The first water-seeking instrument measures the radiation spectrum emitted by atoms on the Martian surface after they are hit by gamma rays from space. The two other instruments look for neutrons emitted by excited atoms.

Analysing the spectrum of neutron emission intensities reveals the surface composition. The strategy identifies hydrogen but water ice is the only hydrogen compound expected near the Martian surface.

"We really have a whopping large signal" for gamma-ray emission from hydrogen near the south pole, Boynton said. The neutron instruments show low emission near the south pole, as expected if neutron-absorbing hydrogen is common near the surface.

He estimates that water ice makes up several percent of the top meter of the Martian surface. The instruments did not show hydrogen near the Martian north pole, but Boynton said that it is now covered by a seasonal layer of carbon-dioxide ice that could conceal hydrogen beneath it.


Landing sites


Another on-board instrument, the thermal imaging camera, is also returning data already - the first infrared close-ups of Mars. It reveals temperature differences with "incredible clarity," said Phillip Christensen of Arizona State University, principal investigator for the camera system.

The camera will map potential sites for NASA's 2003 lander in a few weeks' time, when the satellite flies over them.

One failure is the attempt to revive the Martian Radiation Environment Experiment, which stopped operating in August 2001. Troubleshooting has tested the most likely possibilities without spotting the problem, but principal investigator Frank Cucinoatta of the NASA Johnson Space Center is continuing repair efforts.

 
Jeff Hecht