Meteoritic abundances of fatty acids and potential reaction pathways in planetesimals
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abstract
The origin of fatty acids on the prebiotic Earth is important as they likely
formed the encapsulating membranes of the first protocells. Carbon-rich
meteorites (i.e., carbonaceous chondrites) such as Murchison and Tagish Lake
are well known to contain these molecules, and their delivery to the early
planet by intense early meteorite bombardments constitutes a key prebiotic
source. We collect the fatty acid abundances measured in various carbonaceous
chondrites from the literature and analyze them for patterns and correlations.
Fatty acids in meteorites include straight-chain and branched-chain
monocarboxylic and dicarboxylic acids up to 12 carbons in length---fatty acids
with at least 8 carbons are required to form vesicles, and modern cell
membranes employ lipids with ~12--20 carbons. To understand the origin of
meteoritic fatty acids, we search the literature for abiotic fatty acid
reaction pathways and create a candidate list of 11 reactions that could
potentially produce these fatty acids in meteorite parent bodies.
Straight-chain monocarboxylic acids (SCMA) are the dominant fatty acids in
meteorites, followed by branched-chain monocarboxylic acids (BCMA). SCMA are
most abundant in CM2 and Tagish Lake (ungrouped) meteorites, ranging on average
from 10$^2$ ppb to 4x10$^5$ ppb, and 10$^4$ ppb to 5x10$^6$ ppb, respectively.
In CM, CV, and Tagish Lake meteorites, SCMA abundances generally decrease with
increasing carbon chain length. Conversely, SCMA abundances in CR meteorites
peak at 5 and 6 carbons in length, and decrease on either side of this peak.
This unique CR fatty acid distribution may hint at terrestrial contamination,
or that certain fatty acid reactions mechanisms are active in different
meteorite parent bodies (planetesimals). We identify Fischer-Tropsch-type
synthesis as the most promising pathway for further analysis in the production
of fatty acids in planetesimals.
