Lunar and Planetary Science XLVIII (2017) 2306.pdf

OCEANUS: A NEW FRONTIERS ORBITER TO STUDY TITAN’S POTENTIAL HABITABILITY C. Sotin1, A. Hayes2, M. Malaska1, F. Nimmo3, M. Trainer4, M. Mastrogiuseppe5, J. Soderblom6, P. Tortora7, J. Hof- gartner1, O. Aharonson8, J. W. Barnes9, R. Hodyss1, L. Iess5, R. Kirk10, P.Lavvas11, R. Lorenz12, J.I. Lunine2, E. Mazarico4, A. McEwen13, C. Neish14, C. Nixon, E. Turtle12, V. Vuitton15, R. Yelle13. 1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA. 2Department of Astronomy, Cornell University, Ithaca, NY 14853, USA. 3Department of Earth and Planetary Science, University of California at Santa Cruz, USA. 4NASA Goddard Space Flight Center, Greenbelt, MD, USA. 5Sapienza Universita Di Roma, Ro- ma, Italy. 6MIT, Boston, MA, USA. 7University Bologna, Italy. 8Tel Aviv University, Israel. 9Department of Physics, University of Idaho, Engineering-Physics Building, Moscow, ID 83844, USA. 10U.S. Geological Survey, Astrogeol- ogy Science Center, Flagstaff AZ 86001, USA. 11Groupe de Spectrométrie Moléculaire et Atmosphérique UMR CNRS 7331, Université de Reims Champagne Ardenne. 12Applied Physics Laboratory, John Hopkins University, USA. 13Department of Planetary Sciences, University of Arizona, Tucson, AZ 85721, USA. 14Planetary Science In- stitute, Tucson, AZ 85719, USA. 15Université Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France (Chris- tophe.sotin@jpl.nasa.gov).

Introduction: Cassini has demonstrated that Titan dates for cryvolcanism, impact, and tectonic processes is an organic world of two oceans: surface hydrocarbon that could facilitate exhance with the interior. seas [1,2] that cover part of the north polar region and The New Frontiers 4 AO includes the theme a deep water ocean [3] that decouples the outer ice “Ocean Worlds (Titan and/or Enceladus)” focused on crust from an inner core likely composed of hydrated the search for signs of extant life and/or characterizing silicates [4]. Oceanus is an orbiter that would follow up the potential habitability of Titan and/or Enceladus. on Cassini’s amazing discoveries and assess Titan’s The Titan’s science objectives are (i) Understand the habitability by following the organics through the organic and methanogenic cycle on Titan, especially as methanologic cycle and assessing exchange processes it relates to prebiotic chemistry; and (ii) Investigate the between the atmosphere, surface, and subsurface. subsurface ocean and/or liquid reservoirs, particularly Titan’s reduced nitrogen-rich atmosphere operates their evolution and possible interaction with the sur- as an organic factory [5] where heavy organic mole- face. Oceanus not only addresses these two science cules are produced by a series of reations starting by objectives but would also be responsive to a large the photolysis of methane [6,7]. These organics coat number of the important science questions defined by Titan’s surface and are moved around through a com- the 2013 Decadal Survey. plex source-to-sink sediment transport system analo- Science questions and investigations: The dis- gous to surface processes here on Earth. Titan’s 90-95 coveries of the Cassini missions have triggered several K surface temperature at 1.5 bar surface pressure per- questions related to Titan’s potential habitability. What mit methane and ethane to condense out of the atmos- is the composition of the heavy organic molecules pro- phere and flow as liquids on the surface. As a result, duced in Titan’s atmosphere? Are they prebiotic mole- Titan’s methane-based hydrologic system produces a cules? How much oxygen is incorporated into these rich set of geologic features (dunes, river networks, molecules. As they fall on the surface and are subject polar lakes/seas, etc.) that mimic Eath’s water cycle. to Titan’s sediment transport system, which includes Cassini’s observations of this rich geomorphology is both fluvial and aeolian processes, are there places in hindered by kilometer-scale resolution. Oceanus will the subsurface or surface where this organic material take advantage of a narrow atmospheric window at 5 may have been in contact with liquid water (e.g., im- µm to acquire 25 m/pixel (< 100 m resolution) images pact melt pools, cryovolcanic flows)? Where is the of Titan diverse surface [8]. ethane, a main product of the photolysis of methane, 40 40 The presence of Ar, a product of the decay of K trapped? How thick is the crust? Is it convecting and contained in the silicate core, and methane whose therefore releasing internal heat very efficiently? An- origin is still controversial (primordial methane trapped swers to those questions will not only inform on Titan’s in the interior or methane being a byproduct of the hy- potential habitability but will also shed light on the dration of Titan’s silicate fraction) argue for exchange organic chemistry that occurred on early Earth when processes between the interior and the atmosphere. Earth had a more reduced atmosphere at the time life Like we see here on Earth, these processes will be emerged [REF]. chronicled in the interaction between geological fea- Oceanus will address these questions with three tures on Titan’s surface. Oceanus will investigate spe- high-heritage instruments that will address the po- cific features identified by Cassini as potential candi- tential habitability of Titan: an infrared camera that Lunar and Planetary Science XLVIII (2017) 2306.pdf

