An artist's concept of Mars Odyssey at the Red Planet. Photo: NASA/JPL

When the 2001 Mars Odyssey spacecraft enters orbit around Mars later this year, it will kick off a restructured Mars science program that focuses on the search for water on the Red Planet, past and present.

"The scientific trajectory of the restructured Mars Exploration Program begins a new era of reconnaissance with the Mars Odyssey orbiter," said Jim Garvin, lead scientist for the program, prior to Saturday's successful launch of the spacecraft. "Odyssey will help identify and eventually target those places on Mars where future rovers and landers must visit to unravel the mysteries of the Red Planet."

Mars Odyssey will accomplish this primarily with two instruments, the Gamma Ray Spectrometer (GRS) and the Thermal Emission Imaging System (THEMIS). Combined, the two instruments will give planetary scientists the best maps to date of the composition of the planet's surface, including potential sources of water.

The GRS, mounted on the end of a 6-meter boom that will be deployed after Mars Odyssey enters orbit, will measure gamma rays created by the collision of cosmic radiation with the surface of Mars. Different materials gave different gamma ray "signatures" which the GRS can detect, allowing scientists to measure the elemental composition of the surface.

Included with the GRS package is a pair of neutron detectors mounted on the body of the spacecraft. These detectors measure neutrons of different speeds emitted from the surface, also through collisions with cosmic radiation. Those neutron measurements will allow scientists to measure the amount of hydrogen -- and hydrogen-bearing compounds, notably water -- in the upper meter of the Martian soil. A similar instrument was used on the Lunar Prospector spacecraft to discover water ice deposits within craters near the poles of the Moon.

Such measurements could be key to understanding how much water might exist near the surface of Mars today, either in the form of ice deposits just below the surface or as hydrated minerals. The search for such evidence -- "following the water," as NASA officials have described it -- plays a central role in the science program of the revamped Mars exploration program.

THEMIS, the other major instrument on Mars Odyssey, will also contribute to the search for water. The instrument will map the surface of the planet at 10 infrared wavelengths, which combined will allow scientists to measure the composition of the surface to abundances of 5-10 percent with a resolution of 100 meters.

The data from THEMIS will give scientists their first global maps of the mineral composition of the surface, and allow them to identify minerals that could have been created or altered by water. In addition, THEMIS observations at night could identify regions of geothermal activity on the planet's surface, including areas that could harbor hot springs of water.

In addition to its infrared imaging, THEMIS will also take images at visible light wavelengths of regions of the planet, with an emphasis on features that appear to have been formed by flowing water. Those images will be combined with the mineral composition data to help identify regions of scientific interest for future lander missions.

"For the first time at Mars we will have a spacecraft that is equipped to find evidence for present near-surface water and to map mineral deposits for past water activity," said Mars Odyssey project scientist Steve Saunders. "This mission will give us a basic understanding about the chemistry and mineralogy of the surface."

While GRS and THEMIS will provide the major Martian scientific data from the mission, data from a third instrument may help in the ultimate stage of Martian exploration. The Mars Radiation Environment Experiment (MARIE) will measure the radiation environment around the spacecraft both during the cruise to Mars and after entering orbit, studying the contributions from both the Sun and galactic cosmic rays. The data from MARIE will help scientists and engineers understand how much radiation a human would be exposed to on a manned Mars mission, and how much shielding would be needed to protect them.