Located on the rover, the APXS can sample Martian rocks or patches of soil that are out of the lander's reach, and determine the chemical make-up of whatever it's touching. The Alpha-Proton-X-Ray Spectrometer determines the elemental chemistry of rocks and soils accurately in order to complement and constrain the mineralogical analyses of the other instruments. The rovers' APXS performs elemental analyses of martian surface materials by directly touching a rock or patch of soil. Through the use of alpha particles, protons, and x-rays the APXS will accurately determine a sample's abundances of all rock-forming elements except hydrogen. Analyzing the elemental make-up of martian surface materials provides scientists with information about crustal formation, weathering processes, and water activity on Mars.
Source: NASA, Cornell University