would acquire 25 m pixel size images of Titan’s sur- common to Earth that occur on another body, including face at 1500 km altitude, a radar altimeter that would the nature of Titan’s climate and weather and their time provide a global topography and measurements of the evolution, its geologic processes, the origin of its time-dependent deformation of Titan’s surface, and a unique atmosphere, and analogies between its methane mass spectrometer capable of chareacterizing the pro- cycle and Earth water cycle” (Decadal Survey, 2013). cesses that build the heavy molecules fabricated in Ti- The process interactions observed between geomor- tan’s upper atmosphere as well as determining their phologic features on Titan chronicle the history of its building blocks. In addition, information on the gravity landscape evolution and elucidate the transport of or- field would be obtained from the Doppler shift of the ganics between the subsurface, surface, and atmos- microwave carrier used in the radio link to the ground. phere. Three investigations cover the needs to address Ti- Conclusion: Titan is a moon that resembles a plan- tan’s potential habitability: organic geochemistry, ge- et with active processes at work. The Cassini mission ology and geophysics. With imaging and greatly im- has demonstrated that organics and water, two of the proved measurements of static and time-dependent major ingredients for habitability, are present. By fol- gravity and topography measurements, Oceanus would lowing the organics and the water, the Oceanus orbiter determine how thick and rigid the crust is, its defor- will characterize Titan’s potential habitability. mation history and whether it is convecting. Transport Acknowledgments: This work has been performed from the surface to the interior would greatly increase at the Jet Propulsion Laboratory, California Institute of the habitability of the subsurface ocean, by supplying Technology, under contract to NASA. the building blocks for life to a water-rich environment. References: [1] Stofan E.R. et al. (2007) Nature. The mass spectrometer will perform high–resolution in [2] Sotin C. et al. (2012) Icarus. [3] Iess L. et al. situ measurements of the organic material over a large (2012) Science. [4] Castillo-Rogez J.C. and Lunine J.I. mass range and at different altitudes. It will provide the (2010) Geophys. Res. Lett. [5] Coates A.J. et al. (2007) information required to determine (i) the processes at Geophys. Res. Let. [6] Lavvas P. et al. (2008) Planet. work to form the heavy molecules, (ii) the functional Space Sci. [7] Yung Y.L. et al. (1984) Astrophys. J. [8] group pattern of large molecules providing information Sotin C. et al. (2005) Nature on their composition. Some of these molecules are known to be important for biochemical processes on Fig.1 : A 25 m pixel scale image provides details at the Earth, such as amino acids and nucleobases. Titan dis- level of the images acquire by the Huygens probe a few plays a large variety of geological features that can be kilometers above Titan’s surface. Features such as revisited selectively with a high resolution infrared streams that are observed in the 25 m Huygens images camera. Exploring these features with an order of mag- are unseen in the 250 m RADAR images. nitude increase in spatial resolution and SNR will al- low Oceanus to “Explore and understand the processes