Two gullies that were originally photographed in 1999 and 2001, and imaged again in 2004 and 2005, showed changes consistent with water flowing down the crater walls, according to the study.

In both cases, scientists found bright, light-coloured deposits in the gullies that were not present in the original photos. They concluded that the deposits – possibly mud, salt or frost – were left there when water recently cascaded through the channels.

Other scientists think it possible that gullies like this were caused not by water but by liquid carbon dioxide.