Speculation that there could be some forms of life on the planet Mars was given new impetus on Monday, as scientists from Nasa and elsewhere announced that they can now tie dark streaks seen on the surface of the plant to periodic flows of liquid water.
In contrast to the reputation of Mars as a dustry, lifeless landscape, the scientists reported definite signs of liquid water on the surface of the planet.
"This, I think, gives a focus of where we should more closely," said Alfred McEwen, professor planetary geology at the University of Arizona, who is also the principal investigator of images from a high-resolution on NASA's Mars Reconnaissance Orbiter.
Data from a Nasa satellite shows the features, which appear on slopes, to be associated with salt deposits.
Such salts could alter the freezing and vaporisation points of water in Mars's sparse air, keeping it in a fluid state long enough to move.
Luju Ojha, a PhD student at the Georgia Institute of Technology, and colleagues report the findings in the journal Nature Geoscience.
There are implications for the existence of life on the planet today, because any liquid water raises the possibility that microbes could also be present. And for future astronauts on Mars, the identification of water supplies near the surface would make it easier for them to "live off the land".
"It may decrease the cost - and increase the resilience - of human activity on the planet," co-author Mary-Beth Willhelm, from the Nasa Ames Research Center in California, said in the US space agency media briefing on Monday.
Researchers have long wondered whether liquid water might occasionally flow across the surface today.
It is not a simple proposition, because the temperatures are usually well below zero Celsius and the atmospheric pressure is so low that any liquid H2O will rapidly boil.
But the observation over the last 15 years of gullies and surface streaks that appear to change with the seasons has heightened the speculation.
Mr Ojha has now presented data from the US space agency's Mars Reconnaissance Orbiter that seems to solve the conundrum.
MRO has an instrument called Crism that can determine the chemistry of surface materials.
It has looked at four locations where dark streaks are seen to come and go during Martian summer months. These streaks, called "recurring slope lineae" (RSL), were well known to Mars scientists and were suspected - but not proven - to be associated with trickling water.
"We know from prior investigations that these features form on Mars," Mr Ohja told journalists at the briefing.
"However, the key evidence was missing until now - and that was their chemical identity."
SOURCES: BBC, NYT
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