The Gamma-Ray and Neutron Spectrometer (GRNS) is an instrument aboard the MESSENGER spacecraft, a NASA mission to conduct the first orbital study of Mercury. The purpose of the GRNS is to provide information about the elements that make up Mercury's surface crust. More exactly, it will provide information about the uppermost tens of centimeters of the crust. This instrument measures the numbers and energies of gamma rays and neutrons that reach the MESSENGER probe as it passes near the planet.