Presentation #102.07 in the session Formation of Planets and Satellites.
Carbon is a crucial volatile for a terrestrial planet’s long-term climate and habitability. The bulk silicate Earth is about 4 orders of magnitude depleted in carbon compared to the ISM and solar abundances, and about 1 to 2 orders of magnitude depleted compared to chondritic abundances. The delivery – and destruction – of carbon during the formation of terrestrial planets have long been an open question. Processes at each stage of planet formation are invoked to account for the carbon budgets of the Earth and its precursor materials. These include the dynamic and photochemical evolution of the protoplanetary disc as well as core-mantle differentiation. Recent meteoritic evidence suggests carbon depletion occurred on planetesimals, building blocks of terrestrial planets. Yet detailed modeling of planetesimal carbon outgassing remains lacking. Here we present a 1-D, numerical, thermochemical model that simulates carbon outgassing on planetesimals. The model is an updated version of Peng & Valencia, LPSC 2021. The thermal evolution of the planetesimal is controlled by radiogenic heating, conductive cooling, and latent heat of reaction. C-CO-CO2 reactions are modeled kinetically, and controlled by chondritic oxygen fugacities. The gas generated then percolates through the porous body and is lost to space. For small (<100km) carbonaceous chondrite parent bodies, we found that the equilibrium gas pressure can surpass the local lithostatic pressure at or below ~900 K. To resolve this conflict, we postulate the formation of extensive crack networks, effectively capping the local gas pressure at the lithostatic level. We find this mechanism leading to efficient carbon destruction in small, carbonaceous planetesimals that form early. On the other hand, planetesimals composed of ordinary and enstatite chondrites may be less prone to carbon depletion due to their more reduced conditions. Thus our work can help constrain the possible carriers and timing of carbon delivery to terrestrial planets.