Gamma ray spectrometer

The Gamma Ray Spectrometer (GRS) uses the gamma-ray part of the spectrum to look for the presence of 20 elements from the periodic table, and is used in the exploration of Mars. Its neutron detectors look for water and ice in the soil by measuring neutrons. It is able to measure the abundance and distribution of about 20 primary elements of the periodic table, including silicon, oxygen, iron, magnesium, potassium, aluminum, calcium, sulfur, and carbon. Knowing what elements are at or near the surface will give detailed information about how Mars has changed over time. To determine the elemental makeup of the Martian surface, the experiment uses gamma ray spectrometer and two neutron detectors.

GRS supplies data on the distribution and abundance of chemical elements, much as the Lunar Prospector mission did on the moon. In this case, the chemical element thorium was mapped, with higher concentrations shown in yellow/orange/red in the left-hand side.

How GRS works

When exposed to cosmic rays (charged particles in space that come from the stars, including our sun), chemical elements in soils and rocks emit uniquely identifiable signatures of energy in the form of gamma rays. The gamma ray 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. 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 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.

How are gamma rays and neutrons produced by cosmic rays? Incoming cosmic rays--some of the highest-energy particles--collide with atoms in the soil. 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. 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 HEND and Neutron Spectrometers on GRS directly detect scattered neutrons, and the Gamma Sensor detects the gamma rays.

Water detection

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 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.

GRS will supply data similar to that of the successful Lunar Prospector mission, which told us how much hydrogen, and thus water, is likely on the moon.

The gamma ray spectrometer consists of four main components: the gamma sensor head, the neutron spectrometer, the high energy neutron detector, and the central electronics assembly. The sensor head is separated from the rest of the Odyssey spacecraft by a 6.2 meter (20 ft) boom, which will be extended after Odyssey has entered the mapping orbit at Mars. This maneuver 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 about 100 days of the mapping mission, 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 mapping mission.

GRS specifications

The Gamma Ray Spectrometer weighs 30.5 kilograms (67.2 lb) and uses 32 watts of power. Along with its cooler, the Gamma Ray Spectrometer measures 468 by 534 by 604 mm (18.4 by 21.0 by 23.8 in).

The neutron spectrometer is 173 by 144 by 314 mm (6.8 by 5.7 by 12.4 in).

The high-energy neutron detector measures 303 by 248 by 242 mm (11.9 by 9.8 by 9.5 in). The instrument's central electronics box is 281 by 243 by 234 mm (11.1 by 9.6 by 9.2 in).

External links

• http://mars.jpl.nasa.gov/odyssey/technology/grs.html - NASA Jet Propulsion Laboratory Gamma Ray Spectrometer page
• http://grs8.lpl.arizona.edu/main.jsp - GRS instrument site at the University of Arizona