Does discovery mean alien life could be on Red Planet?
Dr Simon Foster has examined what flowing water on Mars could mean for potential alien life on the planet, as Nasa announces its existence.
The physicist, from Imperial College, said: "The existence of liquid water on the surface of Mars gives us a tantalising glimpse into the prospect of life on the Red Planet.
"It's possible microbial life could still be there underneath the surface.
"If we were ever to go to Mars, it would be almost impossible to take all the resources we need to the planet, so we would have to use what's there, and water would be vital for this. As living organisms, we need liquid water to survive."
Nasa will not be rushing out to search the newly discovered salt water residue for life just yet.
John Grunsfeld, Nasa's associate administrator for science, said: "If I were a microbe on Mars, I would probably not live near one of these sites. I would want to live further north or south, quite far under the surface and where there's more of a freshwater glacier. We only suspect those places exist and we have some scientific evidence that they do."
The discovery of the water flows was made when scientists developed a new technique to analyse chemical maps of the surface of Mars obtained by Nasa's Mars Reconnaissance Orbiter spacecraft.
They found tell-tale fingerprints of salts that form only in the presence of water in narrow channels cut into cliff walls.
The slopes appear during the warm summer months on Mars, then vanish when the temperatures drop. The chemical fingerprints of hydrated minerals did likewise.
Scientists suspected the streaks were cut by flowing water, but previously had been unable to make the measurements.
"I thought there was no hope," said Lujendra Ojha, a graduate student at Georgia Institute of Technology and lead author of the scientific paper.
Mars Reconnaissance Orbiter makes its measurements during the hottest part of the Martian day, so scientists believed any traces of water, or fingerprints from hydrated minerals, would have evaporated.
Also, the chemical-sensing instrument on the orbiting spacecraft cannot home in on details as small as the narrow streaks.
But Ojha and his colleagues created a computer programme that could scrutinise individual pixels. That data was then correlated with high-resolution images of the streaks. Scientists concentrated on the widest streaks and came up with a 100pc match between their locations and detections of hydrated salts.
The discovery "confirms that water is playing a role in these features".
Billions of years ago, Mars lost much of its atmosphere. Several initiatives are under way to determine how much of the planet's water was stripped away and how much remains locked in ice in underground reservoirs.