1Adrien Néri,2François Guyot,1 Bruno Reynard,3Christophe Sotin
Earth and Planetary Science Letters 530, 115920 Link to Article [https://doi.org/10.1016/j.epsl.2019.115920]
1University of Lyon, ENS de Lyon, CNRS, Lyon, France
2Museum National d’Histoire Naturelle, Sorbonne Université, IMPMC, UMR CNRS 7590, IRD UMR206, Paris, France
3Jet Propulsion Laboratory-California Institute of Technology, Pasadena, CA, USA
The inner structure of icy moons comprises ices, liquid water, a silicate rocky core and sometimes an inner metallic core depending on thermal evolution and differentiation. Mineralogy and density models for the silicate part of the icy satellites cores were assessed assuming a carbonaceous chondritic (CI) bulk composition and using a free-energy minimization code and experiments. Densities of other components, solid and liquid sulfides, carbonaceous matter, were evaluated from available equations of state. Model densities for silicates are larger than assessed from magnesian terrestrial minerals, by 200 to 600 kg.m−3 for the hydrated silicates, and 300 to 500 kg.m−3 for the dry silicates, due to the high iron bulk concentration in CI. The stability of Na-phlogopite in the silicate fraction up to 1300 K favors the trapping of most 40K in the rocky/carbonaceous cores with important consequences for modeling of the thermal evolution of icy satellites. We find that CI density models of icy satellite cores taking into account only the silicate and metal/sulfide fraction cannot account for the observed densities and reduced moment of inertia of Titan and Ganymede without adding a lower density component. We propose that this low-density component is carbonaceous matter derived from insoluble organic matter, in proportion of ∼30-40% in volume and 15-20% in mass. This proportion is compatible with contributions from CI and comets, making these primitive bodies including their carbonaceous matter component likely precursors of icy moons, and potentially of most of the objects formed behind the snow line of the solar system.