Washington (AFP)

Mars was not always the arid 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.

So far, researchers believed she had escaped into space.

But according to a new study, it actually ended up largely locked in the Martian crust.

Warning, this finding does not mean "that there is some kind of huge reservoir of water" under the surface of Mars, warns Eva Scheller, researcher at the California Institute of technology and lead author of the study, published Tuesday in the prestigious journal Science.

"We say that the crust is made up of what we call hydrated minerals, that is, minerals that have water in their structure," she told AFP.

At the beginning of its history, Mars had enough liquid water on its surface to cover it below the equivalent of between 100 to 1,500 meters in height, according to scientists.

For comparison, 1,000 meters high across the entire Red Planet is equivalent to half of the Atlantic Ocean, Eva Scheller calculated.

Today there are only 20 to 40 meters high according to this same measurement.

Water present either in the atmosphere or in the form of ice in the polar caps or the Martian subsoil.

So where did the rest go?

- Up to 99% -

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

This phenomenon also exists on Earth, but is more pronounced on Mars due to the low gravity.

Water molecules are made up of oxygen and hydrogen, and "hydrogen atoms are very light," explains Eva Scheller.

"Because of this they can free themselves from the gravitational field of Mars, and escape into space."

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

However, satellite observations and analyzes carried out by the various rovers sent to Mars showed that it had in fact housed many more.

In addition, the study of the level of deuterium on Mars, which makes up a small part of hydrogen and escapes less into space because it is heavier, also did not stick with the theories of atmospheric escape alone. .

The study published Tuesday thus for the first time built a model adding a complementary theory.

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

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

“The loss of water in the Martian crust is at least equal to or greater than atmospheric exhaust,” she says.

Up to 99% of the water that has disappeared from the Martian surface could thus be trapped in the rocks.

- Arid for 3 billion years -

The study shows that "the loss of water in the crust is a very important mechanism for the planets, which dictates when they become arid", underlines the researcher.

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

Plus, knowing that hydrated minerals on Mars are at least three billion years old, this means that the Red Planet had already lost most of its water by that time, according to the study.

“Mars was more or less as we know it today three billion years ago,” says Ms. Scheller.

She hopes to be able to refine the different scenarios considered in her study thanks to the NASA Perseverance rover, which has just arrived on the red planet.

"The Perseverance rover will study exactly those processes and reactions that cause water to become trapped in the crust," she says.

It could turn out to be "the most important piece of the puzzle", providing a definitive answer to the riddle.

© 2021 AFP