Subsurface Water Activity in Mars’ Shallow Alluvial Deposit: Evidence from Tianwen-1 Radar Observations

Early Mars is believed to have hosted extensive liquid water activity; however, the current Martian environment is characterized by aridness and cold, with the majority of water resources thought to be locked in polar ice caps. Increasing evidence from recent Mars exploration missions suggests the presence of deeper subsurface water activity. Our research leverages data from the Tianwen-1 mission, employing high-frequency radar and a climate station onboard the Zhurong rover. We developed a model correlating the loss tangent of subsurface materials with temperature, revealing that within ∼ 5 meters of depth, the loss tangent remains stable until ∼ 239K, after which it rises significantly from ∼ 0.0167 to ~ 0.0448. This result indicates that during Martian spring and summer, diurnal phase transitions between brine ice and liquid brine occur within ∼ 5 meters of the subsurface at the Tianwen-1 landing site (25.066◦ N, 109.926◦ E). This phenomenon is driven by heat transfer through thermal convection from the surface to depths of at least ∼ 2-5 meters, influencing the variation of the loss tangent observed within this range. Our findings contribute to the understanding of the Martian subsurface hydrological cycle, shedding light on the processes that shape surface geology and potential future utilization of Martian water resources.