The Gamma Ray Spectrometer on Mars Odyssey is really three instruments in one, all designed to analyze the chemical composition of the Martian surface. The spectrometer also has the tantalizing capability to detect water, if it exists, at shallow depths beneath Mars' surface.
The multiple-instrument experiment is headed by Professor William Boynton of the University of Arizona's Lunar and Planetary Laboratory, and represents the work of a three-way partnership between the university, the Los Alamos National Laboratory, and Russia's Space Research Institute. The instrument system contains the neutron spectrometer built by the Los Alamos National Laboratory and led by the lab's Dr. William C. Feldman, and the high-energy neutron detector built by Russia's Space Research Institute and headed by principal investigator Dr. Igor Mitrofanov. Dr. Scott Anderson of JPL is the gamma ray spectrometer's investigation scientist.
With the gamma ray spectrometer, says Boynton, "we're mainly there to find out what Mars is made of. Especially hydrogen." Finding hydrogen in various forms could speed the search for the most sought-after hydrogen link of all - H2O - water.
This is Boynton's third try at Mars; the Arizona geochemist previously led teams that designed a similar instrument for the Mars Observer spacecraft, which was lost in 1993, and built an instrument for the Mars Polar Lander mission, lost in 1999, that would have sampled martian soil. He also led the team that built a gamma ray spectrometer for the successful Near Earth Asteroid Flyby mission (NEAR), mission, which orbited an asteroid and completed its mission with a daring landing on the space rock in 2001.
"I'm interested in Mars because it's probably the most fascinating planet in the solar system besides Earth, and probably the only one that could have ever supported other forms of life," says Boynton His other research interests also include geochemical studies relating to the impact event that led to the extinction of the dinosaurs, and studies of meteorites and comets.
His experiment will supply data similar to that of NASA's successful Lunar Prospector mission, which found how much hydrogen, and thus water, is likely to exist on the Moon.
Information provided by the gamma ray spectrometer will be used to determine the amounts of various elements in different regions on Mars. This will include data to produce a global map of water deposits and their depths near the surface, and measurements of the seasonal changes of the polar ice caps. Already, the gamma ray spectrometer has gathered data on bursts of gamma rays from cosmic sources.
Incoming cosmic rays from galaxies and stars are some of the highest-energy particles in the universe. These collide with atoms in the soil of Mars. When atoms are hit with such energy, neutrons are released, which scatter and collide with other atoms. The atoms get "excited" in the process, and emit gamma rays to release the extra energy so they can return to their normal rest state.
By measuring gamma rays coming from the martian surface, it is possible to calculate how abundant various elements are and how they are distributed around the planet's surface. Gamma rays emitted from the nuclei of atoms show up as sharp 'emission lines' on the instrument's spectrum. The energy represented in these emissions show which elements are present, and the intensity of the spectrum produced reveals their concentrations. Some elements like potassium, uranium, and thorium are naturally radioactive and give off gamma rays as they decay, but all elements can be excited by collisions with cosmic rays to produce gamma rays. The spectrometer is expected to add significantly to the growing understanding of the origin and evolution of Mars and the processes shaping it today and in the past.
The high-energy neutron detector and neutron spectrometers on the gamma ray spectrometer directly detect scattered neutrons, and the gamma sensor detects the gamma rays.
By measuring neutrons, it is possible to calculate the abundance of hydrogen on Mars, thus inferring the presence of water. The neutron detectors are sensitive to concentrations of hydrogen in the upper meter (three feet) of the surface. Like a virtual shovel "digging into" the surface, the spectrometer will allow scientists to peer into this shallow subsurface of Mars and measure the amount of hydrogen that exists there. Since hydrogen is most likely present in the form of water ice, the spectrometer will be able to measure directly the amount of permanent ground ice and how it changes with the seasons.
Results from the Mars Pathfinder lander and rover mission of 1997 suggest that rocks on Mars may be rich in silica. Geologists believe the martian highlands may be the source of the silica-rich rocks, and the gamma ray spectrometer will be capable of answering this question. The spectrometer is sensitive to chlorine concentrations, and it will determine whether basins thought to have been lakebeds or ancient ocean bottoms collected significant salt deposits.