Preliminary results from NASA's Mars Odyssey spacecraft indicate that large amounts of ice exist in the red planet's Southern Hemisphere, scientists say. The results represent the first direct measurements to confirm that significant amounts of hydrogen lie beneath the surface of Mars.

An earlier mission by the Mars Global Surveyor revealed surface features thought to have formed as a result of erosion by flowing water but it remained unclear whether the water was still present or had long since vanished. "Now," Steve Saunders of NASA's Jet Propulsion Laboratory says, "we may actually see water rather than guessing where it is or was."

Three gamma-ray spectrographic instruments on board Odyssey searched for signs of hydrogen, which the scientists take as an indication that ice is present. The first technique measured gamma rays (characteristic of particular elements) emitted by components of the Martian soil. The other two instruments detected neutrons, particularly changes to their velocities, caused by hydrogen. The preliminary results suggest that hydrogen is present in the upper few feet of the Martian surface in areas surrounding the planet's South Pole (blue region in image at right). Though the amount of ice cannot be precisely quantified yet, according to principal investigator William Boynton of the University of Arizona, "there is the equivalent of at least several percent water south of 60 degrees latitude."