Abstract
Liquid water was abundant on Mars during the Noachian and Hesperian periods but vanished as the planet transitioned into the cold, dry environment we see today. It is hypothesized that much of this water was either lost to space or stored in the crust. However, the extent of the water reservoir within the crust remains poorly constrained due to a lack of observational evidence. Here, we invert the shear wave velocity structure of the upper crust, identifying a significant low-velocity layer at the base, between depths of 5.4 and 8 km. This zone is interpreted as a high-porosity, water-saturated layer, and is estimated to hold a liquid water volume of 520-780 m of global equivalent layer (GEL). This estimate aligns well with the remaining liquid water volume of 710-920 m GEL, after accounting for water loss to space, crustal hydration and modern water inventory.