Workshop on the Habitability of Icy Worlds (2014) 4061.pdf

LIVING ON THE EDGE: THE HABITABILITY OF TRITON AND OTHER LARGE OBJECTS. William B. McKinnon, Department of and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, MO 63130, 341-966-3989 ([email protected]).

Introduction: The Kuiper belt comprises a vast [e.g., 7] generally fail (the system is too reservoir of bodies whose most direct interaction with dynamically hot), so capture is relegated to earlier pre- inhabited (Earth) and potentially inhabited (, Eu- LHB eras, or is simply taken to be an unlikely event ropa, ) worlds is through impact of short- [e.g., 8]. Late capture is ruled out because circulariza- period, or JFC, . Such impacts can deliver tion would have destabilized ’s irregular satel- as well as biologically essential elements (CHONPS, lites [8,9]. I note that collisional capture early in the etc.) to otherwise barren bodies. The focus here, how- Nice rearrangement may be just as likely, which makes ever, is on the Kuiper belt objects (KBOs) themselves, Triton’s formation distance from the rather uncer- and in particular, the potential habitability of the larg- tain. Regardless of these details, Triton’s post-capture est, dwarf members of this class. I will empha- tidal evolution was likely one of profound internal size Triton, as the largest “KBO” and the one with the heating and thermochemical processing [5,10]. most vigorous geological history (an aspect of critical Habitability: Full differentiation of Triton and importance). While such a discussion is necessarily vigorous hydrothermal activity at the base of a deep speculative, it allows consideration of what we know (~350 MPa) likely followed Triton’s capture, or think we know about habitability in general, and even if its orbit partly circularized while cannibalizing may help clarify issues and indicate where to best Neptune’s (putative) original satellite system [5,9]. place our exploration efforts. This hyperactive era would have persisted for 100 m.y. Living on the Edge … of the : Any and potentially much longer. Substantial conversion of discussion of habitability must begin with the big three accreted NH3 to N2 would have occurred for rock major requirements for life as we know it [e.g., 1]: 1) oxidation states similar to the Earth’s midocean ridges, water; 2) biologically essential elements; and 3) a but as long as oxidation state remained sufficiently source of energy. At condensation in the reducing, copious quantities of simple organics (car- outer Solar System, an abundance of water is a boxylic acids, alcohols) could have been synthesized foregone conclusion. All large KBOs are known to be as well (adding to the original endowment of organics). icy from surface spectra alone, if not bulk . But H2 production (from hydroxylation of silicates) would icy surfaces mask rock-rich interiors. All of the dwarf- have been substantial, affecting the redox evolution of planet-class bodies (, , Triton, …even the high-pressure ocean and -like ) have of ≈2000 kg m–3 or greater, im- above [11]. Notably, reduced P could have released plying rock/ice ratios >1 (only ’s density is into the ocean by reactions with accreted (Fe,Ni)3P. unknown) [2]. Detections of , ethane, and The ultimate concentration of NH3 and CH3OH deter- methanol on various KBO surfaces and the inferred mines whether the ocean stays in contact and interacts presence of (accounting for redness) imply an with the rocky , or whether contact is lost and organic component as well, and if comets are any the ocean is isolated between ice I above and denser guide, this organic component is substantial. ice below. The atmosphere protects the surface from Extensive studies (of , , comets, Neptune’s particles, but ionizing cos- etc.) imply that Triton and other KBOs accreted more- mic rays can still produce surface oxidants, and the or-less equal amounts of (including volatile organ- surface is in communication with the deeper ice shell. ics), less volatile carbonaceous matter (CHON), and References: [1] Hand K.P. et al. (2009) in , refractory “rock” (see references in [2]). Volatile ice Univ. Ariz. Press, 589–629. [2] McKinnon W.B. et al. (2007) compositions are best represented by cometary comae in The Solar System Beyond Neptune, Univ. Ariz. Press, [e.g., 3], while the rock component may resemble the 213–241. [3] Mumma M.J. and Charnley S.B. (2011) ARAA, most primitive carbonaceous chondrites [e.g., 4]: es- 49, 471–524. [4] Zolensky M.E. et al. (2002) MAPS, 37, 737- 761. [5] McKinnon W.B. et al. (1995) in Neptune and Triton, sentially solar in composition (with respect to non- Univ. Ariz. Press, 807–877. [6] Agnor C.B and Hamilton volatile elements) but only partially, aqueously altered. D.P. (2009) , 192, 192–194. [7] Levison H.F. et al. All biologically essential elements are thus included. (2008) Icarus, 196, 258-273. [8] Nogueira E. et al. (2011) Triton: It is widely accepted that Triton is a cap- Icarus, 214, 113–130. [9] Cuk M.C. and Gladman B.R. tured KBO [5], and the leading mechanism at present (2005) Astrophys. J. Lett., 626, L113–L116. [10] Shock E.L. is binary exchange capture [6]. Numerical calculations and McKinnon W.B. (1993) Icarus, 106, 464-477. [11] that attempt to simulate capture in the context of the Lunine J.I. and Nolan M.C. (1992) Icarus, 100, 221–234.