A space craft has detected the best evidence yet of water on Mars.
By Helen Briggs
BBC News Online science reporter
Barely a year ago, Mars Odyssey found signs that the planet has reservoirs of underground ice near its south pole.
Did Mars once have ice-covered oceans?
Scientists at the US space agency (Nasa) estimated there was enough ice to fill Lake Michigan twice.
They said it might be merely the tip of the iceberg and it seems they were right.
New observations by Mars Odyssey and Mars Global Surveyor (another Nasa probe that is mapping the Red Planet) suggest the north pole has about one third more underground ice than the south.
Beneath a shallow crust of dry soil, there appears to be a layer of permanently frozen ground that is up to 75% ice.
"If the conditions were warmer in the past, as they probably were, it may have led to the ice melting to form water which would be much more conducive to the presence of life," says William Boynton, one of a team of Russian and American astronomers behind the discovery.
The finding is exciting not just because it increases the possibility that microbial life could have evolved on a planet other than Earth.
It may also make human exploration of Mars more feasible because astronauts sent to the planet would need a source of water during their stay.
"From the point of view of human missions to Mars, it could be an outstanding discovery," says Bo Maxwell of the UK branch of the Mars Society.
"The big question is exactly how deep these deposits are and how pure they are."
There is now almost overwhelming evidence that Mars has reserves of frozen water hidden just below the surface.
The best evidence so far rests on the distinctive chemical signature of hydrogen in water, detected by an instrument on Mars Odyssey.
Landing a robotic space probe near one of the Martian poles to dig for ice could settle the argument, once and for all.
Ironically, Nasa's Mars Polar Lander, which was lost on descent in 1999, was equipped to do just that.
Geological features suggest water once flowed on Mars
Scientists are now lobbying the agency to send an updated version of the craft back to Mars.
Unlike the ill-fated probe, it would carry biology experiments designed to search for microbial fossils.
"I think we need to revisit what Mars Polar Lander was going to do," says Boynton, of the Lunar and Planetary Laboratory, University of Arizona, Tucson. "To land on the ice and look for organic material that might be indicative of past life."
The latest research, reported in the journal Science, comes as Nasa prepares to launch the second of its Mars Exploration Rovers, named Opportunity.
The robotic explorer is following in the footsteps of its twin, Spirit, and Europe's Mars Express and Beagle 2 mission.
Spirit, Opportunity and Beagle will touch down on Mars after a six-month voyage. They are all heading for the mid-latitudes of Mars rather than the polar regions.
Nevertheless, the news is bound to be greeted with enthusiasm by the project teams, particularly Beagle, which carries experiments to search for life.