Formation of Amino Acids and Carboxylic Acids in Weakly Reducing Planetary Atmospheres by Solar Energetic Particles from the Young Sun

Abstract

Life most likely started during the Hadean Eon; however, the environmental conditions which contributed to the complexity of its chemistry are poorly known. A better understanding of various environmental conditions, including global (heliospheric) and local (atmospheric, surface, and oceanic), along with the internal dynamic conditions of the early Earth, are required to understand the onset of abiogenesis. Herein, we examine the contributions of galactic cosmic rays (GCRs) and solar energetic particles (SEPs) associated with superflares from the young Sun to the formation of amino acids and carboxylic acids in weakly reduced gas mixtures representing the early Earth's atmosphere. We also compare the products with those introduced by lightning events and solar ultraviolet light (UV). In a series of laboratory experiments, we detected and characterized the formation of amino acids and carboxylic acids via proton irradiation of a mixture of carbon dioxide, methane, nitrogen, and water in various mixing ratios. These experiments show the detection of amino acids after acid hydrolysis when 0.5% (v/v) of initial methane was introduced to the gas mixture. In the set of experiments with spark discharges (simulation of lightning flashes) performed for the same gas mixture, we found that at least 15% methane was required to detect the formation of amino acids, and no amino acids were detected in experiments via UV irradiation, even when 50% methane was used. Carboxylic acids were formed in non-reducing gas mixtures (0% methane) by proton irradiation and spark discharges. Hence, we suggest that GCRs and SEP events from the young Sun represent the most effective energy sources for the prebiotic formation of biologically important organic compounds from weakly reducing atmospheres. Since the energy flux of space weather, which generated frequent SEPs from the young Sun in the first 600 million years after the birth of the solar system, was expected to be much greater than that of GCRs, we conclude that SEP-driven energetic protons are the most promising energy sources for the prebiotic production of bioorganic compounds in the atmosphere of the Hadean Earth.

Keywords: amino acids; carboxylic acids; cosmic rays; early Earth; prebiotic synthesis; solar energetic particles; weakly reducing atmosphere.

Figure 1

HPLC chromatograms of amino acids formed by spark discharges in gas mixtures of CO₂, CH₄, N₂, and H₂O in various mixing ratios. (a) rCH₄ = 0, (b) rCH₄ = 0.05, (c) rCH₄ = 0.5. All the samples were analyzed after acid hydrolysis. Compositions are shown in Table 1. ABA means aminobutyric acid.

Figure 2

HPLC chromatograms of amino acids formed by proton irradiation of gas mixtures of CO₂, CH₄, N₂, and H₂O in various mixing ratios. (a) rCH₄ = 0, (b) rCH₄ = 0.01, (c) rCH₄ = 0.02, (d) rCH₄ = 0.05, (e) rCH₄ = 0.2, (f) rCH₄ = 0.5. All the samples were analyzed after acid hydrolysis. Compositions are shown in Table 1. ABA means aminobutyric acid.

Figure 3

Dependence of amino acid yields by spark discharges on methane ratio (rCH₄). (a) Glycine, (b) β-alanine, (c) α-amino butyric acid, (d) γ-aminobutyric acid. All the samples were analyzed after acid hydrolysis. Open symbols show “not detected”.

Figure 4

Dependence of amino acid yields by proton irradiation on methane ratio (rCH₄). (a) Glycine, (b) β-alanine, (c) α-amino butyric acid, (d) γ-aminobutyric acid. All the samples were analyzed after acid hydrolysis. Open symbols show “not detected”.

Figure 5

GC chromatogram of carboxylic acids formed by spark discharge in the gas mixture of CO₂, CH₄, N₂, and H₂O (rCH₄ = 0.25). The sample was analyzed without hydrolysis. C₁: formic acid, C₂: acetic acid, C₃: propanoic acid, iC₄: isobutyric acid, C₄: butyric acid, iC₅: isovaleric acid.

Figure 6

The yields of carboxylic acids formed from the gas mixtures of CO₂, CH₄, N₂, and H₂O by spark discharges (SD) or proton irradiation (PI). The samples were analyzed without hydrolysis. The methane ratio (rCH₄) was 0%, 5%, or 25%, as indicated in the figure. C₁: formic acid, C₂: acetic acid, C₃: propanoic acid, iC₄: isobutyric acid, iC₅: isovaleric acid, C₂di: oxalic acid, C₃di: malonic acid, C₄di: succinic acid, C₅di: glutaric acid.

Figure 7

The energy yields (G-values) of glycine formed from the gas mixtures of CO₂, CH₄, N₂, and H₂O by spark discharges and proton irradiation as a function of rCH₄. Circle: proton irradiation, square: spark discharge, open symbol: not calculated (G was less than 10⁻⁷).

Figure 8

The estimated production rate of glycine by spark discharge (lightning), GCRs, and SEP events in the CO₂-, CH₄-, N₂-, and H₂O-dominated atmosphere of early Earth. Circle: SEPs, triangle: GCRs, square: gas discharge, open symbol: not calculated (less than 10² kg yr⁻¹).