But what appears to be rolling hills covered with vegetation may be sand dunes topped with melting frost, scientists said Tuesday. At first, the scientists thought the tree shapes and other, circular formations might have been caused by small gas pockets exploding under sand dunes. But their latest theory is that the dark shapes are patches of exposed sand standing out among the frost-covered dunes. As spring warms southern Mars, the frost turns directly to gas.
Wind then blows the frost and sand into patterns that resemble trees when viewed from space.
Unlike the monstrous sand dunes observed by the missions to Mars in the 1970s, the new images show smaller dunes, much like those found in the Mojave Desert. Most of the Martian frost consists of frozen carbon dioxide. But when the temperature rises, the carbon dioxide flashes into a gas too quickly to be observed, Malin said. The frost appears at night, when Surveyor can't watch, so whether it forms on the ground or falls as snow from the atmosphere is still a mystery. Two other NASA spacecraft will arrive at Mars later this year.