Dry ice 'smoke' carves up dramatic sand dunes on Mars, Reconnaissance Orbiter shows

The spring changes include outbursts of gas carrying sand, polygonal cracking of the winter ice blanketing the dunes, sand-falls down the faces of the dunes, and dark fans of sand propelled out onto the ice.

"It is a challenge to catch when and how those changes happen, they are so fast," said Ganna Portyankina of the University of Bern in Switzerland.

The process of outrushing gas that carves grooves into the northern dunes resembles the process creating spider-shaped features in far southern Mars, but the spiders have not been seen in the north.

The seasonal dry-ice sheets overlie different types of terrain in the two hemispheres. In the south, diverse terrains include the flat, erodible ground where the spiders form, but in the north, a broad band of sand dunes encircles the permanent north polar ice cap.

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