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Titan: a Hazy Waterworld That We Can Visit 1 Introduction Exoplanets in our Backyard 2020 (LPI Contrib. No. 2195) 3015.pdf TITAN: A HAZY WATERWORLD THAT WE CAN VISIT Jason W. Barnes1, Elizabeth P. Turtle2, Melissa G. Trainer3, Ralph D. Lorenz2, Sarah Horst¨ 4, Shannon M. MacKenzie2, and the Dragonfly Science Team. 1University of Idaho, Moscow, ID, USA ([email protected]), 2Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA, 3NASA Goddard Space Flight Center, Greenbelt, MD, USA, 4Johns Hopkins University, Baltimore, Maryland, USA 1 INTRODUCTION There are precious few examples of planets with both distinct surfaces and thick atmospheres in the Solar Sys- tem: just Venus, Earth, Mars, and Saturn’s moon Titan. These four therefore represent our only opportunities to explore close-up the diversity of physical and chemical processes that we expect occur on trillions of extrasolar planets in the Milky Way. Titan’s contributions to this endeavor derive from its (1) organic chemistry, (2) in- terior water ocean, (3) surface-atmosphere interactions, and (4) hazy methane-rich atmosphere. Titan’s particular strength with respect to exoplan- Figure 1: Artist’s conception of Dragonfly on the surface of Titan, ets derives from its accessibility. The Cassini/Huygens within an interdune in the Shangri-La sand sea. mission explored Titan from Saturn orbit and within the atmosphere, respectively. A future Titan orbiter mission will hopefully follow up on Cassini’s discoveries [e.g. 1], periment in abiotic organic synthesis. Entire planets like providing global imaging and topography, atmospheric Titan may be common in extrasolar systems [3, 4, 5]. measurements and characterization, and gravity probing On Titan, carbon can interact with liquid water on of the interior. the surface. Cryolava flows and impacts create tran- Dragonfly [2] is a Titan lander mission within the sient surficial liquid water environments. When organics NASA New Frontiers program that was selected for (sitting on the surface or falling out of the atmosphere) flight on 2019 June 27. Its science themes include pre- mix with water, the resulting environment simulates what biotic chemistry, habitability, and a search for biosigna- may have happened on the early Earth [6]. Dragonfly tures. To address these themes, Dragonfly will sample seeks to answer the question: How far in complexity both water ice and organic sediments within Titan’s sand space have Titan’s organics progressed in the absence of seas’ dunes and interdunes. Because prebiotic chem- biology? Analyzing previously liquid water that mixed istry or prospective life on Titan might consist of familiar with organics could therefore bring insights into prebi- water-based pathways or use liquid methane/ethane as a otic chemistry unattainable in the terrestrial laboratory solvent, sampling both ice and organics provides for a and potentially shed light on the origin of life and how broad-based approach to either. Dragonfly carries a mass that origin may be replicated in extrasolar systems. spectrometer to determine molecular masses of surface 3 INTERIOR OCEAN materials and a gamma-ray and neutron spectrometer to assess the bulk and inorganic atomic fractions within the Extrasolar terrestrial and super-Earth type planets may regolith, as well as cameras, seismometers, and an atmo- contain significantly higher water mass fractions than spheric sensor suite. Dragonfly is a giant quadcopter: the Earth itself. To that end, Titan and its sister ocean worlds entire lander flies. That aerial mobility makes it possible of the Solar System provide a tangible analog. Dragon- to explore a variety of targets, following up on discover- fly probes Titan’s interior by use of seismometers, elec- ies that we make along the way like rovers do on Mars. tric field measurements, and imaging. Autocorrelation of seismograms of Titanquakes reveal the depth of Titan’s 2 ORGANIC CHEMISTRY solid ice crust, and thereby the depth to the subsurface Titan’s ubiquitous organic compounds created by photol- water ocean mantle [7]. Sufficiently strong quakes could ysis of