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Cold planet: The many ice ages of Mars

PARIS -- Mars has experienced 40 major ice ages over the past five million years, when vast areas of its porous soil froze and thawed again, according to computer simulations released on Wednesday.

The changes mainly result from Mars' angle of rotation, which is so extreme that the Red Planet experiences seasons. These seasons last far longer than Earth's because of the greater distance from the Sun.

Sub-surface ice exists in stable form today in both northern and southern hemispheres of Mars poleward of around 60 degrees latitude.

Historically, though, the extent of the ice has shifted again and again, eastwards and westwards and northwards and southwards, thanks to shifts in exposure to sunlight caused by Mars' rotational tilt, says University of Hawaii astrobiologist Nobert Schorghofer.

As parts of the surface are warmed by the Sun, they surrender moisture to the Martian atmosphere. Eventually, as the surface cools, the evaporation condenses once more into the loose soil, known as regolith.

"Today, this gives rise to pore ice at mid-latitudes and a three-layered depth distribution in the high latitudes of, from top to bottom, a dry layer, pore ice, and a massive ice sheet," says Schorghofer's paper, published by the British journal Nature.

In March, an investigation published in the U.S. journal Science found that there is another sub-surface ice at the Martian south pole alone to cover the entire planet to a depth of 11 meters (35.75 feet).

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