Estimating the capacity for production of formamide by radioactive minerals on the prebiotic Earth

Abstract

Water creates special problems for prebiotic chemistry, as it is thermodynamically favorable for amide and phosphodiester bonds to hydrolyze. The availability of alternative solvents with more favorable properties for the formation of prebiotic molecules on the early Earth may have helped bypass this so-called "water paradox". Formamide (FA) is one such solvent, and can serve as a nucleobase precursor, but it is difficult to envision how FA could have been generated in large quantities or accumulated in terrestrial surface environments. We report here the conversion of aqueous acetonitrile (ACN) via hydrogen cyanide (HCN) as an intermediate into FA by γ-irradiation under conditions mimicking exposure to radioactive minerals. We estimate that a radioactive placer deposit could produce 0.1‒0.8 mol FA km^-2 year^-1. A uraninite fission zone comparable to the Oklo reactors in Gabon can produce 0.1‒1 mol m^-2 year^-1, orders of magnitude greater than other scenarios of FA production or delivery for which reaching sizeable concentrations of FA are problematic. Radioactive mineral deposits may be favorable settings for prebiotic compound formation through emergent geologic processes and FA-mediated organic chemistry.

Figure 1

Factors causing heterogeneous distribution of energy from radioactive placer minerals. (a) Global distribution of major modern heavy mineral placer deposits. (b) Concentration of heavy mineral grains (dark layers) within a typical beach placer deposit; image courtesy C. Bern, United States Geological Survey. (c) Spatial distribution of α, β and γ particles around a single radioactive mineral grain.

Figure 2

GC-MS chromatogram of the most abundant identified products (annotated peaks) resulting from gamma irradiation of mixtures of 30% aqueous ACN. Indicated total doses were delivered at a dose rate of ~3 kGy hr⁻¹.

Figure 3

FA production as a function of total dose and yield from ACN. (a) Concentration of compounds at 2.92, 1.0 and 0.5 kgry hr⁻¹ γ-dose rates versus total dose. (b) FA yield (triangles, mM) and molar conversion efficiency (squares, dimensionless) versus initial acetonitrile concentration for 800 kGy total dose at ~3kGy hr⁻¹ dose rate.

Figure 4

Proposed formamide synthesis and concentration process, depicting localized production and concentration of formamide (HCONH₂) near radioactive mineral deposits on terrestrial surface environments. Note that chemical reactions are not balanced for simplicity.

Figure 5

Formamide (FA) maximum reservoir size for different FA delivery or production mechanisms. FA sources include global comet influx, a single comet event, ideal pH and temperature for oceanic and lake reservoirs from HCN hydrolysis, and radiolysis of ACN and HCN within radioactive placers and uraninite fission zones. Marker size indicates characteristic area of reservoir, with smaller markers indicating more localized processes. Heat map indicates maximum possible FA concentration of the reservoir.