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Project Progress

We have continued our fundamental experimental research into (1) whether or not a photochemical haze may have formed in Mars’ early atmosphere, (2) whether or not such a haze may have warmed or cooled the surface — that is, resulted in a greenhouse or “anti-greenhouse” effect, and (3) whether such an aerosol could have settled to the surface and provided a potentially “false biosignature” in the carbon-13 isotopic composition of organic matter in the martian rock record. Our work over the past year has focussed on the first and third issues. Two new graduate students have followed up on the doctoral work of Mate Adamkovics (supported by NAI funds in previous years under this project) in which we found that particle formation by ultraviolet illumination of CH₄ in a terrestrial-like atmosphere can occur at considerably lower CH₄-to-CO₂ ratios than predicted by photochemical models. This result is significant, with the implication that organic aerosols may have had a larger influence on the climate of early Earth and early Mars than previously estimated by photochemical models that required large amounts of CH₄ before aerosol formed (since CH₄ is likely to have been scarce in the early atmospheres of terrestrial planets). This year, Emily Chu and Philip Croteau have been fine-tuning our mechanistic understanding of particle formation through additional experiments and expanding our experimental photochemical kinetics database by varying the relative amounts of CH₄ and CO₂ and performing critical control experiments to insure that this important experimental result is robust and free from instrumental artifacts. We have also made significant progress in addressing the third issue noted above: Could the organic aerosol formed from the photochemical oxidation of CH₄ be significantly lighter in its carbon-13-to-carbon-12 ratio? If so, then the aerosol could have settled onto the surface and been incorporated into the rock record. Such an inorganic, photochemical signature might then be misinterpreted as a biosignature, as first suggested by Pavlov, Kasting, and co-workers in 2001. Our result so far, made in collaboration with George Cooper at NASA Ames Research Center who analyzed the ¹³C isotopic composition of particles formed in our chamber and brought to his meteoritics laboratory, is that the bulk ¹³C isotopic composition of the organic particles is roughly the same as that of the initial CH₄. This result — especially if it holds up to additional experimental scrutiny in the following year of NAI support — is particularly exciting since it thus far appears that atmospheric photochemistry may be ruled out as a source of isotopically light carbon in the martian rock record and would thus simplify the possible detection of life in martian rocks.