Exobiology and Evolutionary Biology

Photochemistry and Impact-induced Shock Chemistry of Ancient Atmospheres

PI: Kevin Zahnle

This proposal addresses photochemistry and impact-induced shock chemistry in ancient atmosphere of interest to Exobiology. Three basic projects are proposed, two of which are natural outgrowths of work funded by previous Exobiology grants, while the third is newer and more ambitious.

The first task addresses terrestrial atmospheric chemistry at the end of the Archean, after the advent of eukaryotes and oxygenic photosynthesis but before the first appearance of abundant free O2 in the air. This is roughly the period of time when Mass-Independently-Fractionated sulfur (MIF-S) reached its peak. Here I will investigate the hypothesis that giant MIF-S was caused by the atmosphere becoming rich in reduced biogenic sulfur gases, including H2S, CS2, OCS, and DMS. Previous modeling has shown that high fluxes of S-gases probably leads to strong MIF-S. We expect high fluxes of biogenic S gases in the late Archean consequent to biological innovation and the apparent growth of the biologically-available sulfate. This favored sulfate-reducers over methanogens in the competition for substrate, and so would also have favored the emission of S-gases over methane.

The second task addresses sulfur photochemistry on ancient Mars with an emphasis on sulfate formation. This work is pertinent to the histroy of volcanism on Mars and the general observation that sulfates—which are abundant on Mars—appear in general to be younger than phyllosilicates (clays) that seem to indicate an earlier less oxidizing surface environment.
The third task addresses the composition of the atmosphere and surface environments on the Hadean Earth. The Hadean Earth is crucial to Exobiology because it is the one known example of a planet where life began, but the lack of a rock record has made it difficult to characterize what the Hadean was actually like. Here I propose to leverage off our earlier work developing a narrative history of the evolution of the Earth after the Moon-forming impact to address the atmospheric chemistry of the Hadean, emphasizing on the one hand CO as a an interesting prebiotic molecule, and on the other hand exploiting recent theoretical studies suggesting that primitive methane-ammonia atmospheres may not be so unrealistic after all.