Arizona(BBC) : Long thin grooves - called gullies - on the surface of Mars may have been made not by water but by blocks of frozen carbon dioxide - dry ice.  These blocks seem to have flowed down Martian sand dunes on cushions of carbon dioxide gas. Researchers liken these icy sleds to small hovercraft "ploughing furrows as they go". The findings, published in the journal Icarus, were discovered on images from Nasa's Mars Reconnaissance Orbiter.

The grooves - or linear gullies, were on Martian sand dunes and varied in length from only a few hundred metres to 2.5km.

They were found on dunes which are covered by frost during the Martian winter. Every winter Mars' southern polar region is blanketed by a layer of dry ice, up to one metre deep. In the springtime, this seasonal polar cap sublimes - turns directly from solid into gas.