Credit: NASA/JPL/University of Arizona
Another instrument aboard the 2001 Mars Odyssey is a Gamma Ray Spectrometer (GRS). The GRS aboard the Odyssey is actually a set of three instruments. Two of these instruments, the Neutron Spectrometer and the Russian-made High Energy Neutron Detector, detect neutrons released from the surface of the planet. The third instrument (the Gamma Ray Spectrometer) detects gamma ray photons coming from the planet. The information collected by the GRS is used to determine the composition and location of various elements on Mars (such as hydrogen, iron, chlorine, and so on). This also includes the production of a global map of water deposits and other elements on Mars, an estimation of the depth of water deposits (up to one meter deep), and a continuing study of how seasons affect the polar ice caps. The GRS also contributes to the study of cosmic gamma ray bursts.
The gamma ray detector is a large (1.2 kg) high-purity Germanium (Ge) crystal. The crystal is held at a voltage of approximately 3000 volts. Little or no current flows (less than one nanoAmp) unless a high-energy ionizing photon or charged particle strikes it. The electric charge from such a strike is amplified, measured and digitally converted into one of 16,384 (214) channels, or bins. After a specified number of seconds, a histogram is produced, which shows the distribution of events (number of strikes) as a function of energy (channel number).
The GRS is mounted on the end of a long (6-meter) boom attached to the orbiter. The boom is necessary to reduce interference from gamma rays generated by the spacecraft. (All elements in space radiate gamma-rays by the same processes that generate them on the surface of Mars.)
The gamma sensor head contains the detector, a radiative cooler, a low temperature pre-amplifier, a thermal shield with door, and a bracket to mount the sensor head to the end of the boom. The cooler has a door that opens in flight, exposing a radiator, and allowing the sensor to cool to below 90° Kelvin for science data collection. The thermal shield and door are needed to periodically warm up the sensor head to 100° Celsius to anneal radiation damage to the crystal.
The advantage of the high-purity Germanium sensor is that the lines identifying elements in the surface layer of Mars are very sharp. The count rate is very low, but long integration times permit most elements to be determined with a precision of about 10%. The GRS spectra are typically only 30 seconds in duration, but longer accumulation times are achieved by summing up spectra over a particular region of the planet.
The spatial resolution of the instrument is about 300 km; a region this large receives about 6 hours of accumulation time near the equator at the end of the mission and much longer accumulation times near the poles (about a factor of 5 more). Elements that need longer accumulation times can be determined with degraded spatial resolution by summing up spectra over larger regions of the planet. For example, Oxygen, Silicon, Chlorine, Potassium, and Iron can be determined in a 300 km spot, but Nickel and Chromium can only be determined by summing up the data over very large regions; e.g., all of the highlands or all of the lowlands.
The above map shows concentration estimates of equivalent-weight water found in the regions around the equator of Mars. The map is based upon gamma ray data collected for the element hydrogen. Regions of high hydrogen concentration are shown in red while regions of low hydrogen concentration are shown in violet and blue. The highest equatorial concentrations of hydrogen are found around and to the east of Apollineris (left and right center of map) and centered around Arabia Terra (center of map). This hydrogen may be in the form of hydrated minerals or buried ice deposits, but the former is more likely. The white sections at the top and bottom of the map represent regions of the planet with high hydrogen concentration due to large amounts of buried water ice. The locations of the five successful lander missions are marked: Viking 1 (V1), Viking 2 (V2), Pathfinder (PF), Spirit at Gusev (G), and Opportunity at Meridiani (M).