Enceladus and the Icy Moons of Saturn (2016)
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COMPOSITION OF ENCELADUS: ORIGIN AND EVOLUTION. William B. McKinnon1, J. H. Waite, Jr.2, O. Mousis3, J. I. Lunine4 and M. Yu Zolotov5, Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, Saint Louis, MO 63130 (
[email protected]), 2 Southwest Research Institute, San Antonio, TX 78228, 3Aix Marseille Université, CNRS, LAM UMR 7326, 13388, Marseille, France, 4Department of Astronomy and Carl Sagan Institute, Cornell University, Ithaca, NY, 14853, 5 School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287. Introduction: Enceladus is a remarkable world. We infer from Cassini mass spectrometry of plume gas, ice and silica grain analyses, and gravity and libration measurements the existence of an internal ocean in contact with a rock core that may be undergoing hydrothermal activity [1-7]. Such active hydrothermal conditions provide a habitable environment and have been implicated as a region where early life emerged on Earth [e.g., 8]. In this talk we put these compositional analyses and structural inferences into the context of origin and internal evolution models (including those of Titan), and by doing so constrain these models where practical, with the ultimate goals of 1) assessing Enceladus’ habitability and 2) suggesting future measurements that may resolve these issues. Overview of Origins: The saturnian regular icy satellites, as a whole, almost certainly formed in a circumplanetary accretion disk (CPD) that surrounded the growing gas giant at the end of its formation [9]. ProtoSaturn would have been sufficiently massive to have opened a gap in the solar nebula (SN) [10]. Accretion of gas and entrained particles (“pebbles”) then proceeded through streamers, which coalesced and formed a prograde-rotating CPD [e.g., 11,12]. Early in this phase, temperature and pressure conditions in the CPD may have been high enough to induce gas phase chemistry (e.g., CH4 to CO [13]) and devolatilize solids captured from the solar nebula. As the SN dissipates, however, the CPD progressively attenuates and cools, enough so that gas phase chemistry is kinetically inhibited and that the compositions of captured solids captured remain largely unaltered [9]. The D/H ratio in H2O measured in Enceladus’ plumes by INMS is relevant here: D/H is close to the value inferred for Oort Cloud comets (~2.9 × 10-4 [1]; cf. [14]), precluding the possibility that Enceladus’ building blocks condensed in an initially warm and dense saturnian subnebula [15]. In the latter, the D/H ratio acquired by the water ice accreted by Enceladus should be close to the much lower, protosolar value [15]. By inference then, Enceladus largely accreted from rock, ices, and organic matter directly inherited from the solar nebula by the saturnian CPD. Composition: Carbonaceous chondrites are the most primitive chondrite class, so it is often assumed that they are most representative of outer solar system “rock.” CI chondrites contain the fullest complement of elements at solar abundance levels (except H, C, O,
N, and noble gases) [16], and are the class typically associated with outer planet satellite composition [e.g., 17,18]. CIs are the most aqueously altered chondrites, however, and it is not clear if icy moons accreted CItype material or their anhydrous precursors (perhaps both). Comet-like abundances in plume vapor [1] suggest a substantial fraction of organic H-C-O-N-rich cometary material, which would make the bulk composition of Enceladus even closer to solar composition than CI chondrites. On the other hand, evidence from Stardust samples [19] and recent dynamical models [20] argue for some degree of mixing across the solar nebula, so that even inner solar system materials may have ended up incorporated into Enceladus. Evolution: Minor species that have been securely identified, at the percent level or less, in plume vapor are CO2, CH4, and NH3; higher hydrocarbons are also present [1,2; cf. 22]. Salts (principally NaCl and Na carbonate/bicarbonate) have been detected in E-ring and plume ice grains [2]. The presence of all of these in the plume is consistent with a model of an sodic, alkaline ocean with Na+, Cl–, and CO3–2 as the major solutes [6,18]. This, and specifically the possible detection of native H2 [23], are consistent with an actively serpentinizing core [5,6]. A principal question, then, is whether ongoing serpentinization (here taken to generically mean aqueous alteration of Fe-bearing ultramafic minerals) is consistent with a 4.5 billion year old Enceladus, or whether such “serpentinization” is more consistent with a cosmogonically much younger satellite, such as two recent studies have proposed [24,25]. References: [1] Waite J.H. (2009) Nature 460, 487–490. [2] Postberg F. et al. (2011) Nature 474, 620–622. [3] Iess L. et al. (2014) Science 344, 78–80. [4] McKinnon W.B. (2015) GRL 42, doi:10.1002/2015GL063384. [5] Hsu H.-W. et al. (2015) Nature 519, 207–210. [6] Glein C.R. et al. (2015) GCA 162 (2015) 202–219. [7] Thomas P.C. et al. (2016) Icarus 264, 37–47. [8] Sullivan W.T. and Baross J.A. (2007) Planets and Life: The Emerging Science of Astrobiology, CUP. [9] Peale S.J. and Canup R.M. (2015) in Treatise on Geophysics, Elsevier, 559-604. [10] Morbidelli A. and Crida A. (2007) Icarus 191, 158-171. [11] Ayliffe B.A. and Bate M.R. (2009) MNRAS 397, 657–665. [12] Tanigawa T. et al. (2014) ApJ 784, 109. [13] Prinn R.G and Fegley B. (1981) ApJ 249, 308-317. [14] Altwegg K. et al. (2015) Science 620, 1261952. [15] Kavelaars J. J. et al. (2011) ApJL 734, L30. [16] Lodders K. (2003) ApJ 591, 1220–1247. [17] McKinnon W.B. and Zolensky M.E. (2003) Astrobiol. 3, 879–897. [18] Zolotov M. Yu. (2007) GRL 34, L23203. [19] Brownlee D. (2014) AREPS 42, 179–205. [20] Walsh K. et al. (2012) MAPS 1–7. [21] Waite J.H. et al. (2011) EPSC/DPS abs. #61. [22] Postberg F. et al. (2008) Icarus 193, 438-454; [23] Glein C.R. et al. (2016) 47th LPSC, abs. #2885. [24] Charnoz S. et al. (2011) Icarus 216, 535–550. [25] Cuk M. et al. (2016) ApJ 820, 97.
