Outer Solar System Objects with large Bulk Carbon COntent – OSSO_BUCO
Planets and carbon chemistry in the outer Solar System
Numerous icy dwarf planets are observed at the outer edges of the Solar System by the James Webb Space Telescope, revealing a varied carbon chemistry on their surfaces and in their interiors. In this project, we develop global compositional and thermal evolution models that include carbon as a major component to account for these observations. To do so, the properties of carbonaceous matter and its interactions with minerals are characterized at high pressures and temperatures.
Icy Moons and Dwarf Planets: Witnesses to the Early Evolution of the Outer Solar System and potential harbor of prebiotic chemistry
The Galileo mission to Jupiter and the Cassini mission to Saturn unveiled revealed subsurface oceans beneath their icy crusts, rendering these moons potential hosts for prebiotic activity, a possibility that may extend to dwarf planets, particularly during their early evolutionary phases. Among dwarf planets, several large primitive bodies—such as Pluto, Eris (which is more massive than Pluto), and the elongated Haumea—are classified as dwarf planets. With densities up to 2000 kg/m³, these bodies likely possess rocky interiors beneath their icy exteriors. In this study, we aim to explore the implications of carbon-rich compositional models on the structure and dynamics of dwarf planets and small icy moons. Early interior models of icy worlds assumed a water-rich external envelope (hydrosphere) and a rock-rich interior composed of dry silicates and sulfides (SSC), with a variable Fe/Si ratio. Resulting Fe/Si ratios were often non-solar and exhibited significant variations, for which there is currently no clear explanation. Later models incorporated hydrous silicates as a likely core component, suggesting past—and possibly ongoing—aqueous alteration processes. These icy bodies likely host chemical processes involving interactions between water, organics, and rock. Macromolecular compounds have been identified in icy grains ejected from Enceladus, tholins on Pluto’s surface, and organic-rich plumes on Triton. Endogenous organic compounds and their potential aqueous alteration share similarities with those found in Ryugu sampled by JAXA’s Hayabusa2 mission, and carbonaceous chondrites, which have undergone varying degrees of aqueous alteration, with significant prebiotic implications. Our recent work introduces refractory carbonaceous compounds (COM) to account for the density cores while maintaining a CI-chondrite Fe/Si ratio. This approach has refined our estimates of the composition of seven representative bodies among the potential precursors abundant in the outer Solar System. Unlike previous models—which assumed a refractory core composed solely of SSC and resulted in unexplained and variable Fe/Si ratios—our model illustrates a plausible chemical composition for dwarf planets and moons forming in the solar or circumplanetary nebula. It highlights the importance of distinct "snow" (water, CO₂, etc.) and "soot" (carbonaceous compounds) lines, which play a key role in the distribution of volatile and refractory elements during the formation of these bodies. By incorporating carbon as a major component, our models provide a coherent framework to explain, the internal structure of icy bodies, their thermal and geochemical dynamics, their prebiotic potential, particularly through the presence of complex organic compounds and stable aqueous environments. These advancements may also shed light on the formation mechanisms of carbonaceous chondrite parent bodies and the conditions that enabled the emergence of life in the early Solar System.
Two major objectives are identified:
1. How does the carbonaceous component affect the physical properties of the refractory cores and volatile release to the hydrosphere? Modeling of internal density profile and thermal evolution requires that the physical properties of mantle- and core-forming materials are determined. The major source of uncertainty is carbonaceous matter density that can vary from ~1300 in low-density coals to 2300 kg/m3 in crystalline graphite. Systematic investigation of the density and composition of carbonaceous matter analogs will provide reliable composition models of outer Solar system objects. Along with compositional and density variations, there is a drastic increase of thermal conductivity from ~0.4 W/m/K in coals, proxies to the amorphous carbonaceous matter, to ~400 W/m/K in high temperature polycrystalline graphite. The relationship between carbonaceous matter transformations and thermal conductivity will also be investigated. Finally, the compositional change of carbonaceous matter is accompanied by the release of volatiles and hydrocarbons. We will investigate the potential transfer of volatile species to the ocean then surface of the body, where they may be identified by spectroscopic observations by JWST and future in-situ missions. For silicates and sulfides, densities, as well as thermal diffusivities are well known, and phase diagrams can be predicted from thermodynamics.
2. Can thermal evolution models constrain carbon-rich compositional models matching density in moons and dwarf planets? We will explore the evolution of COM for large bodies, Pluto, Eris, Triton where higher pressure and temperature can be reached. We will investigate what could be the causes of the density variations from Pluto (1860 kg/m3) to Eris (2520 kg/m3) while their surfaces have similar infrared spectra. Temperature increase is a likely cause for density increase because it causes dehydration of silicates and evolution of carbonaceous matter to denser forms, as well as variations in ice fraction. Other targets are Ceres and Enceladus with well-characterized shape and reduced moments of inertia following the Dawn and Cassini missions, Haumea, with its elongated Jacobi ellipsoid shape, and Dione, Mimas, Rhea, with less-well constrained MoI. These bodies with radius going from about 200 km to more than 700 km will offer opportunity to explore the role of organic matter at moderate pressure up to a few hundred MPa. Plausible range of composition anchored on the best-characterized bodies will then be used to model potential interior structure and thermal evolution of dwarf planets and Uranus moons, in particular the role of COM in the existence of present or past oceans and the possible consequences of aqueous alteration. Carbon content will be investigated to determine the required concentrations to account for present and future observational constrains.
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.
Project coordination
Bruno Reynard (Laboratoire de géologie de Lyon : Terre, planètes et environnement)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partnership
LPG LABORATOIRE DE PLANETOLOGIE ET GEOSCIENCES
LGL-TPE Laboratoire de géologie de Lyon : Terre, planètes et environnement
Help of the ANR 518,625 euros
Beginning and duration of the scientific project:
December 2023
- 48 Months
