A STREAM once tumbled across the floor of the Martian crater being explored by the Nasa rover Curiosity.
The robot vehicle has found conclusive evidence of “vigorous” water flow some time in the planet’s distant past.
US scientists estimate the stream was between ankle and hip deep and travelled at about three feet per second.
Images from Curiosity’s mast cameras revealed two mounds of rock containing rounded stones ranging in size from a grain of sand to a golf ball. Experts say they could only have been formed by flowing water.
Senior mission scientist Rebecca Williams, from the Planetary Science Institute in Tucson, Arizona, 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.
“This is wonderful ‘concrete’ evidence of water-transported gravels on Mars.
“It is very exciting to have ground truth confirmation of the hypotheses developed from analysing orbital data.
“With this finding, we can now better constrain the amount and duration of water flow activity at this site, a critical step in identifying habitable environments on Mars.”
Curiosity landed in the 96 mile-wide Gale Crater, near the Martian equator, on August 6.
The site of the discovery is between the north rim of the crater and the base of Mount Sharp, which rises to a height of 18,000 feet and is the rover’s ultimate destination.
Scientists think the outcrops resulted from water transporting gravels downslope to the bottom of an alluvial fan.
This is a fan-shaped layer of sediments formed when a fast-flowing stream spreads out across a flat plain.
Other materials accumulating on top of the gravels eventually cemented them together.
“From the size of gravels it carried, we can interpret the water was moving about three feet per second, with a depth somewhere between ankle and hip deep,” said Curiosity science co-investigator Dr William Dietrich from the University of California at Berkeley.