THEY鈥橵E found it at last鈥攖he Mars Odyssey satellite has discovered a vast field of water ice just under the surface at the south pole. The planet鈥檚 polar ice caps consist mainly of frozen carbon dioxide, but the discovery that the ice also contains significant amounts of water is encouraging news in the search for life on Mars.
Mars Odyssey was launched in April 2001, reached the Red Planet in October, and started collecting data last month. One of the mission鈥檚 top priorities was to find water. On an earlier mission, the Mars Global Surveyor revealed sharply etched rock features that might have been the handiwork of flowing water, but it could not tell if the water was still there or had vanished early in Martian history.
This time the spacecraft鈥檚 gamma-ray spectrometer looked for hydrogen by analysing emissions from atoms on the Martian surface after they were hit by gamma rays from space. Hydrogen absorbs neutrons, so low emissions of neutrons compared to other radiation is a sign that the element is present. Water is the only compound containing hydrogen you鈥檇 expect to find near the Martian surface.
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鈥淭here鈥檚 a lot of ice on Mars. We really have a whopping signal [from hydrogen],鈥� Bill Boynton of the University of Arizona told a NASA press conference, explaining that Odyssey measured low neutron emissions near the south pole.
He estimates that water ice makes up several per cent of the top metre of the planet鈥檚 surface from the Martian south pole to 60 degrees south. Although the instruments didn鈥檛 register hydrogen near the north pole, Boynton says it might simply be hidden under a seasonal layer of carbon dioxide ice.
Another on-board instrument, Odyssey鈥檚 thermal imaging camera, is also sending back data, in the form of the first close-up infrared images of Mars. They reveal temperature differences with 鈥渋ncredible clarity鈥�, says Phillip Christensen of Arizona State University, principal investigator for the camera system. In a few weeks鈥� time the camera will scout out possible landing sites for NASA鈥檚 2003 lander.