Evidence for Weak Attenuation in Mars’s Deep Mantle

Surface or shallow subsurface water and ice have been reported on Mars, but sustaining life requires more than just the presence of liquid water. A mechanism to preserve water over geological timescales is essential, and a deep-water reservoir could fulfill this role. However, the volatile content of Mars’ deeper mantle remains poorly constrained. Using seismic data from global tectonic marsquakes and meteorite impacts recorded by the InSight mission, we observed weak attenuation in Mars’ deep mantle (500–1500 km) relative to Earth’s. This weak attenuation likely results from lower water content, larger grain size, and/or reduced oxygen fugacity in the martian mantle. Assuming mantle mineral grain sizes on Mars are similar to those on Earth, Mars’ upper mantle appears relatively dry, with water content estimated at less than 13–24% of Earth’s. If deep water exists on Mars today, it is most likely confined to the basal mantle layer (~ 1550–1700 km) at the core-mantle boundary, potentially the only viable deep-water reservoir for the red planet.