These dunes on a crater floor have a frost of carbon dioxide ice. In the spring, the ice melts directly into vapor. The dark streaks in this image may mark places where the gas has escaped. This image was taken by by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter on Jan. 1, 2010.

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This image shows an area within Proctor Crater that has both dunes and ripples. The smaller, brighter ridges are ripples made of very fine sand. The larger, darker forms are dunes made of dust from dark volcanic rocks. This image was taken by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter in February 2009.

This image looks remarkably like groves of trees growing among Martian dunes. But, the trees are an optical illusion. They are actually dark streaks of sediment on the downwind side of the dunes. They were created by escaping gas from the evaporating carbon dioxide ice below. The bottom of the ice melts into vapor and moves toward holes in the ice, carrying dark sediment along with it that is then deposited when the gas escapes.

The angle of this image of Victoria Crater reveals layering at the top of the crater walls. The pattern on the floor of the crater is made of sand dunes. Tracks from NASA’s Opportunity rover can be seen on the left side of the crater. This image was taken in July 2009 by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter.

This image is of the carbon dioxide ice cap at the south pole of Mars. The pattern is formed by the ice vaporizing. Scientists think that as the ice cap melts from the bottom up, the carbon dioxide turns directly into gas. It flows beneath the ice to openings, eroding the ground below into a spiderlike network of troughs. The flowing gas also carries dust that escapes with it and settles into fan-shaped deposits on top of the ice.