Mysterious features on Mars that ebb and flow with the seasons are almost certainly caused by water flowing on the planet today, scientists believe.
The dark streaks appear on sloping ground during late spring and disappear by autumn.
New evidence strongly suggests that they are formed by salty water flowing down hillsides, crater rims and canyon walls. The streaks are narrow, typically less than five metres (16.4ft) wide, but can be hundreds of metres long.
Scientists at the American space agency Nasa say the discovery raises the tantalising possibility of primitive life forms similar to "extremophile" microbes that survive hostile conditions on Earth living on present-day Mars.
Speaking at a televised press conference, John Grunsfeld, astronaut and associate administrator of the agency's science mission directorate in Washington DC said: "The really exciting thing about this is our view of Mars has been really about seeking chemical fossils of past life on Mars.
"The existence of liquid water, even if it's super-salty briny water, gives the possibility that if there's life on Mars we have a way to describe how it might survive.
"We are now at a point technologically ... that we have the capability to go there and ask this question - is there life on Mars? - and answer it."
Experts have speculated for some time that water might be involved in the formation of the streak features, known as recurring slope lineae (RSL), but evidence has been lacking until now.
The discovery was made using high resolution satellite imagery to probe RSLs at four locations on Mars. In each case the "streaks" were found to contain salt minerals very likely to have precipitated from flowing briny water. Similar salts were absent from the surrounding terrain.
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."
Mars is a cold barren desert today but is thought to have been warmer and wetter in the distant past, with a thicker atmosphere, rivers and oceans. Around three billion years ago two thirds of the planet's northern hemisphere was covered by a huge ocean up to a mile deep, scientists believe.
Then somehow Mars lost its surface water. Much of it is thought to have evaporated into space, but a portion remains locked in the polar icecaps and possibly in pockets underground.
Nasa planetary science director Dr Jim Green said Mars now had to be seen in a new light.
"Mars is not the dry, arid planet that we thought of in the past," he said. "Under certain circumstances liquid water has been found on Mars."
The research is based on an analysis of high resolution spectral data from Nasa's Mars Reconnaissance Orbiter spacecraft.
Spectroscopy involves looking for chemical "fingerprints" by measuring what wavelengths of light an unknown material absorbs and reflects.
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.
Dr Joe Michalski, a Mars expert at London's Natural History Museum, 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 hyper-arid, 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."
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.
Finding surface water on Mars would not only have implications for life on the planet, but could also have a big impact on the planning of future manned missions.
Coincidentally, 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.