Workshop on the Habitability of Icy Worlds (2014) 4011.pdf

STRUCTURE AND DYNAMICS OF GANYMEDE: IMPLICATIONS FOR ITS POTENTIAL HABITABILITY. O. Grasset1, 11Planetology and Geodynamics, University of Nantes, CNRS, France; olivi- [email protected].

Introduction: Ganymede is the largest moon of the processes are indeed possible[3] but not as clear-cut as Solar System, with great relevance to astrobiological the exchanges envisaged for Europa and that probably studies because it satisfies a large number or prerequi- prevailed until recent times. sites for habitability. A review of its properties, espe- Based on Galileo data, there is no evidence of pre- cially regarding its habitability will be given. sent activity, or recent features which could suggest the existence of shallow reservoirs. No evidence for recent Main characteristics of the moon: Ganymede is cryovolcanic resurfacing is identified thus far. Howev- the largest satellite in the Solar System (2631 km in er, locally restricted scalloped depressions called pate- radius). In the Jovian system, it holds a key position rae adjacent to Ganymede’s bright terrain, which could because it features old, densely-cratered terrain, similar represent caldera-like features[7] are interpreted as cry- to most of Callisto’s surface, but also widespread tec- ovolcanic features that appear in Ganymede’s past tonically resurfaced regions, resembling a large part of history. In fact, the geologic record on Ganymede does Europa’s surface. Ganymede displays a wide range of not support the existence of shallow liquid reservoirs at surface ages, which reveal a geological record of sev- present. Still, these occurrences cannot totally be ruled eral billions of years, and a great variety in geological out because most of the Galileo data was acquired at and geomorphic units. These features are the surface medium spatial resolution, impeding the detection of signature of internal heat release during Ganymede’s small features. evolution. Ganymede is also the only satellite and - with Mercury and the Earth- one of only three solid Conclusions: Following Galileo’s discovery 400 bodies in the Solar System that generate a magnetic years ago of the Galilean satellites, our knowledge of dipole field at the present time. A description of our the large icy moons within our Solar System has con- current knowledge of Ganymede’s characteristics can tinued to grow. On Ganymede, Galileo has demon- be found in Grasset et al.[1], and references therein. As strated that the largest moon of our solar system is of indicated by its small moment of inertia factor of strong interest as a planetary body and also as a poten- 0.3115, Ganymede is a highly differentiated body. In- tial habitat. That is why the ESA JUICE mission will terior structure models consistent with the gravity go in orbit around the moon after a tour of three years field, bulk density, and magnetic constraints include (i) in the Jovian system. It is the necessary step to charac- an iron-rich core (at least part of which being liquid to terise this remarkable moon and to explore in details its generate the intrinsic magnetic dipole field), (ii) a sili- potential habitability. cate shell, (iii) an hydrosphere which may be at least 500 km thick (about 50 %wt.), and a very tenuous at- mosphere[2-4]. The composition of the atmosphere in- References: [5-6] cludes O, O2, and possibly ozone (O3) . [1] Grasset O. et al. (2013) Planet. Space Sci., 78, doi: 10.1016/j.pss.2012.12.002. Potential habitability of the liquid reservoirs in [2] Kivelson, M.G. et al. (2002) Icarus 157. Ganymede: Two kind of habitable zones should be [3] Sohl F et al. (2010) Space Sci. Rev., 153. considered: a deep and global liquid layer, and also [4] Anderson J. D. et al. (2001) Icarus, 153 (1). shallow liquid reservoirs in the icy crust. On Gany- [5] Hall D. et al. (1998), Astroph. J. 99. mede, the hydrosphere must be split into a high- [6] Noll K.S. et al. (1996) Science 273. pressure ice layer consisting of various water-rich [7] Stephan K. et al. (2013) Astroph. Sp. Sci. L., 356. high-pressure ices denser than liquid water, the subsur- face water ocean, and an ice-I layer forming the outer crust of the satellite. The liquid layer could be up to 100 km thick. It has been suggested that the depth of the ocean should be close to 150 km[2]. Chemical and energy exchanges between the rocky layer and the ocean at present, which are crucial for habitability, cannot be ruled out but imply efficient transport pro- cesses through the thick high - pressure icy layer. Such