Perchlorate brine formation from frost at the Viking 2 landing site.

Abstract

The presence and stability of brines on Mars's surface remain a significant mystery in planetary exploration. Previous mechanisms proposed for brine formation include melting of ice-salt mixtures and salt deliquescence. However, melting lacks a recharge mechanism, and deliquescence is impeded by Mars's extreme surface aridity. This study explores an underexplored process: the role of seasonal frost in brine formation. Utilizing meteorological data from the Viking 2 lander-the only mission, with Phoenix, to observe in situ water frost formation-I demonstrate that brines can form over approximately 30 sols at the end of winter as frost sublimates. The stable brines exhibit a water activity upper limit of 0.52, corresponding to the eutectic point of calcium perchlorate, a salt detected in various Martian regions, likely including the Viking 2 landing site. Consequently, I conclude that calcium perchlorate can generate small amounts of liquid brine in contact with frost for brief periods. The seasonal nature of frost suggests these brines recur and may leave long-term imprints. Therefore, frost-covered regions are prime candidates for future habitability and astrobiological exploration.