Water ice was expected in the polar caps, since they represent the largest known reservoirs of water on Mars. Estimates suggest that if they melted, they would cover the planet in a layer of water up to 33 metres deep.
But as it scanned the region around the south pole, MARSIS also turned up an unexpected ice source. It lies near the southern polar cap and is underneath a region of Martian surface that shows no visible signs of ice. The radar signals reveal what appear to be relatively thin layers of underground water ice layers that may contain water of a equivalent to half of that locked up in the entire southern polar cap.
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