Origin-of-life Molecules in the Atmosphere after Big Impacts on the Early Earth

Abstract

The origin of life on Earth would benefit from a prebiotic atmosphere that produced nitriles, like HCN, which enable ribonucleotide synthesis. However, geochemical evidence suggests that Hadean air was relatively oxidizing with negligible photochemical production of prebiotic molecules. These paradoxes are resolved by iron-rich asteroid impacts that transiently reduced the entire atmosphere, allowing nitriles to form in subsequent photochemistry. Here we investigate impact-generated reducing atmospheres using new time-dependent, coupled atmospheric chemistry and climate models that account for gas-phase reactions and surface catalysis. The resulting H _2 -, CH _4 -, and NH _3 -rich atmospheres persist for millions of years, until the hydrogen escapes to space. The HCN and HCCCN production and rainout to the surface can reach 10 ^9 molecules cm ^−2 s ^−1 in hazy atmospheres with a mole ratio of CH _4 /CO _2 > 0.1. Smaller CH _4 /CO _2 ratios produce HCN rainout rates of 0.1 is 4 × 10 ^20 –5 × 10 ^21 kg (570–1330 km diameter), depending on how efficiently iron reacts with a steam atmosphere, the extent of atmospheric equilibration with an impact-induced melt pond, and the surface area of nickel that catalyzes CH _4 production. Alternatively, if steam permeates and deeply oxidizes the crust, impactors of ∼10 ^20 kg could be effective. Atmospheres with copious nitriles have >360 K surface temperatures, perhaps posing a challenge for RNA longevity, although cloud albedo can produce cooler climates. Regardless, postimpact cyanide can be stockpiled and used in prebiotic schemes after hydrogen has escaped to space.

Keywords

• Earth (planet)
• Earth atmosphere
• Astrobiology
• Atmospheric composition
• Pre-biotic astrochemistry