The present-day distribution of surface and subsurface water on Mars reflects a complex history of exchange between the planet’s surface and interior since its formation. Constraining the water content of the deep mantle is essential for estimating the planet’s total water budget and understanding its long-term geologic and hydrologic evolution. Yet, the abundance of water at depth remains loosely constrained. Here, we analyze seismic data from global tectonic marsquakes and meteorite impacts recorded by the InSight mission, and identify evidence for weaker attenuation in Mars’ deep mantle (500–1500 km), in contrast to Earth. This low attenuation likely reflects fundamental differences in mantle properties between the two planets, including water content, grain size, oxygen fugacity, and/or temperature. Isolating the role of water from other parameters is key to quantifying Mars’ deep water reservoir and provides new constraints on the volatile inventory and interior evolution of the red planet.
