 Altitude [km] Altitude [km] OCS 90% of 20 20 atmos. mass HCI HS SO 0 0 5 10 15 20 10-10 10-8 10-6 10-4 10-2 Wavelength [μm] Mass mixing ratio DD002 Figure 2: Greater understanding of Venus will provide higher fidelity simulations and data interpretation of what an exo-Venus might resemble to a future astrophysical observatory such as JWST or others planned for the 2030s versus the poorly-constrained Venus atmosphere (right), where most of the trace gas contents are uncertain, especially below ~45 km (i.e., below the clouds) [Kane et al., 2019]. A conceptual DEAP mission could survey details of the composition from 70 km to the surface to quantify what future transiting exoplanetary spectroscopy telescopes (JWST etc.) can evaluate beyond our solar system. Deep-atmosphere data is needed to constrain and validate models attempting to understand whole-atmosphere conditions of Venus-like exoplanets. The history of habitability at Venus? One of the most exciting emerging justifications for Venus cloud-deck compositional measurements are their critical role in assessing past or present habitability (and potential for biological activity). Cloud-deck microbial metabolism has become increasingly recognized as a significant venue for biol- ogy in a variety of Earth environments, including the stratosphere. Limaye and others (2018) sum- marized the case for scenario on present-day Venus, with specific indicator species such as phosphine (PH3) as a detectable biosignature at Venus and in spectroscopy of exoplanets [Sousa-Silva et al., 2020]. This scenario implies the in situ detectability of biogenic trace gases within the environmentally habit- able cloud deck (~50-60 km altitude) today. Chemical signatures dating for Venus’ oceanic period (or 2 Deep Atmosphere of Venus Probe as a Mission Priority for the Upcoming Decade more recently) would be detectable with suitably sensitive analytical instrumentation of the type that have conducted related investigations on Mars as part of the Curiosity rover (as an example) for the past 8+ years [Trainer et al., 2019]. Such instrumentation was largely non-existent 20 years ago but on the basis of investments that high sensitivity mass spectrometers (far exceeding the level of sensitivity enabled by remote sensing) on such missions as Curiosity (SAM) and Cassini/Huygens (INMS and GCMS), bringing such sensors to the “samples” throughout the Venus atmosphere (Figure 1) is now possible. GOALS AND RELEVANCE We define the comprehensive survey of the definitive composition, dynamics, and environment of the Venus atmosphere (~70 km to surface) as a primary science goal for Venus exploration in the next decade [V&V, 2011; VEXAG Goals, 2020].
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