Radiation-Driven Formation of Reactive Oxygen Species in Oxychlorine-Containing Mars Surface Analogues

The present study demonstrates that γ-radiolyzed perchlorate-containing Mars soil salt analogues (in a CO atmosphere) generate upon H O wetting the reactive oxygen species (ROS) superoxide radical (O ), hydrogen peroxide (H O ), and hydroxyl radicals ( OH). This study also validates that analogue radiolysis forms oxychlorine species that, in turn, can UV-photolyze to OH upon UV photolysis. This investigation was made possible by the development of a new assay for inorganic-origin O and H O determination and by the modification of a previous assay for soil OH. Results show that radiolyzed Mg(ClO ) generates H O and OH; and when included as part of a mixture analogous to the salt composition of samples analyzed at the Mars Phoenix site, the analogue generated O , H O , and OH, with OH levels 150-fold higher than in the radiolyzed Mg(ClO ) samples. Radiolyzed Mars Phoenix site salt analogue that did not contain Mg(ClO ) generated only OH also at 150-fold higher concentration than Mg(ClO ) alone. Additionally, UV photolysis of the perchlorate γ radiolysis product chlorite (ClO ) generated the oxychlorine products trihalide (Cl ), chlorine dioxide (ClO ), and hypochlorite (ClO ), with the formation of OH by UV photolysis of ClO . While the generation of ROS may have contributed in part to CO production in the Viking Labeled Release (LR) experiment and O (g) release in the Viking Gas Exchange (GEx) experiment, our results indicate that they are not likely to be the major contributor to the LR and GEx results. However, due to their highly reactive nature, they are expected to play a significant role in the alteration of organics on Mars. Additionally, experiments with hypochlorite show that the thermal stability of NaClO is in the range of the thermal stability observed for thermally liable oxidant responsible for the Viking LR results. Key Words: Mars-Oxygen-Salts-Radiation-Habitability. Astrobiology 17, 319-336.