Density of carbonaceous organic matter in icy bodies

Carbonaceous organic matter (COM), between 20 and 50%, is needed to model the rocky core of icy bodies (Neri et al, 2020, Reynard & Sotin. 2023), and account for their mass, moment of inertia, and density as measured by the Cassini, Galileo and Juno missions. However, the effects of temperature and pressure on COM at conditions of the rocky cores (up to ~7 GPa and 1300 K) have not been taken into account in these preliminary models. We present an experimental characterization of the evolution of COM at elevated temperature and pressure to describe its density evolution during thermal evolution of icy bodies. Carbonaceous organic matter undergoes important transformations both in terms of composition and structure when subjected to an increase in temperature and pressure. These transformations are characterized by the loss of heteroatoms (O, H, N, S) and a structural rearrangement which lead to a variation of density from 1200 kg/m³ at 300 K to 2300kg/m³ at 1300 K.

We first performed determinations of the ambient temperature compressibility of COM analogs using diamond anvil cell experiments. Kerogens and glassy carbons were used as analogs of COM. Second, we adapted the Vitrimat kinetic model of kerogen chemical and density evolution as a function of temperature and time (Burnham 2019). This modification accounts for chemical evolution determined on samples heated at temperatures of 473-723 K for times ranging from seconds to 100 days, and at various pressures (0.2-2.5 GPa). Combining these two studies allows us to describe the density evolution of COM as a function of time, temperature and pressure, assuming it behaves like kerogens.

Thermo-chemical evolution models are coupled with this equation to determine the time evolution of the density structure of icy bodies and compare it with available observations. In addition to density evolution of COM in the core, heteroatoms are released as volatiles (mainly H₂O, CO₂, CH₄). They may form new species in the core (carbonates) and the high-pressure ice level (clathrates), reach the ocean, and be released to the upper ice level, then to space. Models will also provide estimates of volatile fluxes and formation of new compounds on the density structure.

Improvements of density determinations of COM analogs will provide accurate models for predicting the density and thermal evolutions compatible improved determinations of internal structures of icy moons from the JUICE and future Europa Clipper and Dragonfly missions, and observation of dwarf planets by JWST.

 

Burnham, A. K. Kinetic models of vitrinite, kerogen, and bitumen reflectance. Organic Geochemistry 131, 50-59 (2019). https://doi.org/https://doi.org/10.1016/j.orggeochem.2019.03.007

Néri, A., Guyot, F., Reynard, B. & Sotin, C. A carbonaceous chondrite and cometary origin for icy moons of Jupiter and Saturn. Earth and Planetary Science Letters 530, 115920 (2020). https://doi.org/https://doi.org/10.1016/j.epsl.2019.115920

Reynard, B. & Sotin, C. Carbon-rich icy moons and dwarf planets. Earth and Planetary Science Letters 612, 118172 (2023). https://doi.org/https://doi.org/10.1016/j.epsl.2023.118172