Modeling carbon outgassing from chondritic planetesimals

Abstract

The thermochemical evolution of planetesimals is an underprobed stage of volatile delivery to terrestrial planets during their formation, and may contribute to the volatile depletion of the Earth relative to primitive chondrites. We have developed a model of C outgassing from porous, chondritic planetesimals. Our model tracks the thermal evolution and the production of CO/CO2 gas using the redox states of ordinary and enstatite chondrites (OC and EC, respectively, collectively the "NCs"), and CI and CV carbonaceous chondrites ("CCs"). We posit the formation of global fractures when local gas pressure exceeds confinement levels, which vent the excess directly to space, leading to efficient C depletion. We also account for sintering and the enthalpy of dehydration from wet carbonaceous chondrite bodies. We find that C depletion is more efficient on CC planetesimals than NCs due to the former's oxidized environment: for 10-100 km planetesimals formed at 2 Myr after CAI formation, > 50% of C is depleted in almost all CC bodies while < 50% is depleted in almost all NC bodies. Both the largest and the smallest bodies tend to preserve more C, the former due to sintering locking condensed C in against escape, while the latter due to efficient conductive cooling. Earlier accreted planetesimals deplete more C: bodies formed before ~ My deplete most of their C. Our results favor NC planetesimals as the C carriers during terrestrial planets' accretion. Terrestrial planets likely accreted from a mix of C-depleted and C-rich bodies from both CC and NC reservoirs.