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.Comment: 19 pages, 7 figures, to appear in Faraday Disc., vol 168, 2014, DOI:
10.1039/C4FD00003
