Prebiotic organic synthesis in early Earth and Mars atmospheres: Laboratory experiments with quantitative determination of products formed in a cold plasma flow reactor

Abstract

The goal of this study was to explore prebiotic chemistry in a range of plausible early Earth and Mars atmospheres. To achieve this laboratory continuous flow plasma irradiation experiments were performed on N 2/H 2/CO/CO 2 gas mixtures chosen to represent mildly reducing early Earth and Mars atmospheres derived from a secondary volcanic outgassing of volatiles in chemical equilibrium with magmas near present day oxidation state. Under mildly reducing conditions (91.79% N 2, 5.89% H 2, 2.21% CO, and 0.11% CO 2), simple nitriles are produced in the gas phase with yield ( G in molecules per 100 eV), for the key prebiotic marker molecule HCN at G ∼ 1 × 10 −3 (0.1 nmol J −1). In this atmosphere localized HCN concentrations possibly could approach the 10 −2 M needed for HCN oligomerization. Yields under mildly oxidizing conditions (45.5% N 2, 0.1% H 2, 27.2% CO, 27.2% CO 2) are significantly less as expected, with HCN at G ∼ 3 × 10 −5 ( 3 × 10 −3 nmol J −1 ). Yields in a Triton atmosphere which can be plausibly extrapolated to represent what might be produced in trace CH 4 conditions (99.9% N 2, 0.1% CH 4) are significant with HCN at G ∼ 1 × 10 −2 (1 nmol J −1) and tholins produced. Recently higher methane abundance atmospheres have been examined for their greenhouse warming potential, and higher abundance hydrogen atmospheres have been proposed based on a low early Earth exosphere temperature. A reducing (64.04% N 2, 28.8% H 2, 3.60% CO 2, and 3.56% CH 4), representing a high CH 4 and H 2 abundance early Earth atmosphere had HCN yields of G ∼ 5 × 10 −3 (0.5 nmol J −1). Tholins generated in high methane hydrogen gas mixtures is much less than in a similar mixture without hydrogen. The same mixture with the oxidizing component CO 2 removed (66.43% N 2, 29.88% H 2, 0% CO 2, and 3.69% CH 4) had HCN yields of G ∼ 1 × 10 −3 (0.1 nmol J −1) but more significant tholin yields.