Scientists may have identified what could be the best place to look for life on the Red Planet. It is the Russell Crater in Mars' southern hemisphere.
By Dr David Whitehouse
BBC News Online science editor
Observations of the region made during the local autumn and spring, when frost covers the dunes and then recedes, indicate liquid water could be present on the surface at certain times of the year.
The Russell Crater dunefield
Detailed analysis suggests this water could be mixing with soil to create frequent mudflows.
"The water we believe is there means that it could be the best place we know of so far where you could dig into the surface to look for life," researcher Dr Dennis Reiss told BBC News Online.
"There are extraordinary features in Russell Crater, features resembling terrestrial mudflows," said Dr Reiss, of the German Aerospace Centre.
He has studied high-resolution images from the Mars Global Surveyor spacecraft in orbit around the planet. He has concluded the mudflows could be the youngest features on Mars, perhaps even being formed from time to time during the present day.
In particular, Dr Reiss measured the temperature of the surface along with its reflectivity, technically called its albedo.
These two quantities told him a lot about what is going on in the dunes.
The albedo indicates if the surface is frozen or thawed and the temperature provides clues about what types of ice and liquid are present.
When spring comes
When winter arrives at Russell Crater, water vapour and carbon dioxide condense out of Mars' thin atmosphere and frost the dunes.
The albedo and temperature readings indicate that during the spring thaw, the frozen carbon dioxide sublimates - turns into a gas - leaving a thawed surface containing liquid water.
Mudflows might occur today
"It could be that in the first couple of centimetres of the surface there is liquid water, possibly even on the surface as well," Dr Reiss said.
"In this place, for a few hours each day, just after noon in the summer, there could be liquid water on the surface of Mars."
Such a possibility is exciting for those wishing to look for life on the planet.
It is generally believed that to find life on Mars, one should try to find water.
However, none of the spacecraft set for a Mars landing this year are due to go anywhere near the Russell Crater.
The European Beagle 2 will land on the arid plain of Issidis, about 10 degrees north of the equator.
It is a site chosen to be warm enough for Beagle to work and low enough for Beagle's parachutes to allow a safe landing.
The two US rovers, also due for launch next month, will touch down near the equator, halfway around the planet from each other.
Dennis Reiss and Ralf Jaumann publish their work on Russell Crater in Geophysical Research Letters.