Image copyright NASA Image caption Curiosity has been exploring Gale Crater since 2012

Nasa's Curiosity rover has found that water can exist as a liquid near the Martian surface.

Mars should be too cold to support liquid water at the surface, but salts in the soil lower its freezing point - allowing briny films to form.

The results lend credence to a theory that dark streaks seen on features such as crater walls could be formed by flowing water.

The results are published in the journal Nature Geoscience.

Scientists think thin films of water form when salts in the soil, called perchlorates, absorb water vapour from the atmosphere.

The temperature of these liquid films is about -70C - too cold to support any of the microbial life forms that we know about.

Forming in the top 15cm of the Martian soil, the brines would also be exposed to high levels of cosmic radiation - another challenge to life.

But it's still possible that organisms could exist somewhere beneath the surface on Mars, where conditions are more favourable.

Evaporation cycle

The researchers drew together different lines of evidence collected over a Martian year, and from different instruments carried by the Curiosity rover.

The Rover Environmental Monitoring System (REMS) - essentially the vehicle's weather station - measured the relative humidity and temperature at the rover's landing site of Gale Crater.

Scientists were also able to estimate the subsurface water content using data from an instrument called Dynamic Albedo of Neutrons (DAN). These data were consistent with water in the soil being bound to perchlorates.

Finally, the Sample Analysis at Mars (SAM) instrument gave the researchers the content of water vapour in the atmosphere.

Image copyright NASA Image caption Gale Crater once hosted a lake with conditions that could have been favourable to life

Image copyright NASA Image caption Curiosity's Mast and deck where REMS sensors are located

The results show conditions were right for the brines to form during winter nights at the Martian equator, where Curiosity landed. But the liquid evaporates during the Martian day when temperatures rise.

Javier Martin-Torres, a co-investigator on the Curiosity mission and lead scientist on REMS, told BBC News the detection was indirect but convincing: "What we see are the conditions for the formation of brines on the surface. It's similar to when people were discovering the first exoplanets.

"They were not seeing the planets, but they were able to see the gravitational effects on the star.

"These perchlorate salts have a property called deliquescence. They take the water vapour from the atmosphere and absorb it to produce the brines."

He added: "We see a daily water cycle - which is very important. This cycle is maintained by the brine. On Earth we have an exchange between the atmosphere and the ground through rain. But we don't have this on Mars."

Image copyright NASA Image caption Streaks known as recurring slope lineae may be caused by seeping water

Image copyright Martin Torres / Zorzano Image caption Scientists see a daily water cycle maintained by the brines

While one might think that liquid water would form at warmer temperatures, the formation of brines is the result of an interaction between temperature and atmospheric pressure. It happens that the sweet spot for formation of these liquid films is at colder temperatures.

The fact that the scientists see evidence for these brines at the Martian equator - where conditions are least favourable - means that they might be more persistent at higher latitudes, in areas where the humidity is higher and temperatures are lower.

In these regions they might even be present all year round.

Dark streaks on slopes seen by orbiting spacecraft have long been thought to be the product of running water seeping from the Martian soil. But this interpretation has been contested.

"It's speculation at this point... but these observations at least support or go in this direction," said Dr Martin-Torres.

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