  The possible meteorite parent body origin of Earth's pregenetic nucleobases
is substantiated by the guanine (G), adenine (A) and uracil (U) measured in
various meteorites. Cytosine (C) and thymine (T) however are absent in
meteorites, making the emergence of a RNA and later RNA/DNA/protein world
problematic. We investigate the meteorite parent body (planetesimal) origin of
all nucleobases by computationally modeling 18 reactions that potentially
contribute to nucleobase formation in such environments. Out of this list, we
identify the two most important reactions for each nucleobase and find that
these involve small molecules such as HCN, CO, NH3, and water that ultimately
arise from the protoplanetary disks in which planetesimals are built. The
primary result of this study is that cytosine is unlikely to persist within
meteorite parent bodies due to aqueous deamination. Thymine has a
thermodynamically favourable reaction pathway from uracil, formaldehyde and
formic acid, but likely did not persist within planetesimals containing H2O2
due to an oxidation reaction with this molecule. Finally, while FT synthesis is
found to be the dominant source of nucleobases within our model planetesimal,
NC synthesis may still be significant under certain chemical conditions (e.g.
within CR2 parent bodies). We discuss several major consequences of our results
for the origin of the RNA world.
