The Gamma Ray Spectrometer (GRS) instrument formed part of the geochemistry investigation of the MESSENGER mission and yielded information about the elemental composition of the planet's surface. GRS detected gamma-ray emissions in the 0.1 - 10 MeV range, allowing the identification of certain elements and their abundances to be determined. Gamma ray measurements were used to infer the composition of Mercury's surface over localized regions using established techniques, such as used recently onthe Lunar Prospector and Mars Odyssey missions.

The GRS detector was a coaxial germanium crystal 50 mm in diameter and 50 mm in length, chosen for its superior energy resolution and the ability to anneal the detector to remove accumulated radiation damage. The detector was rigidly clamped in a hermetically sealed Al capsule pressurized with clean, dry nitrogen. The capsule was cooled to an operating temperature in the 80-95 K range by a mechanical cryocooler. A plastic scintillator anti-coincidence shield surrounded the germanium detector in its sides and back, for rejection of cosmic-ray background. Galactic cosmic rays continuously bombard the surface of Mercury, and through interactions with the surface, gamma rays of discrete energies that are characteristic of specific elements are created. A fraction of these gamma rays, as well as those from the decay of radiogenic elements escape from the surface, where they can be detected by the orbiting GRS. Gamma-ray fluxes are measurable at altitudes up to 1000 km and for gamma rays up to about 10 MeV that emanate from depths of up to tens of centimeter beneath the surface. Detected fluxes are generally low and require numerous orbital passes over a specific region to obtain a statistically well-defined energy spectrum. The measurements of elements such as Fe, Si, Mg, Na, Al, Ca, Ti, K and Th by GRS provided insight into distinguishing between different formation models for Mercuryas well as other planetary evolution issues.