  The widespread presence of ribonucleic acid (RNA) catalysts and cofactors in
Earth's biosphere today suggests that RNA was the first biopolymer to support
Darwinian evolution. However, most "path-hypotheses" to generate building
blocks for RNA require reduced nitrogen-containing compounds not made in useful
amounts in the CO2-N2-H2O atmospheres of the Hadean. We review models for
Earth's impact history that invoke a single ~10^23 kg impactor (Moneta) to
account for measured amounts of platinum, gold, and other siderophilic
("iron-loving") elements on the Earth and Moon. If it were the last sterilizing
impactor, Moneta would have reduced the atmosphere but not its mantle, opening
a "window of opportunity" for RNA synthesis, a period when RNA precursors
rained from the atmosphere to land holding oxidized minerals that stabilize
advanced RNA precursors and RNA. Surprisingly, this combination of physics,
geology, and chemistry suggests a time when RNA formation was most probable,
~120 +/- 100 million years after Moneta's impact, or ~4.36 +/- 0.1 billion
years ago. Uncertainties in this time are driven by uncertainties in rates of
productive atmosphere loss and amounts of sub-aerial land.
