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These 'kidney beans' are actually sand dunes frozen in place by carbon dioxide frost

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NASA 's Mars Reconnaissance Orbiter imaged dunes frozen in place by carbon dioxide frost.

Sand dunes on Mars behave like those on Earth , migrate as winds blow sand from one side to the other.

Frost settles on them during the Martian winter in the northern hemisphere prevents them from moving until they thaw.

The amount of frost depends on the tilt of Mars on its axis.