The formation and stability of the superoxide radical (O2−) on rock‐forming minerals: Band gaps, hydroxylation state, and implications for Mars oxidant chemistry

Abstract

We have examined the generation and stability of O2− on TiO2 and albite, a common Na feldspar. We were not able to produce thermally stable superoxide on albite, in contrast to the results of Yen et al., who reported the generation of O2− that was stable up to 200°C on labradorite, another common feldspar. The superoxide radical did form under UV irradiation at 77 K on albite that was not dehydrated but decayed rapidly at room temperature. On dehydrated albite, no superoxide signal was observed. We studied the kinetics of O2− decomposition on hydrated albite and derived an activation energy, Ea = 15.2 kJ/mol. Assuming pseudo first‐order kinetics, a simple thermal model of Mars' regolith demonstrates that the surface O2− population does not go to zero overnight; superoxide extinction at the surface is only complete when the seasonal CO2 cap covers the surface and surface photolysis is inhibited. Depending on the specific quantum efficiency of the e−/h+ generation process, a finite, non‐equilibrium population of O2− should be observable on Martian surface materials throughout the Martian year. However, on the basis of our inability to generate stable O2− on hydrated albite via direct UV irradiation, we do not believe that this mechanism is capable of explaining the O2 release in the Viking Gas Exchange (GEx) results, since O2 release in that case was observed even after samples had been stored for 143 sols in the dark at 10°C, then heated to 145°C for 3 hours. At least two other potential pathways to the generation of O2− are identified in this article. The first possibility is that metal oxides common on the Martian surface, particularly hematite, may be photoactive on Mars and play a role analogous to TiO2 in surface catalysis. Secondly, we found that superoxide that formed during the sorption or drying of a 1% H2O2 solution on TiO2, and potentially other oxides seems to be stable indefinitely.