Nasa scientists have discovered new evidence that briny water flows on Mars during its warmest months, raising chances that life could exist on the Red Planet, the space agency said on Thursday.

Nasa first found signs of water on Mars more than a decade ago, but earlier indications were that any existing water would be frozen and concentrated at the poles.

Recently analysed images from Nasa's Mars Reconnaissance Orbiter satellite show dark, finger-like features that extend down some slopes and crater walls on the planet during its late spring through summer, fading in the Martian winter.

"This is the best evidence we have to date of a liquid water occurring today on Mars," said Philip Christensen, a geophysicist at Arizona State University, Tempe, in a Nasa panel announcing the findings in Washington.

Nasa scientists believe that if there is liquid water on Mars, it would be highly salty and lie beneath the surface.

That would explain why it would not freeze in the planet's frigid surface temperatures, which can fall to around 200 degrees below zero Fahrenheit (minus 128 degrees Celsius), or evaporate in its low air pressure.

"It is more like a syrup, maybe, in how it flows," said Alfred McEwen of the University of Arizona, Tucson, principal investigator in charge of a special camera on the Mars orbiter called a High Resolution Imaging Science Experiment.   

He also is the lead author of a report on evidence of water flows published on Thursday in the journal Science.   

The scientists on Nasa's panel stressed that liquid water is more likely to sustain life than ice, underscoring the importance of the latest discovery.   

Past Nasa discoveries revealed evidence of ancient shorelines and riverbanks on Mars. And analysis of gullies on the Red Planet five years ago turned up fresh mineral deposits that suggested recent water flows, but provided no categorical proof of that, scientists said.

The latest discovery is more difficult to explain away as evidence of anything but contemporary water flows, said Michael Meyer, Mars exploration program lead scientist at Nasa. - Reuters