Nasa's Mars rover has discovered gravel once carried by the waters of an ancient stream that "ran vigorously" through the area, the US space agency said on Thursday.
Scientists had previously found evidence of water on the Red Planet, but this is the first time stream bed gravel has been discovered. The rocky Hottah outcrop looks "like someone jack-hammered up a slab of city sidewalk, but it's really a tilted block of an ancient stream bed," project scientist John Grotzinger said in a statement.
The pictures transmitted by Curiosity show the pebbles have been cemented into layers of conglomerate rock at a site between the north rim of the Gale Crater and the base of Mount Sharp, where Curiosity is heading. The sizes and the shapes of the rocks give an idea of the speed and the depth of the stream, Nasa said.
"The shapes tell you they were transported, and the sizes tell you they couldn't be transported by wind," said Curiosity scientist Rebecca Williams.
"This is the first time we're actually seeing water-transported gravel on Mars," said William Dietrich of the University of California, Berkeley. And thanks to imagery previously captured from Mars's orbit, the scientists said they can see an alluvial fan of material washed down from the rim - with many apparent channels uphill of Link and Hottah.