Accumulation of volatiles under salt crusts in the highly evaporative Qaidam basin: Implications for salt crust fluid processes on Mars

The behavior of volatiles is critically important for understanding crustal fluids and the potential existence of a subsurface biosphere on Mars. However, our knowledge of the volatile cycle on Mars is limited by insufficient data from landed rovers and orbiter sensors. Halite salt crusts are widespread in the Qaidam Basin on the northern Tibetan Plateau due to strong evaporation under hyperarid climate conditions. We observed that the halite-dominated salt crust in the desiccated playa area diverts fluids percolating from depth to the surface, leading to the formation of raised polygonal rims enriched in gypsum. We drilled through the salt crust using a hand mill and measured the instantaneous gas concentrations and compositions. Beneath the halite salt crust, significantly higher concentrations of H₂O, CO₂, and CH₄ were detected compared with levels in the atmospheric background and at the polygonal rims. The thickness of the salt crust ranges from approximately 0.3 to 1 m, with halite content primarily between 5 and 30 wt%, and is comparable in scale to the thickness (typically <3 m) and abundance (10–25 wt%) of chloride deposits on Mars. These results suggest that similar salt crust formation should also be common in Martian crater basins subjected to long-term evaporation under hyperarid conditions. Furthermore, such salt crusts could trap deep volatiles, including potential biogenic gases, which may be detectable by gas spectrometers aboard Mars landers.