Exploring the Origins of Carbon in Terrestrial Worlds

TL;DR

This paper investigates the origins of carbon in terrestrial worlds by comparing organic content in meteorites and the interstellar medium, proposing a chemical processing mechanism in protoplanetary disks that could supply organic carbon to forming planets.

Contribution

It introduces a model where CO destruction in warm, partially ionized disks supplies organic carbon to planetesimals, explaining the carbon source for Earth's building blocks.

Findings

Organic carbon in meteorites is only partly from star formation chemistry.

CO in the ISM is largely in gas or ice form, with reduced abundance in disks.

Gas-phase CO destruction can provide organic carbon to planetesimals.

Abstract

Given the central role of carbon in the chemistry of life, it is a fundamental question as to how carbon is supplied to the Earth, in what form and when. We provide an accounting of carbon found in solar system bodies, in particular a comparison between the organic content of meteorites and that in identified organics in the dense interstellar medium (ISM). Based on this accounting identified organics created by the chemistry of star formation could contain at most ~15% of the organic carbon content in primitive meteorites and significantly less for cometary organics, which represent the putative contributors to starting materials for the Earth. In the ISM ~30% of the elemental carbon is found in CO, either in the gas or ices, with a typical abundance of ~10^-4 (relative to H2). Recent observations of the TW Hya disk find that the gas phase abundance of CO is reduced by an order of magnitude compared to this value. We explore a solution where the volatile CO is destroyed via a gas phase processes, providing an additional source of carbon for organic material to be incorporated into planetesimals and cometesimals. This chemical processing mechanism requires warm grains (> 20 K), partially ionized gas, and sufficiently small <10 micron grains, i.e. a larger total grain surface area, such that freeze-out is efficient. Under these conditions static (non-turbulent) chemical models predict that a large fraction of the carbon nominally sequestered in CO can be the source of carbon for a wide variety of organics that are present as ice coatings on the surfaces of warm pre-planetesimal dust grains.