One of the chief scientific goals that 2001 Mars Odyssey will focus on is mapping the chemicals and minerals that make up the Martian surface. As on Earth, the geology and elements that form the Martian planet chronicle its history. And while neither elements, the building blocks of minerals, nor minerals, the building blocks of rocks, can convey the entire story of a planet's evolution, both contribute significant pieces to the puzzle. These factors have profound implications for understanding the evolution of Mars' climate and the role of water on the planet, the potential origin and evidence of life, and the possibilities that may exist for future human exploration.

Other major goals of the Odyssey mission are to:

• Determine the abundance of hydrogen, most likely in the form of water ice, in the shallow subsurface
• Globally map the elements that make up the surface
• Acquire high-resolution thermal infrared images of surface minerals
• Provide information about the structure of the Martian surface
• Record the radiation environment in low Mars orbit as it relates to radiation-related risk to human exploration

During the 917-day science mission, Odyssey will also serve as a communication relay for U.S. or international scientific orbiters and landers in 2003 and 2004. After this period, the orbiter will be available as a communication relay for an additional 457 days, making for a total mission duration of 1,374 days, or two Martian years. Science operations may still continue during the communication relay-only phase depending on remaining orbiter resources.

The orbiter carries three science instruments: a thermal infrared imaging system, a gamma ray spectrometer and a radiation environment experiment.These are all calibrated during the spacecraft's cruise phrase on its way to Mars. Opportunities for data collection are assigned on a time-phased basis depending on when conditions are most favorable for specific instruments.

Thermal Emission Imaging System

This instrument is responsible for determining Mars' surface mineralogy. Unlike our eyes, which can only detect visible light waves, a small portion of the electromagnetic spectrum, the instrument can see in both visible and infrared, thus collecting imaging data that has been previously invisible the scientists.

In the infrared spectrum, the instrument uses 10 spectral bands to help detect minerals within the Martian terrain. These spectral bands, similar to ranges of colors, serve as signatures, or spectral fingerprints, of particular types of geological materials.

Minerals, such as carbonates, silicates, hydroxides, sulfates, hydrothermal silica, oxides and phosphates, all show up as different colors in the infrared spectrum. This multispectral method allows researchers to detect in particular the presence of minerals that form in water and understand those minerals in their proper geological context. Remote-sensing studies of natural surfaces, together with laboratory measurements, have demonstrated that 10 spectral bands are sufficient to detect minerals at abundances of five to 10 percent. In addition, the use of 10 infrared spectral bands can determine the absolute mineral abundance in a specific location within 15 percent.

The instrument's multispectral approach will also provide data on localized deposits associated with hydrothermal and subsurface water and enable 100-meter (328-feet) resolution mapping of the entire planet. In essence, this allows a broad geological survey of the planet for the purpose of identifying minerals, with 100 meters (328 feet) of Martian terrain captured in each pixel, or single point, of every image. It will also allow the instrument to search for thermal spots during the night that could result in discovering hot springs on Mars.

Using visible imaging in five spectral bands, the experiment will also take 18-meter (59-feet) resolution mineralogical and structural measurements specifically to determine the geological record of past liquid environments. More than 15,000 images each 20 by 20 kilometers (12 by 12 miles) will be acquired for Martian surface studies. These more detailed data will be used in conjunction with mineral maps to identify potential future Martian landing sites. These image will provide an important bridge between the data acquired by the Viking missions and the high-resolution images captured by Mars Global Surveyor.

The instrument weighs 11.2 kilograms (24.7 pounds); is 54.5 centimeters (21.5 inches) long, 34.9 centimeters (13.7 inches) tall and 28.6 centimeters (11.3 inches) wide; and runs on 17 watts of electrical power.

The principal investigator for the instrument is Dr. Philip Christensen of Arizona State University in Tempe.

