Outer Solar System Objects with large Bulk Carbon COntent

Funding : ANR PRC, (reference 270840)

Coordinator: Bruno Reynard, LGL-TPE

Host Institution : CNRS

Duration : 2024 – 2027

Résumé du projet

The moons and dwarf planets witnessed the early stages of formation of the outer Solar system. Classical silicate-iron sulfide mixture densities are inconsistent with low densities inferred for the rocky core (2400-2500 kg/m3) of Titan, Enceladus, Dione, and Ceres. We recently proposed that compositions including the overlooked, abundant carbon component provide a viable model for the low densities of refractory cores. Models point to fractions of C-rich organic matter up to 25 wt% (~40% in volume) in the refractory cores, resulting in close to Solar C/Si and C/O ratios.

Forming small carbon-rich planets is a so-far unexplored scenario. Metamorphic and thermal evolution of a C-rich core differs from that of a silicate-sulfide core. It can release C-rich fluids to outer icy layers, and may fuel prebiotic activity, a crucial step to the origin of life if dwarf planets like Ceres accreted to Earth as a late veneer. With this major carbon-rich component, the present structure of large outer Solar system objects is tied to 1) composition through the C/Si (the carbonaceous matter to silicate-sulfide ratio in the core) and C/O ratios (with oxygen from the silicate and ice fraction in the planetary body), and 2) density increase associated with reactions occurring at increasing temperature in carbonaceous matter and silicates in the 600-1200 K range. We propose a coupled experimental and geodynamic approach to explore the density and reactions of carbon and silicate-sulfide, and the plausible compositional range and thermal evolution of these carbon-rich bodies. The models will be used to propose observations to test them and define the conditions of formation of dwarf planets and moons in the solar nebula, such as their position with respect to the so-called “snow” and “soot” lines.