Abstract

Using a new photochemical model, the HCN chemistry in Earth's early atmosphere is revisited. We find that HCN production in a CH 4-rich early atmosphere could have been efficient, similar to the results of a previous study (Zahnle, 1986). For an assumed CH 4 mixing ratio of 1000 ppmv, HCN surface deposition increases from 2 × 10 9 cm −2 s −1 at fCO 2 = 3% to more than 1 × 10 10 cm −2 s −1 (30 Tg/yr) at fCO 2 = 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 × 10 7 cm −2 s −1 or lower, thereby providing a modest source of HCN for prebiotic synthesis.

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