The MESSENGER Gamma-Ray Spectrometer: Calibration and operations

The MESSENGER spacecraft carried a suite of geochemical remote-sensing devices that included a Gamma-Ray Spectrometer (GRS). The GRS underwent an extensive series of tests and calibrations prior to launch in order to characterize the response of the system, information that is necessary to derive elemental composition from gamma-ray measurements. From orbit about Mercury, the GRS collected gamma-ray data that were used to characterize the elemental composition of Mercury's near-surface materials. We report here the details of the ground calibration and the characterization and operation of the GRS during both the cruise and orbital phases of the mission. This information provides the basis for characterizing gamma-ray emissions from Mercury and deriving elemental composition data from those measurements.