The plausibility of origins scenarios requiring two impactors

TL;DR

This study evaluates the likelihood of early Earth prebiotic chemistry scenarios involving two impactors, concluding such events are very unlikely unless specific environmental and chemical stability conditions are met.

Contribution

It provides a quantitative assessment of double impact scenarios for prebiotic chemistry, highlighting their improbability and the conditions needed for their plausibility.

Findings

Double impact scenarios are only possible >4 Gya.

Such scenarios are extremely unlikely for prebiotic chemistry.

Stability of ferrocyanide salts is crucial for scenario plausibility.

Abstract

Hydrogen cyanide delivered by cometary impactors can be concentrated as ferrocyanide salts, which may support the initial stages of prebiotic chemistry on the early Earth. One way to achieve the conditions required for a variety of prebiotic scenarios, requiring for example the formation of cyanamide and cyanoacetylene, is through the arrival of a secondary impactor. In this work, we consider the bombardment of the early Earth, and quantitatively evaluate the likelihood of origins scenarios that invoke double impacts. Such scenarios are found to be possible only at very early times ($>\,$4Gya), and are extremely unlikely settings for the initial stages of prebiotic chemistry, unless (i) ferrocyanide salts are stable on 1000yr timescales in crater environments, (ii) there was a particularly high impact rate on the Hadean Earth, and (iii) environmental conditions on the Hadean Earth were conducive to successful cometary delivery (i.e., limited oceanic coverage, and low ($\lesssim 1$bar) atmospheric surface pressure). Whilst environmental conditions on the early Earth remain subject to debate, this work highlights the need to measure the typical lifetime of ferrocyanide salts in geochemically realistic environments, which will determine the plausibility of double impact scenarios.