Oxidation of organic macromolecules by hydrogen peroxide: Implications for stability of biomarkers on Mars

An assessment of the stability of organic macromolecules in the presence of oxidants believed to exist on the surface of Mars is critical in order to evaluate their utility as martian biomarkers. Two types of organic macromolecules-tholins produced from plasma discharge in a 99.9:0.1 N-2/CH4 atmosphere (McDonald et al. 1994, Icarus 108, 137), and terrestrial humic acid--were subjected to oxidation by 30% H2O2 in water at three temperatures and the overall observed rate constant k for oxidation to volatile species determined for each temperature. The temperature dependence of k for tholin oxidation by H2O2, assuming pseudo-first-order kinetics, is log k = 1.98 - 912/T, with k in days(-1) and T in Kelvins. The half-lives for oxidative destruction of tholins calculated from this relationship are 0.35 yr at equatorial mean temperature (215 K) and 24 yr at polar mean temperature (150 K). For humic acid, log k = 14.77 - 4500/T, resulting in half-lives of 2700 yr at 215 K and 3.2 x 10(12) yr at 150 K. These half-lives are only estimates, given significant uncertainties in the application of psuedo-first-order kinetic analysis and extrapolation over wide temperature ranges. The data suggest, however, that some organic macromolecules may be stable against oxidation on the martian surface, at least in the polar regions, over the entire history of Mars. (C) 1998 Academic Press.