  Giant impacts can generate transient hydrogen-rich atmospheres, reducing
atmospheric carbon. The reduced carbon will form hazes that rain out onto the
surface and can become incorporated into the crust. Once heated, a large
fraction of the carbon would be converted into graphite. The result is that
local regions of the Hadean crust were plausibly saturated with graphite. We
explore the consequences of such a crust for a prebiotic surface hydrothermal
vent scenario. We model a surface vent fed by nitrogen-rich volcanic gas from
high-temperature magmas passing through graphite-saturated crust. We consider
this occurring at pressures of 1-1000 bar and temperatures of 1500-1700 degC.
The equilibrium with graphite purifies the left-over gas, resulting in
substantial quantities of nitriles (0.1% HCN and 1 ppm HC3N) and isonitriles
(0.01% HNC) relevant for prebiotic chemistry. We use these results to predict
gas-phase concentrations of methyl isonitrile of ~ 1 ppm. Methyl isocyanide can
participate in the non-enzymatic activation and ligation of the monomeric
building blocks of life, and surface, or shallow, hydrothermal environments
provide its only known equilibrium geochemical source.
