The GRS system is composed of a Gamma subsystem, the Neutron spectrometer (NS), built by the Los Alamos National Laboratory (LANL), and the High-Energy Neutron Detector (HEND), built by Russia's Space Research Institute (IKI).
This instrument suite detects and counts gamma rays and neutrons from the Martian surface. By associating the spectral distribution of the gamma rays with known nuclear transitions lines, it is possible to calculate the ratio of elemental abundances on the surface. By counting the number of neutrons as a function of energy it is possible to calculate the hydrogen abundance thus inferring the presence of water.
The data permit a variety of Martian geoscience and life science problems to be addressed including the crustal composition, weathering processes, volcanism, and volatile reservoirs.
In addition, the instrument suite is also sensitive to gamma ray and particle fluxes from non-Martian sources and will address problems of astrophysical interest including gamma-ray bursts, the extragalactic cosmic ray background, and solar activity.
You may learn more about how the individual instruments work on the following pages: