The surface of Mars may have absorbed the water that once flowed on the Red Planet, thereby making it uninhabitable, scientists from the University of Oxford in the United Kingdom have suggested. Although the surface is now barren and frozen, evidence suggests that it was once a warmer planet where water flowed freely, the researchers said.

Earlier, scientists had proposed that the planet’s interior cooled much more quickly than the Earth’s. That caused its magnetic field to diminish around four billion years ago, allowing water vapour to escape. But estimates of the amount of water Mars initially had are larger than estimates of how much would have escaped to space. A team of scientists, led by a researcher at Oxford Jon Wade, has now said the water that did not evaporate, but may have been absorbed by the surface of Mars, Ars Technica reported.

Basalt rocks in Mars are different from the ones found on Earth because of higher levels of oxygen. When terrestrial basalts absorb water into their minerals, the density decreases, they become a little buoyant and are unlikely to sink deep. But the different chemical composition of Martian basalts largely cancels out this buoyancy effect, scientists said, thus allowing them to absorb about 25% more water.