Nasa says its rover on Mars has found more evidence that the Red Planet once had water.
Curiosity has discovered gravel once carried by the waters of an ancient stream that "ran vigorously" through the area, says the US space agency .
Evidence of water has been found before but this is the first time gravel from a stream bed 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.
Curiosity, which has been exploring Mars since early August, also investigated a second outcrop known as Link.
The pictures it transmitted back to earth 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. They were transported by water flow," said scientist Rebecca Williams.
Scientists estimate the water was moving at a brisk pace of three feet per second, somewhere between ankle and hip deep.
"This is the first time we're actually seeing water-transported gravel on Mars," said Curiosity science co-investigator William Dietrich, of the University of California, Berkeley .
"This is a transition from speculation about the size of stream bed material to direct observation of it."
Curiosity is on a two-year mission to investigate whether it is possible to live on Mars and to learn whether conditions there might have been able to support life in the past.
