Photocatalytic oxidation of organic compounds on Mars

Abstract

THE non-detection of organic compounds on Mars¹ is interesting because there are at least two mechanisms that can produce a contemporary accumulation of these compounds on Mars—photochemical synthesis and meteoritic infall. The synthesis of organic compounds from carbon monoxide and water absorbed in inorganic matrices under UV irradiation and simulated martian conditions has been demonstrated¹. By comparison, our Moon has a surface composition that is 1.1% carbonaceous chondrite, due to a meteoritic infall rate about three times less than that on Mars². As much as 5% of carbonaceous chondrite material is organic³. Assuming a meteoritic influx rate three times that of the Moon, and a reasonable mixing rate with the martian regolith, organic constituents of carbonaceous chondrites should be diluted in the martian soil to concentrations detectable by the Viking gas chromatograph–mass spectrometer (GCMS)¹. The results of the Viking GCMS experiments were, therefore, rather surprising. No organic molecules were present in the Viking GCMS samples above concentrations of parts per 10⁹ (ref. 1). Mechanisms have been proposed in which peroxides, superoxides and ozonides formed under UV irradiation oxidise organic compounds^4,5. Glow discharge was suggested as the ‘scavenger’ of martian organic matter⁶. Here, we propose an alternative mechanism for the destruction of organics on Mars—UV-stimulated catalytic oxidation. Our proposal differs from previous ones in that all conditions required for photocatalytic oxidation of organics—gaseous oxygen⁷, UV irradiation⁸, and titanium dioxide^9,10—have been found to be present in the martian environment.