Mars may hold water ice under surface

Clue is hydrogen deposits in planet's southern hemisphere

Scientists say they may have solved the mystery of where Mars' water went. Large amounts of hydrogen have been detected under the surface -- perhaps in the form of water ice.

PASADENA, California (CNN) - Instruments aboard the Mars Odyssey spacecraft have detected substantial amounts of hydrogen just under the surface of the planet Mars, according to mission scientists.

They say they believe the hydrogen is most likely in the form of water ice.

Researchers announced preliminary findings from Mars Odyssey in March, when early data from an instrument called the gamma ray spectrometer showed evidence of vast deposits of hydrogen in Mars' southern hemisphere.

Full scientific details are to be published in the May 31 edition of the journal Science. Additionally, NASA plans to hold a news conference on Thursday to announce more specifics about the quantities and distribution of the ice.

EXTRA INFORMATION Our CNN.com interactive presentation on the Mars Odyssey lets you conduct an instrument check  on the spacecraft's features.

Experts say the confirmation of ice on Mars could answer a question that's nagged them for years: Despite the fact that the surface of Mars now appears dry as bone, much physical evidence -- including channels on the landscape -- suggests that water once flowed there.

Researchers have long been puzzled about where the water went. Now, the new findings suggest that it could have gone underground.

Does the possible presence of water on Mars mean life exists there? Scientists say it raises the chances, but it's by no means conclusive.

If life exists there now, it's most likely in the form of some type of microbe. Experts say the best chance for finding conclusive evidence of such life would involve a "sample return mission." A unmanned probe would land on the planet, scoop up a soil sample that might harbor these Martian microbes, and then return it to earth for analysis.

The new Mars Odyssey data on water distribution could help scientists target such a probe to land in a promising area. NASA has discussed launching a sample return mission this decade, but so far none is scheduled.

Mars Odyssey was launched on April 7, 2001, and reached Mars in October. The craft began returning data in February and is scheduled to remain on duty in Mars orbit until August 2004.