Bolide impacts and the oxidation state of carbon in the Earth's early atmosphere

Abstract

A one-dimensional photochemical model was used to examine the effect of bolide impacts on the oxidation state of Earth's primitive atmosphere. The impact rate should have been high prior to 3.8 Ga before present, based on evidence derived from the Moon. Impacts of comets or carbonaceous asteroids should have enhanced the atmospheric CO/CO2 ratio by bringing in CO ice and/or organic carbon that can be oxidized to CO in the impact plume. Ordinary chondritic impactors would contain elemental iron that could have reacted with ambient CO2 to give CO. Nitric oxide (NO) should also have been produced by reaction between ambient CO2 and N2 in the hot impact plumes. High NO concentrations increase the atmospheric CO/CO2 ratio by increasing the rainout rate of oxidized gases. According to the model, atmospheric CO/CO2 ratios of unity or greater are possible during the first several hundred million years of Earth's history, provided that dissolved CO was not rapidly oxidized to bicarbonate in the ocean. Specifically, high atmospheric CO/CO2 ratios are possible if either: (1) the climate was cool (like today's climate), so that hydration of dissolved CO to formate was slow, or (2) the formate formed from CO was efficiently converted into volatile, reduced carbon compounds, such as methane. A high atmospheric CO/CO2 ratio may have helped to facilitate prebiotic synthesis by enhancing the production rates of hydrogen cyanide and formaldehyde. Formaldehyde may have been produced even more efficiently by photochemical reduction of bicarbonate and formate in Fe(++)-rich surface waters.

The evolution of Earth's biogeochemical nitrogen cycle

“…Kuhn and Atreya, 1979). In the absence of this TiO 2 catalysis mechanism, N 2 oxidation to NO x species in post-impact plumes becomes the major source (~10 12 mol/yr) (Kasting, 1990), which, however, could have been insignificant over stretches of many millions of years when no impacts occurred and it would have declined rapidly after ~3.8 Gyr (Kasting, 1990). Hydrothermal reduction of N 2 to NH 4 + could have generated 10 9 -10 12 mol/yr of fixed nitrogen, according to some scaled laboratory estimates (Smirnov et al, 2008); however, those calculations assume an arbitrary reaction yield and an unlimited supply of the FeNi alloy that acts as a reductant or catalyst.…”

UV shielding of NH3and O2by organic hazes in the Archean atmosphere

“…The questions we are addressing are still first-order ones, and we feel that our approach to the modeling is commensurate with this level of accuracy. The photochemical model used here is a one-dimensional (horizontally averaged) model that has been adapted from one used in previous studies of the primitive terrestrial atmosphere [e.g., Kasting, 1982;Kasting et al, 1983;Zahnle, 1986;Kasting, 1990]. The model includes 72 chemical species involved in 337 reactions.…”

“…Reactions and rate constants are listed in Table 1. In addition to the reactions listed here, we also included lightning production of NO, 02, and CO in the troposphere, as described by Kasting [1990].…”

“…Volcanic outgassing of SO2 and H2 was included; the surface fluxes were set equal to 3.5 x 109 and 2.5 x 10 •ø cm -2 s -• respectively [Kasting, 1990]. At the upper boundary, zero flux was assumed for most species.…”

Formation of amino acid precursors with large molecular weight in dense clouds and their relevance to origins of bio-homochirality

“…Many kinds of amino acids were detected in the products when various kinds of energies including ultraviolet light (Sagan & Khare 1971) and heat (Harada & Fox 1964). It is suggested, however, that the primitive atmosphere of the Earth was not strongly reduced, but only slightly reduced: Carbon dioxide, carbon monoxide, nitrogen and water were among major constituents of the primitive Earth atmosphere (Kasting 1990). It is not easy to form bioorganic compounds from slightly reduced gases by such major energies 466 Kensei Kobayashi et al as ultraviolet light, spark discharge and heat.…”