Reevaluation of Perchlorate in Gale Crater Rocks Suggests Geologically Recent Perchlorate Addition

Perchlorate (ClO4-) was discovered in Martian soil by the Phoenix lander, with important implications for potential Martian biology, photochemistry, aqueous chemistry, and the chlorine cycle on Mars. Perchlorate was subsequently reported in both loose sediment and bedrock samples analyzed by the Sample Analysis at Mars instrument onboard the Curiosity rover in Gale crater based on a release of O-2 at 200-500 degrees C. However, the continually wet paleoenvironment recorded by the sedimentary rocks in Gale crater was not conducive to the deposition of highly soluble salts. Furthermore, the preservation of ancient perchlorate to the modern day is unexpected due to its low thermodynamic stability and radiolytic decomposition associated with its long exposure to radioactivity and cosmic radiation. We therefore investigate alternative sources of O-2 in Sample Analysis at Mars analyses including superoxides, sulfates, nitrate, and nanophase iron and manganese oxides. Geochemical evidence and oxygen release patterns observed by Curiosity are inconsistent with each of these alternatives. We conclude that perchlorate is indeed the most likely source of the detected O-2 release at 200-500 degrees C, but contend that it is unlikely to be ancient. Rather than being associated with the lacustrine or early diagenetic environment, the most likely origin of perchlorate in the bedrock is late stage addition by downward percolation of water through rock pore space during transient wetting events in the Amazonian. The conclusion that the observed perchlorate in Gale crater is most likely Amazonian suggests the presence of recent liquid water at the modern surface. Plain Language Summary Perchlorate is a chemical found on Mars that dissolves easily in water, which can keep water from freezing at the cold temperatures found on Mars's surface. It was first discovered in soil, and has been found by the Curiosity rover in rock at Gale crater. Perchlorate should have flowed out of the sandy lake bottom in ancient Gale crater before it turned into rock. Perchlorate is also easily destroyed, so even if it were included in the rock originally, perchlorate should have disintegrated in the following similar to 3.5 billion years. Because of these dilemmas with finding perchlorate in ancient rocks, we have reevaluated the evidence for perchlorate as observed by Curiosity. Of a number of other chemicals which could explain the data, we find that perchlorate is still the best fit and is probably present. Due to the problems with perchlorate being included in the rock originally and surviving until today, we conclude that it has been added to the rocks recently. This recent addition could happen by perchlorate dissolving in water and flowing into the rock. Such a process suggests that liquid water was present in Gale crater long after Mars became cold and dry.