Workshop on the Habitability of Icy Worlds (2014) 4014.pdf

HABITABILITY POTENTIAL OF ICY MOONS: A COMPARATIVE STUDY. A. Solomonidou1,2, A. Coustenis1, Th. Encrenaz1, F. Sohl3, H. Hussmann3, G. Bampasidis1,2, F. W. Wagner3, F. Raulin4, D. Schulze- Makuch5, and R. M. C. Lopes6, 1LESIA - Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot –Meudon, France ([email protected]), 2National and Kapodistrian University of Athens, Department of Physics, Athens, Greece, 3DLR, Institute of Planetary Research, Berlin, Germany, 4LISA-IPSL, CNRS/UPEC & Univ. Paris Diderot, Créteil, France, 5Washington State University, School of Earth and Environmental Sciences, 6 Washington, USA, Jet Propulsion Laboratory, Pasadena, California, USA.

Introduction: Looking for habitable conditions in the equivalent of hydrothermal systems, the simultane- the outer solar system our research focuses on the natu- ous presence of water, geodynamic interactions, ral satellites rather than the planets themselves. Indeed, chemical energy sources and a diversity of key chemi- the habitable zone as traditionally defined may be cal elements may fulfill the basic conditions for habi- larger than originally conceived. The strong gravita- tability. tional pull caused by the giant planets may produce Titan has been suggested to be a possible cryovol- enough energy to sufficiently heat the interiors of or- canic world due to the presence of local complex vol- biting icy moons. The outer solar system satellites then canic-like geomorphology and the indications of sur- provide a conceptual basis within which new theories face albedo changes with time [7,8]. Such dynamic for understanding habitability can be constructed. activity that would most probably include tidal heating, Measurements from the ground but also by the Voy- possible internal convection, and ice tectonics, is be- ager, Galileo and the Cassini spacecrafts revealed the lieved to be a pre-requisite of a habitable planetary potential of these satellites in this context, and our un- body as it allows the recycling of minerals and poten- derstanding of habitability in the solar system and be- tial nutrients and provides localized energy sources. In yond can be greatly enhanced by investigating several a recent study by Sohl et al. [2013], we have shown of these bodies together [1]. Their environments seem that tidal forces are a constant and significant source of to satisfy many of the “classical” criteria for habitabil- internal deformation on Titan and the interior liquid ity (liquid water, energy sources to sustain metabolism water ocean can be relatively warm for reasonable and chemical compounds that can be used as nutrients amounts of ammonia concentrations, thus completing over a period of time long enough to allow the devel- the set of parameters needed for a truly habitable plan- opment of life). etary body. Satellites of the gaseous giants : Indeed, several of Such habitability indications from bodies at dis- the moons show promising conditions for habitability tances of 10 AU, are essential discoveries brought to and the development and/or maintenance of life. Eu- us by space exploration and which have recently revo- ropa, Callisto and Ganymede may be hiding, under lutionized our perception of habitability in the solar their icy crust, putative undersurface liquid water system. oceans [3] which, in the case of Europa [2], may be in In the solar system’s neighborhood, such potential direct contact with a silicate mantle floor and kept habitats can only be investigated with appropriate de- warm by tidally generated heat [4]. Titan and Ence- signed space missions, like JUICE-Laplace (JUpiter ladus, Saturn’s satellites, were found by the Cassini- ICy moon Explorer) for Ganymede and Europa [9]. Huygens mission to possess active organic chemistries JUICE is an ESA mission to Jupiter and its icy moons, with seasonal variations, unique geological features recently selected to launch in 2022. and possibly internal liquid water oceans. Titan’s rigid References: crust and the probable existence of a subsurface ocean [1] Coustenis, A., Encrenaz, Th., in “Life Beyond create an analogy with terrestrial-type plate tectonics, Earth : the search for habitable worlds in the Uni- at least surficial [5], while Enceladus’ plumes find an verse”, Cambridge Univ. Press, 2013. [2] Patterson, analogue in geysers. As revealed by Cassini the liquid G.W., et al.: AGU P41F-09, 2011. [3] Grasset, O., et hydrocarbon lakes [6] distributed mainly at polar lati- al.: Astrobiology 13, 991-1004, 2013. [4] Sohl, F., et tudes on Titan are ideal isolated environments to look al.: submitted. [5] Solomonidou, A., et al.: PSS 77, for biomarkers. Currently, for Titan and Enceladus, 104-117, 2013. [6] Stofan, E.R., et al.: Nature 445, 61- geophysical models try to explain the possible exis- 64, 2007. [7] Solomonidou, A., et al.: submitted (a). tence of an oceanic layer that decouples the mantle [8] Solomonidou, A., et al: submitted (b). [9] Grasset, from the icy crust. If the silicate mantles of Europa and O., et al.: PSS, 78, 1-21, 2013. Ganymede and the liquid sources of Titan and Ence- ladus are geologically active as on Earth, giving rise to