Mars Express has become the first spacecraft to detect reserves of water ice deep beneath the surface of the Red Planet, experts have announced.
Two radar passes revealed a buried Martian impact basin
The Marsis radar experiment carried onboard appears to have discovered water ice more than 1km below ground.
It is thought the greatest reservoir of retained water on Mars could be found beneath the surface, perhaps providing a habitat for microbial life.
The US-European Marsis team report their findings in the journal Science.
Underground layered deposits at the planet's north pole have an upper unit thought to be dominated by water ice. This water ice is believed to be nearly pure, with only about 2% contamination by dust.
Beneath this ice layer is a lower unit containing sand cemented with water ice.
Mars Advanced Radar for Subsurface and Ionosphere Sounding (Marsis) data from Chryse Planitia, in the planet's northern equatorial region, reveal an almost circular structure about 250km across, lying about 2km underground that the authors propose is a buried impact basin.
Chryse Planitia is thought to have been shaped by massive outflow of floodwaters from the Valles Marineris region and other areas of the northern highlands.
A linear feature in the presumed crater is though to represent the floor of the impact basin. The strength of the reflection possibly suggests there is a body of water ice preserved in the impact basin dating from the time of the outflow.
Deployment of the radar was delayed for just over a year
"If the water got trapped there it could still be there even billions of years later," Jeff Plaut, Marsis principal investigator at Nasa's Jet Propulsion Laboratory, told the BBC News website.
He added: "We have found no convincing evidence yet for subsurface liquid water."
Roberto Seu of the University of Rome "La Sapienza" told the BBC News website: "Due to the complexity of the data processing, we are trying to confirm our hypotheses with further analyses.
"We need to be careful, Marsis is an unprecedented instrument, so there are no other data sets to compare it with."
The science team will begin using the radar experiment to search for liquid water in late December.
During this period of science operations, which will last until March 2006, Marsis will collect data from the southern polar region and mid-latitude regions.
Data collection with Marsis is to a large extent dependent on the orbit of Mars Express staying on the night side of the planet. Nighttime conditions are ideal for radar sounding; the planet's ionosphere hinders the technique and its effects are strongest during the day.
Two 20m-long (65ft) hollow fibreglass "dipole" booms comprise the instrument's primary antenna, while a 7m-long (23ft) "monopole" boom acts as a receive-only antenna.
Marsis sends out pulses of radio waves from the primary antenna to the planet's surface and analyses the time delay and strength of the waves that return.
Analysis of those waves that penetrate the soil and bounce back give information on transitions between materials with different electrical properties, such as rock and water.