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ROADMAP for the EXPLORATION of DWARF PLANET CERES. J. C. Castillo-Rogez1, C

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ROADMAP for the EXPLORATION of DWARF PLANET CERES. J. C. Castillo-Rogez1, C Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989) 8077.pdf ROADMAP FOR THE EXPLORATION OF DWARF PLANET CERES. J. C. Castillo-Rogez1, C. A. Ray- 1 2 3 4 1 mond , C. T. Russell , A. S. Rivkin , M. Neveu , Ceres afficionados all over the world. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA ([email protected]), 2 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA. 3Applied Physics Laboratory, John Hopkins Universi- ty, Laurel, MD. 4School of Earth and Space Exploration, Arizona State University, Tempe, AZ. Introduction: Ceres, the largest asteroid, and only was identified prior to Dawn’s arrival [1] and have led dwarf planet found in the inner solar system, offers a Ceres to turn from a “credible” possible ocean world to playground for testing hypotheses pertaining to the a “candidate” ocean world [9]. Specifically, in the early Solar system evolution as well as the habitability frame of the Roadmap for Ocean Worlds Goals, Dawn potential of large volatile-rich bodies. The Dawn mis- brought positive answers to the following questions: sion has revolutionized our undertanding of Ceres in a Goal 1 (Identify Ocean Worlds), A.1 Is there remnant decade that has also seen major breakthroughs in solar radiogenic heating? B.1 Do signatures of geologic system dynamical modeling, cosmochemistry, and the activity indicate the possible presence of a subsurface rise of ocean worlds. Probably the most significant ocean? B.7 Can the surface composition be linked with finding from the Dawn mission is unambiguous evi- the presence of a sub-surface ocean? dence for oceanic material right on Ceres’ surface as- sociated at least in one place with a recent cryovolcan- Dawn’s discoveries at Ceres also introduced new ic feature. This goes above and beyond pre-Dawn pre- evidence (or context) for addressing questions of broad dictions. This and other discoveries from the Dawn interest. First, the presence of ammonia adds to the mission are raising new questions and setting the stage story of early Solar system migration although alterna- for future exploration, as described in this presentation. tive scenarios are possible [10]. Also the nature of oce- anic material on Ceres’ surface, including sodium car- Post-Dawn State of Knowledge of Ceres: Ceres is bonate [11], a species found only on Earth and Encela- one of the best explored solar system bodies thanks to dus’ plumes [12], can help better understand the geo- the extensive observation campaign achieved by the chemical processes ongoing in other ice rich bodies. Dawn Mission. The combination of mineralogical, Indeed, per its size and water abundance, Ceres be- elemental, geological, and geophysical observations set longs to a class of objects that could host relatively standards for future missions. These led to key find- alkaline conditions as was suggested for Europa [13] ings, including the confirmation that Ceres has been and inferred from Cassini observations of Enceladus subjected to the hydrothermal processing of its materi- [14]. It has been suggested that the deep oceanic mate- als at the global scale, likely fueled by short-lived ra- rial could be exposed via the removal of an ice shell dioiostope heat [1]; the discovery that that environment via impact-induced sublimation [15]. This combined involved ammonia- and carbon-rich compounds, point- with clues for carbon suggests that the study of Ceres’ ing to an origin of Ceres’ materials from the outer solar surface directly addresses the ROW Goal II B.3 system; a geology driven in part by volatile abundance “Characterize the ice-ocean interface” and offers a in multiple forms, including ground ice, persistently playground for testing hypotheses aboud the chemical shadowed regions, and icy regolith toward high lati- evolution and habitability potential of Ocean Worlds. tudes [2, 3, 4]; the likely role of brines in driving cry- ovolcanism in the form of several outstanding features Key Open Questions: Workings and Life: The (Ahuna mons and Occator bright spots, as well as po- next step in the assessment of Ceres’ astrobiology sig- tential ancient features in the same vein) [5]; and the nificance is to evaluate the extent of liquid in its interi- signature of volatile activity driven by solar wind [6]. or. This is a difficult endeavor for bodies that are not Dawn’s observations have been complemented subject to tidal deformation and sources of seismologi- over the past years by investigations with the Hubble cal activity. This question might be addressed by stud- Space Telescope leading to the finding of abundant ying the interaction of Ceres with the solar wind alt- carbon on Ceres’ surface, as well as, potentially, sulfur hough this remains to be quantified. Comparison be- rich species [7]. The discovery of water vapor by the tween images returned by Dawn and a future mission Herschel Space Observatory [8] is consistent with the could be used to search