The surface of Mars may still hold water - albeit the saltiest variety possible - according to scientists, after Nasa's Curiosity rover found evidence of liquid brine on the planet.
Researchers have detected the presence of a chemical substance in the Martian soil that absorbs water vapour from the atmosphere to form a brine that remains a liquid even when temperatures fall below the freezing point of water.
Although liquid water is deemed essential for life, the researchers said the discovery had no immediate implications for the possible existence of microbial life forms on Mars, as cosmic radiation would make it too hazardous for living organisms to survive. But they said future missions to Mars would have to consider taking precautions against any salt water in the Martian soil, as the brine could be "very corrosive" and could eat into any permanent structures built on the surface.
There is now overwhelming evidence that rivers and lakes once existed on Mars many hundreds of millions of years in the past, but that most of this water was lost into space.
Scientists estimate that Mars had about seven times as much water 4.5 billion years ago as it does now, but this disappeared when the planet lost its protective magnetic field.
However, the Curiosity rover, which has been exploring the Gale crater on Mars just south of the equator since 2012, has discovered evidence for the existence of water vapour in the Martian atmosphere which can be absorbed by a perchlorate substance on the planet's surface to form a salty brine which acts as an antifreeze.
"We have discovered calcium perchlorate in the soil and, under the right conditions, it absorbs water vapour from the atmosphere," said Morten Madsen of the Niels Bohr Institute at the University of Copenhagen, who was part of the research team that analysed the Curiosity data.
"Our measurements from the Curiosity rover's weather monitoring station show that these conditions exist at night and just after sunrise in the winter.
"Based on measurements of humidity and temperature at a height of 1.6m and at the surface of the planet, we can estimate the amount of water that is absorbed.
"When night falls, some of the water vapour in the atmosphere condenses as frost, but calcium perchlorate is very absorbent and it forms a brine with the water, so the freezing point is lowered and the frost can turn into a liquid.
"The soil is porous so what we are seeing is the water seeps down through the soil. Over time, other salts may dissolve in the soil and now they are liquid, they can move and precipitate elsewhere under the surface."
The study, published in the journal Nature Geoscience, suggests that in the past there was a large standing lake filling the Gale crater, which would account for the layers of sediments detected by Curiosity.
"The sediment plates on the bottom are level, so everything indicates that the entire Gale crater may have been a large lake," Professor Madsen said.
• There is evidence that rivers and lakes once existed on Mars many hundreds of millions of years ago.
• Scientists estimate that Mars had about seven times as much water 4.5 billion years ago.
• It disappeared when the planet lost its protective magnetic field.
• Curiosity rover has discovered evidence for the existence of water vapour in the Martian atmosphere.
• This can be absorbed by a perchlorate substance on the planet's surface to form a salty brine which acts as an antifreeze.