Gamma Ray Spectrometer

This instrument plays a lead role in determining the elemental makeup of the Martian surface. Using a gamma ray spectrometer and two neutron detectors, the experiment detects and studies gamma rays and neutrons emitted from the planet's surface. When exposed to cosmic rays, all chemical elements emit gamma rays with distinct signatures. This spectrometer looks at these signatures, or energies, coming from the elements present in the Martian soil. 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.

By measuring neutrons, it is possible to calculate Mars' hydrogen abundance, thus inferring the presence of water. The neutron detectors are sensitive to concentrations of hydrogen in the upper meter of the surface.

Gamma rays, emitted from the nuclei of atoms, show up as sharp emission lines on the instrument's spectrum. While the energy represented in these emissions determines which elements are present, the intensity of the spectrum reveals the elements' concentrations. The spectrometer will send a reading to Earth every 20 seconds. This data will be collected over time and used to build up a full-planet map of elemental abundances and their distributions.

The spectrometer's data, collected at 300-kilometer (186-mile) resolution, will enable researchers to address many questions and problems regarding Martian geoscience and life science, including crust and mantle composition, weathering processes and volcanism. The spectrometer is expected to add significantly to the growing understanding of the origin and evolution of Mars and of the processes shaping it today and in the past.

The gamma ray spectrometer consists of two main components: the sensor head and the central electronics assembly. The sensor head is separated from the rest of the Odyssey spacecraft by a 6-meter (20-feet) boom, which will be extended after Odyssey has entered the mapping orbit at Mars. This is done to minimize interference from any gamma rays coming from the spacecraft itself. The initial spectrometer activity, lasting between 15 and 40 days, will perform an instrument calibration before the boom is deployed. After 100 days in orbit, the boom will deploy and remain in this position for the duration of the mission. The two neutron detectors -- the neutron spectrometer and the high-energy neutron detector -- are mounted on the main spacecraft structure and will operate continuously throughout the mission.

The instrument weighs 30.2 kilograms (66.6 pounds) and uses 32 watts of power. Along with its cooler, the gamma ray spectrometer measures 46.8 centimeters (18.4 inches) long, 53.4 centimeters (21.0 inches) tall and 60.4 centimeters (23.8 inches) wide. The neutron spectrometer is 17.3 centimeters (6.8 inches) long, 14.4 centimeters (5.7 inches) tall and 31.4 centimeters (12.4 inches) wide. The high-energy neutron detector measures 30.3 centimeters (11.9 inches) long, 24.8 centimeters (9.8 inches) tall and 24.2 centimeters (9.5 inches) wide. The instrument's central electronics box is 28.1 centimeters (11.1 inches) long, 24.3 centimeters (9.6 inches) tall and 23.4 centimeters (9.2 inches) wide.

The principal investigator for the gamma ray spectrometer is Dr. William Boynton of the University of Arizona.

Martian Radiation Environment Experiment

This instrument characterizes aspects of the radiation environment both on the way to Mars and in the Martian orbit. Since space radiation presents an extreme hazard to crews of interplanetary missions, the experiment will attempt to predict anticipated radiation doses that would be experienced by future astronauts and help determine possi-ble effects of Martian radiation on human beings.

Space radiation comes from two sources -- energetic particles from the Sun and galactic cosmic rays from beyond our solar system. Both kinds of radiation can trigger can-cer and cause damage to the central nervous system. A spectrometer inside the instrument will measure the energy from these radiation sources. As the spacecraft orbits the red planet, the spectrometer sweeps through the sky and measures the radiation field.

The instrument, with a 68-degree field of view, is designed to continuously collect data during Odyssey's cruise from Earth to Mars. It can stores large amounts of data for downlink whenever possible, and will operate throughout the entire science mission. The instrument weighs 3.3 kilograms (7.3 pounds) and uses 7 watts of power. It measures 29.4 centimeters (11.6 inches) long, 23.2 centimeters (9.1 inches) tall and 10.8 centimeters (4.3 inches) wide.

The principal investigator for the radiation environment experiment is Dr. Gautum Badhwar of NASA's Johnson Space Center.