For the first time, large quantities of frozen water have been discovered in the martian soil. Using the gamma ray spectrometer on NASA's 2001 Mars Odyssey spacecraft, researchers confirmed the existence of ice in an expanse of martian soil the size of the United States, centered on the planet's south pole. "We saw a whopping large signal," says William Boynton of the University of Arizona in Tucson, the instrument's principal investigator.
Mars Odyssey's observations started on 19 February. The initial results, based on analysis of just the first few days of data, were presented 1 March at a press conference in Pasadena, California. They include high-resolution visible and infrared images, from which scientists should be able to deduce the mineral composition of the martian surface. But while it will take some time before the camera data are fully analyzed, the importance of the gamma ray spectrometer results was immediately obvious.
When high-energy cosmic rays hit the martian surface, they send neutrons shooting out of the nuclei of atoms there. The neutrons then slow down as they collide with other atoms in the soil. Hydrogen atoms are extremely efficient in slowing down neutrons because they have about the same mass. Eventually, the neutrons are recaptured by various nuclei, in reactions that produce gamma rays with characteristic energies. The detectors on the gamma ray spectrometer measured both a strong depletion of fast neutrons in the region around the south pole and an abundance of gamma rays at an energy characteristic of hydrogen. The amount of hydrogen detected, combined with the temperature and pressure on Mars, makes water by far the most likely candidate, Boynton says.
"The result confirms what many people already were expecting," says Agustin Chicarro of the European Space Agency (ESA), who is the project scientist for ESA's Mars Express mission, due to arrive at Mars by the end of 2003. Previously, geologic formations such as river beds and gullies suggested that the red planet was once a warmer, wetter place. But the new findings are the most direct evidence of water to date. "It shows that Mars had lots of water in its early history, and it opens up possibilities for long-term human presence on Mars," Chicarro says.