Chemical effects of large impacts on the Earth's primitive atmosphere

Intense bombardment of the moon and terrestrial planets ~3.9-4.0 × 10⁹ years ago^1,2 could have caused the chemical reprocessing of the Earth's primitive atmosphere³. In particular, the shock heating and rapid quenching caused by the impact of large bodies into the atmosphere could produce molecules such as HCN and H₂CO⁴ which are important precursors for the abiotic synthesis of complex organic molecules^5-7. Here we model the production of HCN and H₂CO by thermochemical equilibrium and chemical kinetic calculations of the composition of shocked air parcels for a wide range of temperatures, pressures and initial compositions. For atmospheres with C/O >=1, our results suggest that bolide impacts cause HCN volume mixing ratios of approximately 10^-3 to 10^-5 in the impact region and global average ratios of 10^-5 to 10^-12. The corresponding H₂CO mixing ratios in the impact region are 10^-7 to 10^-9 no-global mixing can occur, however, as H₂CO is rapidly destroyed or rained out of the atmosphere within days to hours. Rainout to the oceans of 3-15% of the HCN produced can provide ~(3-14) × 10¹¹ mol HCN per year. This is somewhat larger than other predicted sources of HCN⁸ and H₂CO⁹ on the primitive Earth.