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Pieces of ice and sand at the north pole of Mars reveal an ancient climate that oscillated between hot and cold.
Mars, now dry and dusty, still holds ice at its poles, and the evidence strongly suggests that it was once a planet where water flowed freely on the surface.
The Mars Reconnaissance Orbiter's shallow radar (SHARAD) scrupulously dug out the northern pack ice and spotted layers of ice and sand buried beneath it. The discovery adds a nuance to the history of the planet's climate and reveals why some of its water remains trapped in the ice instead of being lost in space.
Under the ice
The northern polar region of Mars includes a site called Cavi Unit, composed of ice and sand, buried beneath the more visible northern ice cap. SHARAD can scan up to one and a half kilometers in the ice and visualize the different layers. What he saw was alternating layers of sand and ice.
Researchers know that Mars, like the Earth, is wobbling in its orbit for tens of thousands of years. And while it wobbles, tilted toward or from the Sun, the climate also "wobbles", becoming increasingly hot and cold. The researchers therefore believe that the ice in the Cavi unit was deposited during the colder ice ages on Mars. Previously, planetary scientists had assumed that ice would melt during the hottest periods of March. But SHARAD revealed that the ice was instead covered with sand, which isolated it from the sun's rays. This prevented it from melting or being completely lost, just like much of the surface water of Mars when its atmosphere cleared.
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