Revisiting HCN formation in Earthʹs early atmosphere

Using a new photochemical model, the HCN chemistry in Earthʹs 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.