Preservation of Microorganisms (Chroococcidiopsis sp. 029) in Salt Minerals under Low Atmospheric Pressure: Application to Life Detection on Mars

Abstract Salt minerals on Mars represent a promising target for investigating potential past surface and subsurface life. Terrestrial salt minerals have been shown to incorporate microorganisms within crystals. However, the effect of Mars’s low atmospheric pressure on the preservation of microorganisms in salt minerals during their formation remains unclear. Here we investigated the interactions between microorganisms ( Chroococcidiopsis sp. 029) and crystals of halite (NaCl), epsomite (MgSO 4 ·7H 2 O), and gypsum (CaSO 4 ·2H 2 O) during the rapid evaporation of brines under simulated Martian atmospheric pressure. Parallel experiments were conducted under terrestrial pressure for comparison. Halite, epsomite, and gypsum formed under both terrestrial and low-pressure conditions, though crystal morphologies varied depending on the pressure. Microorganisms were identified within fluid inclusions or as solid inclusions in the crystal matrix. Halite crystals exhibited a greater propensity to incorporate cells under low pressure compared to terrestrial pressure, while the entrapment of cells in epsomite was similar under both conditions. In contrast, significantly fewer cells were trapped in gypsum crystals under low pressure. The results demonstrate the feasibility of cell entrapment in salt minerals formed by rapid evaporation under low-pressure conditions on Mars, and atmospheric pressure exerts distinct influences on different types of salts. The variation in fluid inclusion size in halite under different pressures even shows promise as a possible paleobarometer. Our findings suggest that halite is the most promising candidate for preserving potential Martian life and could be an excellent target for future Mars sample return missions.