for the signature of a deep liq- detection of many ice-rich sites, suggesting that ice is uid layer in Ceres’ rotation [16] and possibly also re- present below a thin regolith and regularly exposed via veal telling changes in surface properties. Indeed the landslides and small impacts. key to evaluating Ceres’ internal structure might come These pieces of information allow for a fresh as- from the long-term observation of the faculae (bright sessment of Ceres’ astrobiological significance, which deposits) observed in the Occator crater. The exposure Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989) 8077.pdf age of those deposits appears inconsistent with the The answers to these questions would drive the ~100My age of the crater and may indicate that the third step in Ceres’ exploration, with regard to better reservoir involved in the formation of these features is understanding “how life might exist at each ocean not yet at thermal equilibrium. world and search for life” [ROW Goal IV]. Explora- If pursuing the exploration of Ceres in the context tion strategies developed for Mars may be applicable of the Roadmap for Ocean Worlds, a future mission to there, in particular planetary protection technologies. Ceres could address the following questions, e.g., Finally, the exploration of Ceres and large icy sat- Goal II (Characterize the Ocean), A.1 What is the ellites requires a theoretical framework and experi- thickness, salinity, density and composition of the mental progress to assess, e.g., the stability and ther- ocean? How do these properties vary spatially and /or mophysical properties of salt-rich materials, the phys- temporally? Goal III. (Characterize the Habitability), ics driving endogenic processes in a (relatively) small A.1 What environments possess redox disequilibria, in gravity body, exogenic processes altering its surface, what forms, in what magnitude, how rapidly dissipated and the development, thriving, and preservation of life by abiotic reactions, and how rapidly replenished by and biosignatures in salt-rich environments. local processes? B.1 What is the inventory of organic compounds, what are their sources and sinks, and Ceres as a Stepping Stone for the Exploration of what is their stability with respect to the local envi- Ocean Worlds: Ceres represents a critical data point ronment? B.2 What is the abundance and chemical for understanding the chemical evolution of volatile- form of nitrogen, oxygen, phosphorus, sulfur, and in- rich worlds and especially their potential for forming organic carbon, what are their sources and sinks, and and preserving organic compounds. With it low gravity are there processes of irreversible loss or sequestra- and relative begnin environment, Ceres also offers tion relative to the liquid environment? easy surface access (in comparison to Mars or Europa) whereas the roundtrip light-time to/from Ceres requires Origins: Despite the evidence for ammonia and the introduction of semi-autonomous techniques for carbon compounds the origin of Ceres remains uncer- advanced surface operations. Hence a long-term explo- tain; several competing theories can explain an origin ration program of Ceres is compelling, not just for the of Ceres at its current location with supplies of solar anticipated science return, but also because it will system planetesimals [17] or even from ammonia-rich help us practice and hone new technologies of rele- organics formed in the inner solar system [18] These vance to the future exploration of ocean worlds, such various hypotheses may be addressed via isotopic as surface operations, planetary protection, and end-to- chemistry of low-z elements, and especially hydrogen, end sample collection and return to Earth. oxygen, and nitrogen isotopes. However, the extensive hydrogeochemistry that modified Ceres’ materials also Acknowledgements: This work is being carried out at likely altered their original isotopic signature. Hence the Jet Propulsion Laboratory, California Institute of Tech- nology, under contract to NASA. answers to volatile migration might be better addressed References: [1] Castillo-Rogez, J. C., McCord, T. at more primordial objects (e.g., comets, smaller C- B. (2010) Icarus 205, 443-459; [2] Schmidt, B., et al., type asteroids, main-belt comets). submitted to Nat. Geosc.; [3] Schorghofer, N., et al. (2016) GRL 13, 6783-6789; [4] Prettyman, T., et al. A Roadmap for Ceres Exploration: The in situ (2016) Nature, in press; [5] Ruesch, O., et al. (2016) investigation of outstanding landmarks is an obvious Science 353, 6303; [6] Russell, C. T., et al. (2016) Sci- next step in the exploration of Ceres and might be ac- ence 353, 6303; [7] Hendrix, A. R., et al. (2016) GRL complished within the constraints of the Discovery 17, 8920-8927; [8] Kueppers, M., et al. (2014) Nature program. Key objectives could focus on assessing hab- 505, 525-527; [9] Hendrix, A. R., Hurford, T. A., and itability (the natural next step in the ROW framework) the ROW Team (2016), Planetary Visions 2050 Work- by investigating the chemical fingerprints contained in shop; [10] De Sanctis, C., et al.
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