The Mars Odyssey has spent ten days gathering data
By Dr David Whitehouse
BBC News Online science editor
Vast amounts of water-ice may have been discovered beneath the surface of Mars.
After just 10 days of gathering data, the Mars Odyssey spacecraft has found significant quantities of frozen hydrogen located near the southern polar region.
A Gamma Ray Spectrometer is mapping the planet
Researchers say that the hydrogen is most likely due to substantial quantities of ice, although the amount of ice cannot be quantified yet.
William Boynton, of the University of Arizona, US, said: "If this is confirmed, it is fantastic. There is the equivalent of at least several percent water south of 60 degrees latitude."
For many years, scientists have speculated that near-surface water may exist on Mars.
There is evidence that water had flowed on its surface in vast quantities in the distant past and there is some evidence that it has done so far more recently.
Now, the Gamma Ray Spectrometer instruments on board Mars Odyssey have made the first direct measurements that confirm there are significant amounts of hydrogen and probably water - just beneath the surface.
These instruments have detected just the type of signal expected from a large amount of ice.
"We are delighted with the quality of data we're seeing," said Dr Steve Saunders, Odyssey project scientist.
Ice shows up blue on the Gamma Ray Spectrometer
"We will use it to build on what we have learned from Mars Global Surveyor and other missions. Now, we may actually see water rather than guessing where it is or was.
"With the thermal images, we are able to examine surface geology from a new perspective."
The researchers have determined the amount of hydrogen in the soil by two different techniques.
One relies on the ability of hydrogen to slow down, or moderate, neutrons and the other relies on the fact that hydrogen can absorb a neutron and then emit a gamma ray of characteristic energy.
Scientists have seen both of these effects in the initial data from the Mars Odyssey spacecraft.
The first maps produced by this data indicate that there are substantial amounts of hydrogen, and by inference water ice, on Mars.
A map of the southern hemisphere of Mars from the equator at the outer edge to the South Pole shows water ice marked as blue. It indicates that all areas south of 60 degrees south latitude could contain large amounts of ice.
Over the next few months, scientists will make a more detailed assessment of the data.
Mars Odyssey will also continue to collect new data over the next couple of years, which should allow researchers to make more detailed maps.
These will show where and how much ice is present on Mars, as well as maps of other elements and minerals.
Dr Jim Garvin, Lead Scientist of the Mars Exploration Program at Nasa, said: "These preliminary Odyssey observations are the 'tip of the iceberg' of the science results that are soon to come, so stay tuned."