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G. D. McDonald (JPL)
Organic macromolecules have been delivered to Mars by carbonaceous chondrites over the history of the planet, and may have also been produced by biology during Mars' early history. In order to evaluate the stability of these potential biomarkers against oxidative destruction on the surface of Mars, humic acid was exposed to gaseous hydrogen peroxide at several temperatures. The extent of oxidative degradation of the humic acid as a function of time was determined by gas chromatographic measurement of carbon dioxide production. The observed first-order rate constant k for humic acid oxidation by dry hydrogen peroxide varies with temperature according to the equation log k = 13.55 - 5270/T, with k in days(-1) and T in Kelvins. Based on these data, it is estimated that organic macromolecules produced by Martian biology 4 Ga ago could still be present in measurable amounts in regions of the planet where the oxidation rate constant for organic material is approximately 10(-12) d(-1). This rate constant corresponds to an effective oxidation temperature of around 209K, and thus a maximum temperature of approximately 220K or less. The only areas of the Martian surface where temperatures likely do not exceed 220K are near the poles, at latitudes higher than roughly ± 80°.
The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.