The spatial distribution of carbon dust in the early solar nebula and the carbon content of planetesimals

TL;DR

This study models how carbonaceous material in the early solar nebula was processed during planetary formation, explaining the low carbon content in terrestrial planets through thermal destruction during chondrule formation.

Contribution

It provides a detailed model of carbon processing in the solar nebula, linking thermal events to the observed carbon distribution in planetary bodies.

Findings

Carbon hydrocarbons are removed at 250-400 K

Amorphous carbon survives up to 1200 K

Chondrule formation is key to low planetary carbon content

Abstract

A high fraction of carbon bound in solid carbonaceous material is observed to exist in bodies formed in the cold outskirts of the solar nebula, while bodies in the terrestrial planets region contain nearly none. We study the fate of the carbonaceous material during the spiral-in of matter as the sun accretes matter from the solar nebula. From observational data on the composition of the dust component in comets and interplanetary dust particles, and from data on pyrolysis experiments, we construct a model for the composition of the pristine carbonaceous material in the outer parts of the solar nebula. We study the pyrolysis of the refractory and volatile organic component and the concomitant release of high-molecular-weight hydrocarbons under quiescent conditions of disk evolution where matter migrates inwards. We also study the decomposition and oxidation of the carbonaceous material during violent flash heating events, which are thought to be responsible for the formation of chondrules. It is found that the complex hydrocarbon components are removed from the solid disk matter at temperatures between 250 and 400 K, while the amorphous carbon component survives up to 1200 K. Without efficient carbon destruction during flash-heating associated with chondrule formation the carbon abundance of terrestrial planets, except for Mercury, would be not as low as it is found in cosmochemical studies. Chondrule formation seems to be a process that is crucial for the carbon-poor composition of the material of terrestrial planets.