Mars was not always the dry planet we know today: billions of years ago, it was home to lakes, rivers, and even oceans. But the question of where all this water went was a mystery.

Until now, researchers believed that she had escaped into space. But according to a new study, it actually took off in a massive Martian crust.

Warning, this discovery does not mean “there is a large pool of water below the surface of Mars”, warns Eva Schillerler, a California Institute of Technology researcher and lead author of the study, in the prestigious journal Science on Tuesday Published.

“We say that the crust is made up of what we call hydrated minerals, ie minerals that have water in their composition,” he told AFP.

According to scientists, at the beginning of its history, there was enough liquid water to cover it below the height of 100 to 1,500 meters on the surface of Mars.

For comparison, the entire red planet is equivalent to half of the 1,000-meter-high Atlantic Ocean, Eva Shilper calculated.

Today it is only 20 to 40 meters high according to this measurement. Water is either present in the atmosphere or as ice in polar caps or in the Martian subsoil.

So where did the rest go?

– up to 99% –

Until now, researchers had considered this water loss to be due to atmospheric exhaust.

This phenomenon is also present on Earth, but is more pronounced on Mars due to lower gravity.

Eva Scheller explains that water molecules are made up of oxygen and hydrogen and “hydrogen atoms are very light.” “Because of this they can free themselves from Mars’ gravitational field, and escape into space.”

But this explanation was not enough to explain the loss of small amounts of water only.

However, observations of satellites as well as analyzes by various rovers sent to Mars suggest that it was indeed too much shelter.

In addition, the study of deuterium levels on Mars, which makes up a small fraction of hydrogen and escapes less into space because it is heavier, also does not stick to the principles of atmospheric escape alone.

Thus the study published on Tuesday for the first time produced a model linking a complementary theory.

“When a stone interacts with water, there are a series of very complex chemical reactions that form a hydrated mineral,” explains Eva Scheller.

According to the researcher, clay is a very common example of such a mineral, and is also the most common on Mars.

“The water loss in the Martian crust is at least equal to or greater than the atmospheric exit,” she says. Up to 99% of the water disappearing from the Martian surface can thus be trapped in rocks.

– For 3 billion years –

The study suggests that “the lack of water in the crust is a very important mechanism for the planets, which determines when they become dry”, underlines the researcher.

This process also occurs on Earth, but thanks to plate tectonics (which does not exist on Mars), trapped water is recycled, via volcanic phenomena.

Furthermore, knowing that hydrated minerals on Mars are at least three billion years old, this means that the red planet had lost most of its water by that time.

“That’s what Mars knew more than three billion years ago,” says Ms. Scheller.

She hopes to be able to refine the various scenarios considered for her study for the NASA Fortitude Rover, which have just arrived on the red planet.

“The Persistence Rover will actually study the processes and reactions that cause water to get trapped in the crust,” she says. This may be the “most important piece of the puzzle”, providing a definitive answer to the puzzle.