A rolling stone gathers no moss – but on Mars it can nevertheless cloak itself in mystery. This NASA image shows the track of a boulder that rolled across the Nili Fossae region of Mars. For now it is anyone's guess what set the rock in motion.
This false-colour picture (click on it for higher resolution) was posted on 7 June to the Beautiful Mars Tumblr feed, a collection of high-resolution shots from the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter. It shows dark, jagged tracks left in the soil by a lumpy boulder, probably a few metres wide, as it tumbled down a slope. A handful of smaller rocks (seen on the right) also made dotted lines as they rolled.
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