  There are two dominant and contrasting classes of origin of life scenarios:
those predicting that life emerged in submarine hydrothermal systems, where
chemical disequilibrium can provide an energy source for nascent life; and
those predicting that life emerged within subaerial environments, where UV
catalysis of reactions may occur to form the building blocks of life. Here, we
describe a prebiotically plausible environment that draws on the strengths of
both scenarios: surface hydrothermal vents. We show how key feedstock molecules
for prebiotic chemistry can be produced in abundance in shallow and surficial
hydrothermal systems. We calculate the chemistry of volcanic gases feeding
these vents over a range of pressures and basalt C/N/O contents. If
ultra-reducing carbon-rich nitrogen-rich gases interact with subsurface water
at a volcanic vent they result in 1 mM to 1 M concentrations of diacetylene,
acetylene, cyanoacetylene, hydrogen cyanide, bisulfite, hydrogen sulfide and
soluble iron in vent water. One key feedstock molecule, cyanamide, is not
formed in significant quantities within this scenario, suggesting that it may
need to be delivered exogenously, or formed from hydrogen cyanide either via
organometallic compounds, or by some as yet-unknown chemical synthesis. Given
the likely ubiquity of surface hydrothermal vents on young, hot, terrestrial
planets, these results identify a prebiotically plausible local geochemical
environment, which is also amenable to future lab-based simulation.
