Critical synthesis of Martian perchlorate research: Bridging scattered mission data, theories, experimental...

As the critical and abundant resource on Mars, the unique perchlorate-rich composition of Martian regolith profoundly influences the Martian geochemical cycle. Elevated perchlorate levels and their role in Martian chlorine cycling are central for addressing organic matter preservation, microbial survival, and interpreting brine formation at subfreezing temperatures in Martian surface environments. A major consensus, emerged from over two decades of experimental analog studies and observational data by Mars probes, is that perchlorates are not static end products but part of an active redox cycle induced by Martian UV and cosmic radiation, with regolith mineralogy being a major confounding factor. However, significant gaps exist in the current literature regarding their dominant formation pathway, decomposition mechanisms, lower depth distribution (> 10 cm), and the existence of other oxychlorines. To add to these challenges, the lack of detailed analysis of Martian regolith samples, particularly for lower-oxidation-state oxychlorines, necessitates reliance on laboratory analogy studies that often poorly analogues for the environments revealed by Mars surface and subsurface conditions. This critical narrative establishes a framework to synthesize existing knowledge and gaps, prioritize regolith sampling and data gathering missions on Mars, and recommend analytical priorities for the laboratory analysis of planned Mars sample returns. Meanwhile, resolving the formation and decomposition of perchlorates through rigorous laboratory analog studies with protocols demonstrating robust Mars-environment relevance could reveal unexplored aspects of the Martian chlorine redox cycle. It is crucial to manage the challenges perchlorates pose for Martian colonists, while harnessing their potential to provide oxygen and enable liquid brines.