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R.C. Quinn (SETI Institute), A.P. Zent (NASA Ames)
Lack of unambiguous spectral evidence for carbonates on Mars has led to the suggestion that they maybe unstable on the martian surface. Mukhin et al. (1996) conducted experiments in which natural calcite was exposed under vacuum to UV light. They reported a quantum yield for the photo-induced decomposition of carbonate of 10-5 molecules/photon and a threshold effect at 350-400 nm. This is surprising because of the high threshold effect and because previous work (Booth and Kieffer 1978) had shown that carbonates form under conditions similar to those on Mars even with UV light present. Additionally, the Mukhin results may not be applicable to Mars because of the low partial pressure of CO2 in their system.
We have experimentally investigated the UV photo-induced decomposition of calcium carbonate under a simulated Martian atmosphere to assess more realistically the efficacy of the process on Mars. In these experiments, 13C labeled CO2 was used to allow products of carbonate decomposition to be differentiated from the simulated Mars atmosphere. No 13C-carbon dioxide release was observed in experiments run at temperatures below 323K, indicating either that UV decomposition of carbonate does not occur in a 10 mbar Mars atmosphere, or that the decomposition did not exceed the system lower limit of detection. Extrapolating the lower detection limit (3x10-8 molecules/photon) to an upper limit of carbonate decomposition at the surface of Mars yields a maximum UV photodecomposition loss rate from bulk calcite of about 2 nm/yr. We conclude that it is unlikely that the apparent absence of carbonate on the martian surface is due to UV photodecomposition of carbonates in the current environment.
Support for this work was provided by the NASA Exobiology Program.
References:
Booth, M. C., H. H. Kieffer, J. Geophys. Res., 83, 1809-1815, 1976
Mukhin, L.M., A.P. Koscheev, Y.P. Dikov, J. Huth, H. Wanke, Nature, 379, 141-143, 1996.