The Curiosity Mars rover has made its first major discovery: solid geological evidence of an ancient water stream on the surface of the Red Planet.
We have had suggestions of old river beds on Mars since the early Seventies, thanks to images from the Mariner 9 mission, and we also know that Mars has ice at its poles and a small amount of water vapour in its atmosphere. However, Curiosity has now provided conclusive evidence of a fast-flowing and lengthy stream at some point in the past.
The evidence comes from gravels surveyed by the rover. The gravels are cemented together in a conglomerate that now looks like a rocky outcrop. The scientists believe it looks that way due to being hit by a meteorite, long after it dried up.
According to the Planetary Science Institute (PSI) in Arizona, the rocks vary in size from grains of sand to golf-ball-sized pebbles and, crucially, some are rounded.
"The shapes tell you they were transported and the sizes tell you they couldn't be transported by wind," PSI senior scientist Rebecca Williams, a co-investigator on the Curiosity mission, said in a statement. "They were transported by water flow."
Williams added that the discovery would help scientists work out the amount of water that flowed at the site — right now, they reckon it moved at just under a metre a second and was between ankle- and hip-deep — and for how long it flowed.
Ultimately, this will aid their quest to figure out which bits of Mars might have once been habitable.