The Gamma Ray Spectrometer (GRS)

The Gamma Ray Spectrometer (GRS): "The Gamma Ray Spectrometer (GRS) instrument suite is a set of instruments aboard the 2001 Mars odyssey spacecraft 2001 Mars Odyssey.Two of these instruments (the Neutron Spectrometer and the High Energy Neutron Detector) detect neutrons released from the surface of the planet. The third instrument (the Gamma Ray Spectrometer) detects gamma ray photons coming from the planet. 
Together, these instruments help us learn about the composition of the Martian surface. The information collected by GRS will be used to help determine the composition and location of the elements on Mars. This includes production of a global map of water deposits and other elements on Mars; an estimation of the depth of water deposits; and continued study of how seasons affect the polar ice caps." Read more at the University of Arizona 2001 Mars Odyssey Lunar and Planetary Lab.