Meteorites and the RNA World: A Thermodynamic Model of Nucleobase Synthesis within Planetesimals

TL;DR

This study models nucleobase formation in meteorite parent bodies, revealing which reactions could produce key bases and explaining the absence of cytosine and thymine in meteorites, impacting theories on the origin of the RNA world.

Contribution

It provides a thermodynamic model identifying key reactions for nucleobase synthesis in planetesimals, explaining the absence of certain bases and their implications for the origin of life.

Findings

Cytosine unlikely to persist due to deamination

Thymine formation from uracil is thermodynamically favorable but likely unstable in oxidizing conditions

FT synthesis is the dominant pathway for nucleobase formation in planetesimals

Abstract

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.