Near-surface liquid water on Mars inferred from seasonal marsquakes.

Exploring the presence and behavior of water on Mars is critical for understanding the planet's geological evolution, hydrological processes, and potential habitability, which has been a central objective of past and ongoing Martian exploration missions. Martian geomorphology provides evidence of ancient groundwater activity, but confirmation of present-day liquid water remains limited. Here, we infer near-surface brines confined to meter-scale depths in regions north of about 30 °N in the northern hemisphere, based on the analysis of seasonal variations in marsquake seismicity and thermal modeling. The absence of seasonal marsquakes during colder periods and their abrupt resurgence in warmer seasons can be explained by ice-to-brine phase transitions. Our findings reveal a mechanism whereby seasonal melting of subsurface ice elevates pore pressure and lubricates faults, leading to a reduction in frictional strength, and ultimately inducing marsquakes. This mechanism accounts for the seasonal variation, clustering, high seismic b-values, and shallow focal depths of seasonal marsquakes. Additionally, we estimate that the melting point of briny ice on Mars is below approximately 250 ± 13 K, advancing our understanding of Mars' present-day brine cycle and near-surface hydrological processes.