Revisiting HCN formation in Earth's early atmosphere

Using a new photochemical model, the HCN chemistry in Earth's early atmosphere is revisited. We find that HCN production in a CH4-rich early atmosphere could have been efficient, similar to the results of a previous study (Zahnle, 1986). For an assumed CH4 mixing ratio of 1000 ppmv, HCN surface deposition increases from 2 × 109 cm−2 s−1 at fCO2 = 3% to more than 1 × 1010 cm−2 s−1 (30 Tg/yr) at fCO2 = 0.3% and 1%. These conditions may well have applied throughout much of the Archean eon, 3.8–2.5 Ga. Prior to the origin of life and the advent of methanogens, HCN production rates would likely have been at 1 × 107 cm−2 s−1 or lower, thereby providing a modest source of HCN for prebiotic synthesis.

Research highlights
► In this work we study the HCN chemistry in early Earth atmosphere.
► Archean Earth (3.8-2.5 Ga) atmosphere probably contained much more methane than today.
► HCN production in methane-rich early Earth atmosphere could have been efficient.