atmospheric methane set it apart from other So- reveal further details of interior structure, allowing for lar System planets. The complexity of Titan’s organics validation of models and serving as a reference point for exceed that of everywhere else in the Solar System other extrasolar waterworld interior structure as well. We also than Earth itself. Titan therefore serves as an ongoing ex- constrain the depth to the ocean using Titan’s Schumann Exoplanets in our Backyard 2020 (LPI Contrib. No. 2195) 3015.pdf 2 like Titan’s may be ubiquitous among extrasolar planets [14]; indeed, the lack of spectroscopic absorption fea- tures in some transiting exoplanets has been attributed to high haze opacity. In-situ observation of that haze pro- duction at Titan [and in the lab][]2018NatAs...2..303H allows us to constrain the physical and chemical pro- cesses at work in a way that complements our very-low- resolution transit spectroscopy of extrasolar planet atmo- spheres [like GJ1214b 15]. 6 CONCLUSION Titan is an important extrasolar planet analog because it shares important aspects but also because we can visit Ti- tan by spacecraft in our lifetimes. Cassini and future Ti- tan orbiters will provide global atmospheric and surface measurements at high resolution. Dragonfly will explore Titan as a rotorcraft in a way similar to that achieved at Mars with rovers. As a result, we expect for insights gleaned from its surface mission to be broadly relevant across planetary science discliplines, and in particular Figure 2: Cassini T104 flyby mosaic of Titan’s north pole, showing a specular reflection off of Kraken Mare, a liquid hydrocarbon sea. for its discoveries to show relevance to the interiors, sur- faces, and atmospheres of solid- and liquid-surface ex- trasolar planets. resonance [8]. Dragonfly imaging of landing sites can References reveal faulting and other geological process as may mix [1] C. Sotin, et al. (2017) in LPSC vol. 48 2306. surface organics with liquid water. [2] R. D. Lorenz, et al. (2018) Johns Hopkins APL 4 SURFACE & ATMOSPHERE Technical Digest 374–387. [3] A. E. Gilliam, et al. (2011) Planetary and Space As the only location in the universe with surface lakes Science 59(9):835 ISSN 0032-0633 doi. other than Earth, Titan allows us to observe how oceano- [4] J. Checlair, et al. (2016) Planetary and Space Sci- graphic processes operate in an extraterrestrial environ- ence 129:1 doi. ment [e.g. 9]. Use of sun glints to detect extrasolar plan- [5] J. M. Lora, et al. (2018) ApJ 853(1):58 doi. etary oceans [10], for instance, has been tested at Titan arXiv:1712.04069. from Cassini [11]. [6] C. D. Neish, et al. (2018) Astrobiology 18:571 doi. Because Dragonfly will land in Titan’s equatorial [7] S. D. Vance, et al. (2018) Astrobiology 18(1):37 desert, far from the north polar seas, its primary contri- doi. bution lies in the characterization of surface and atmo- [8] C. Beghin,´ et al. (2007) Icarus 191(1):251 doi. spheric processes in-situ. We will not be sending landers [9] R. D. Lorenz (2014) GRL 41(16):5764 doi. to any extrasolar planets while any of us is alive. Drag- [10] T. D. Robinson, et al. (2010) ApJ 721:L67 doi. onfly will, however, constrain surface alteration mecha- arXiv:1008.3864. nisms, determine their rates, and allow us a second data [11] J. W. Barnes, et al. (2013) ApJ 777:161 doi. point for how those processes may vary with differing [12] M. G. Trainer, et al. (2018) Astrobiology Science gravity, atmospheric density, and composition. Strategy Whitepaper. 5 HAZY METHANE ATMOSPHERE [13] G. Arney, et al. (2016) Astrobiology 16(11):873 doi.arXiv:1610.04515. Titan’s nitrogen-methane atmosphere, and the inevitable [14] T. D. Robinson, et al. (2014) PNAS 111(25):9042 organic haze that results from its exposure to sunlight, doi.arXiv:1406.3314. provides insights in to the conditions of early Earth [12]. [15] Z. K. Berta, et al. (2012) ApJ 747(1):35 doi. Prior to the formation of life, methane in Earth’s atmo- arXiv:1111.5621. sphere may have seeded the surface with organics by similar photolysis under the faint young Sun [13]. Hazes.
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