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Role of organics in the thermo-chemical evolution of icy moons and dwarf planets

Presentation #509.01 in the session Icy Moons Interior and Ice Shell (Oral Presentation)

Published onOct 23, 2023
Role of organics in the thermo-chemical evolution of icy moons and dwarf planets

Density and moment of inertia (MoI) of icy moons and dwarf planets suggest the presence of a low-density carbonaceous component in their refractory cores [1, 2]. The initial carbonaceous compound would be composed of organic molecules that are abundant in primitive solar system bodies. This hypothesis is tested using thermal evolution models. Organics degrade quickly and the carbonaceous compound becomes denser as temperature increases. In a homogeneous accretion scenario where the three components (silicates and sulfides, water ice, and organics) are mixed in solar proportions, ices differentiated from the carbon-rich refractory core, while hydration of silicates could take place. The thermal evolution models take into account the thermal characteristics of organics, in particular their low thermal conductivity, and the degradation of those organics as the refractory core heats up. Models also include the leaching of radioactive elements, in particular K, during the differentiation phase. All models provide the present-day observed values of mass and MoI. The presence of carbonaceous matter slows down the heating rate of the refractory core. Originally hydrated silicates are only partially dehydrated in the refractory cores of most moons. Viable scenarios point to a difference in formation or evolution between Ganymede and Titan in spite of their similar size and mass. Fully dehydrated mineralogy, inferred in Europa and possibly the densest dwarf planet Eris, require heterogeneous accretion near the water snow line of the solar or circumplanetary nebula. Progressive gas release from slowly warming carbonaceous matter-rich cores may sustain up to present-day the replenishment of ice-oceanic layers in organics and volatiles. It accounts for the observation of nitrogen, light hydrocarbons and complex organic molecules at the surface, in the atmospheres, or in plumes emanating from moons and dwarf planets.

References: [1] Reynard B. & C. Sotin, Earth Planet Sci Lett,612; 2023. [2] A. Néri, et al, Earth Planet Sci Lett, 530; 2020.

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