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 on
           the 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 Mercury
           as well as other planetary evolution issues.