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Most of the water on Mars is gone, and it may be the fault of the rocks. Billions of years ago, the Red Planet probably had just as much water as Earth, but now it’s all gone. Tiny variations in the chemistry of the two planets may have allowed Mars’s rocks to suck up the water before they sank deep under the surface.

It has been suggested that Mars lost its water when it lost the magnetic field that protected it from the harsh solar wind. Once that was gone, the planet’s early atmosphere was stripped away and escaped into space along with most of the surface water. But recent research has shown that this process may be unable to account for the loss of all the water.

In an effort to figure out what else could have caused this loss, at the University of Oxford and his colleagues simulated the planet’s early geological reactions with water. They found that because Martian rocks are full of iron oxide, they can drink in about 25 per cent more water than similar rubble on Earth.

“Very subtle changes in chemistry can have very big impacts on whether you keep water around on the surface of the planet,” says Wade. “Mars rocks can hold much more water than terrestrial rocks. So, Mars has drunk its water, and Earth hasn’t.”

Once the water soaked into the rock, it couldn’t seep back out. Iron oxide doesn’t work like a sponge: it would have incorporated only the oxygen from water, while the hydrogen was lost to space. The rocks then would have sunk into the Martian mantle, stealing the planet’s water for good.

As the hydrated rocks sank, so did the planet’s chances for life. “You could initiate life on Mars, but for life to evolve into complex animals, stuff like us, you probably have to have water for billions of years,” says Wade. “If you run out of water, you run out of life.”

With almost all the water sucked away into the ground early in Mars’s history, it is unlikely that complex life could ever have evolved there, even if the planet had held onto its magnetic field and atmosphere. Mars may have been doomed by its chemistry from the start.

Read more: Boiling water on Mars could make the planet’s sand levitate