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

We have examined the generation and stability of O 2 − on TiO 2 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 O 2 − 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 O 2 − decomposition on hydrated albite and derived an activation energy, E a = 15.2 kJ/mol. Assuming pseudo first‐order kinetics, a simple thermal model of Mars' regolith demonstrates that the surface O 2 − population does not go to zero overnight; superoxide extinction at the surface is only complete when the seasonal CO 2 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 O 2 − should be observable on Martian surface materials throughout the Martian year. However, on the basis of our inability to generate stable O 2 − on hydrated albite via direct UV irradiation, we do not believe that this mechanism is capable of explaining the O 2 release in the Viking Gas Exchange (GEx) results, since O 2 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 O 2 − 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 TiO 2 in surface catalysis. Secondly, we found that superoxide that formed during the sorption or drying of a 1% H 2 O 2 solution on TiO 2 , and potentially other oxides seems to be stable indefinitely.