HCN Production via Impact Ejecta Reentry During the Late Heavy Bombardment

Abstract

Major impact events have shaped the Earth as we know it. The Late Heavy Bombardment is of particular interest because it immediately precedes the first evidence of life. The reentry of impact ejecta creates numerous chemical by‐products, including biotic precursors such as HCN. This work examines the production of HCN during the Late Heavy Bombardment in more detail. We stochastically simulate the range of impacts on the early Earth and use models developed from existing studies to predict the corresponding ejecta properties. Using multiphase flow methods and finite‐rate equilibrium chemistry, we then find the HCN production due to the resulting atmospheric heating. We use Direct Simulation Monte Carlo to develop a correction factor to account for increased yields due to thermochemical nonequilibrium. We then model 1‐D atmospheric turbulent diffusion to find the time accurate transport of HCN to lower altitudes and ultimately surface water. Existing works estimate the necessary HCN molarity threshold to promote polymerization that is 0.01 M. For a mixing depth of 100 m, we find that the Late Heavy Bombardment will produce at least one impact event above this threshold with probability 24.1% for an oxidized atmosphere and 56.3% for a partially reduced atmosphere. For a mixing depth of 10 m, the probability is 79.5% for an oxidized atmosphere and 96.9% for a partially reduced atmosphere. Therefore, Late Heavy Bombardment impact ejecta is likely an HCN source sufficient for polymerization in shallow bodies of water, particularly if the atmosphere were in a partially reduced state.

When Did Life Likely Emerge on Earth in an RNA‐First Process?

“…This atmosphere would then have had a redox state fully compatible with the formation of HCN, HCCCN, and the other reduced primary RNA precursors cited above (Parkos et al. (2018) and references therein) . Metallic iron also reduces CO 2 to CO and methane, and reduces N 2 to NH 3 .…”

“…The iron core from a 10 23 kg Moneta impactor could have reduced up to three ocean masses, delivering as much as 90 bars of H 2 to the atmosphere. This atmosphere would then have had a redox state fully compatible with the formation of HCN, HCCCN, and the other reduced primary RNA precursors cited above (Parkos et al (2018) and references therein). [89] Metallic iron also reduces CO 2 to CO and methane, and reduces N 2 to NH 3 .…”

“…This atmosphere would then have had a redox state fully compatible with the formation of HCN, HCCCN, and the other reduced primary RNA precursors cited above (Parkos et al (2018) and references therein). [89] Metallic iron also reduces CO 2 to CO and methane, and reduces N 2 to NH 3 . The resulting atmosphere would then have become a productive source of large amounts of HCN, HCCCN, H 2 NCN 2 , glycolaldehyde, and other primary precursors that are inaccessible in the standard Hadean atmosphere.…”

The Emergence of Life

“…In one study, the Miller-Urey experiments could produce RNA nucleobases in discharges and laser-driven plasma impact simulations under reducing conditions with formamide as an intermediate (Ferus et al 2017). One report makes a compelling argument for the occurrence of short-lived reducing atmospheres which could have supported the formation of organic matter (Genda et al 2017;Parkos et al 2018). However, the reducing atmosphere scenario of early Earth (on which the Oparin-Haldane hypothesis is based) have been challenged (Abelson 1966;Trail et al 2011).…”

Provisioning the origin and early evolution of life

“…Big impactors, which have the potential to deliver significant amounts of material, vaporise on impact and their organic cargo is largely atomised, along with a portion of the surface and atmosphere of Earth, ultimately giving species such as NO . , CO and HCN [6,7]. Even in carbonaceous chondrites, which are the richest in 'organics', nucleotides have not been detected, levels of amino acids and lipids needed for life are very low and there are a huge number, likely millions, of other compounds present [8].…”