Mars, the next frontier in space exploration, still poses many questions for scientists. The planet was once more hospitable, characterized by a warm and wet climate with liquid oceans. But today Mars is cold and dry, with most water now located below the surface. Understanding how much water is stored offers critical information for energy exploration, as well as life sustainability on the planet.
A research group from Tohoku University has helped shed light on this by improving an existing Mars climate model. The enhanced model accommodates the various properties of Martian regolith, or the loose deposits of solid rock that comprise Martian soil.
Mirai Kobayashi says current models fail to account for the fact that laboratory experiments have demonstrated that the water-holding capacity of the regolith is strongly influenced by its adsorption coefficient.
"Models to date that estimate the distribution of surface and subsurface water on Mars assume that its regolith properties are uniform. This contrasts with observations made by orbiters and landers, which suggest that Martian regolith has globally non-uniform physical properties."
The model estimated Mars's subsurface water distribution down to 2 meters from the surface. Like a sponge, highly absorptive regolith in Mars's mid- and low latitudes retains substantial amounts of absorbed water. Some of this water, the findings showed, remains on the surface of the regolith as stable adsorbed water.
The study also showed that the soil on Mars could keep ice near the surface in the middle and lower areas because water vapor moves more slowly there. This means the soil helps trap water for a long time by slowing down how water vapor spreads, which is important for understanding the change in water on Mars over time.
"Our study stresses the importance of incorporating absorption and inhomogeneity of Martian regolith in forecasting Mars's surface water," says Takeshi Kuroda, who led the team alongside Kobayashi, Arihiro Kamada and Naoki Terada. "The model can also be used to study how water on Mars has changed, and how it may have moved deeper underground near the planet's mantle."
With several Mars exploration missions underway, including the Japan-led Martian Moons eXploration (MMX) and the international Mars Ice Mapper (MIM) projects, the model is expected to complement further studies that can lead to subsurface water maps of Mars.
The results were published in the Journal of Geophysical Research: Planets on February 24, 2025.