Flowing liquid water is almost certainly responsible for mysterious features on Mars that change with the seasons, scientists believe.
Satellite images have identified narrow streaks, typically less than five metres (16.4 ft) wide, that appear on slopes during warm seasons, lengthen, and then fade when conditions become cooler.
Experts have speculated that water might be involved in the formation of the gully-like features, known as recurring slope lineae (RSL), but only now has evidence supporting the theory come to light.
A new high resolution technique has revealed that RSLs at four locations on Mars contain salt minerals that precipitate from briny water.
The salts, which are absent from the surrounding terrain, are thought to have been left by water flowing down the sides of hills or crater rims.
Writing in the journal Nature Geoscience, the scientists led by Phd student Lujendra Ojha, from the Georgia Institute of Technology in Atlanta, US, concluded: "Our findings strongly support the hypothesis that recurring slope lineae form as a result of contemporary water activity on Mars."
If confirmed the discovery has major implications for the chances of finding life on Mars, and future human exploration of the Red Planet.
'Reconnaissance Orbiter spacecraft'
Mars is a cold barren desert today but is thought to have been warmer and wetter billions of years ago, with a thicker atmosphere, rivers and oceans.
Much of the planet's water is believed to have evaporated into space, but some remains locked in the polar icecaps and possibly in pockets underground.
The new research is based on an analysis of spectral data from the American space agency Nasa's Mars Reconnaissance Orbiter spacecraft.
Breaking down reflected light into its different wavelengths provides a chemical "fingerprint" of what a substance is made of. The Mars scientists devised a new method that allowed chemical signatures to be extracted from individual image pixels, providing a much higher level of resolution than had been achieved before.
At four locations, Palikir Crater, Horowitz Crater, Hale Crater, and Coprates Chasma - a huge Martian canyon - they found evidence of RSL salt deposits. The most common salts were magnesium perchlorate, magnesium chlorate and sodium perchlorate, all of which are consistent with flowing briny water.
The scientists wrote: "Water is essential to life as we know it. The presence of liquid water on Mars today has astrobiological, geologic and hydrologic implications and may affect future human exploration."
'Wherever we find water, we find life'
Just where the water has come from still remains an unsolved mystery.
Theories include the melting of near-surface ice, absorption from the thin Martian atmosphere, and seasonal discharges from local aquifers, layers of water-bearing rock.
"It is conceivable that RSL are forming in different parts of Mars through different formation mechanisms," said the scientists.
Dr Joe Michalski, a Mars expert at London's Natural History Museum London, said: "These results provide strong evidence that salty water occasionally flows on the Martian surface, even today. We know from the study of extremophiles on Earth that life can not only survive, but thrive in conditions that are hyperarid, very saline or otherwise 'extreme' in comparison to what is habitable to a human. In fact, on Earth, wherever we find water, we find life.
"This finding is yet another example of water on Mars, but a hugely important one because it points to environments that could potentially be habitable to certain kinds of bacteria, even today."
News of the discovery comes four days after the new Ridley Scott movie The Martian had its UK premiere in London. The film stars Matt Damon as astronaut Mark Watney who finds himself marooned on the Red Planet.