Large fraction of insoluble organic matter inside Ganymede and Titan

Models of the interior structure of Titan and Ganymede are constrained by Cassini and Galileo data, respectively. Gravity and magnetic data suggest that Ganymede is fully differentiated with an inner liquid iron core, a silicate mantle and a hydrosphere including an ocean sandwiched between an icy crust and a high pressure ice (HP ice) layer [1]. The larger value of the reduced moment of inertia for Titan suggests that its interior is less differentiated [2,3]. We have calculated a series of density profiles assuming that the hydrosphere is composed of H₂O and that the silicate (plus iron rich core for Ganymede) has an elementary distribution equivalent to that of carbonaceous chondrites. Mineralogy and density of the silicate fraction are assessed using a free-energy minimization code. We use equations of state (EoS) for the different H₂O phases of water [4]. The temperature profile is determined assuming conduction in the silicate core which is internally heated by the decay of long-lived radioactive elements, convection in the HP ice layer [5], adiabatic profile in the ocean and either convection or conduction in the ice crust. All models have values of radius and mass measured by the Cassini and Galileo missions. The models differ by the value of the radius of the interface between the hydrosphere and the silicate core. We find that these models, for both Ganymede and Titan, provide reduced moment of inertia (RMoI) much too small. The presence of a low-density component in the silicate core is required to match mass, radius and RMoI. We propose that this component is the Insoluble Organic Matter (IOM) that could occupy up to 40% of the volume of the silicate shell. It implies that the icy moons of Jupiter and Saturn formed from a mix of carbonaceous chondrites and comets [6].

[1] Hussman et al.(2015) Treatise on Geophysics, 10: Physics of Terrestrial Planets and Moons, 605-635. [2] Iess L. et al. (2012) Science, 337, 457. [3] Castillo-Rogez J.C. and Lunine J.I. (2010). Geophys. Res. Lett., 37, L20205. [4] Vance S. et al. (2014) Planet Space Sci., 96, 62-70. [5] Kalousova K. and Sotin C. (2018) Geophys. Res. Lett., 45. [6] Neri A. et al. (2019) EPSL, in revision.