The alpha particle X-ray spectrometer, or APXS, provides a close-up analysis of the abundances of elements that make up rocks and soil. The APXS is most often used on space missions because it is compact, has low power requirements, and operates in very cold temperatures in a near vacuum.
An APXS contains a radioactive source that shoots alpha particles at a rock sample. When the particles hit the nuclei of atoms inside the rock, the alpha particles can bounce off, be absorbed, or release x-rays.
Detectors in the spectrometer measure the energy of alpha particles, protons and X-rays coming off the sample, allowing the machine to determine what elements are present and in what
APXS mounted on the Arm of the Mars Exploration Rover, Spirit.
The APXS will provide spectral data, which can be compared to samples in the Mission Control Database Library for identification.
Above is a spectral graph of sandstone. Note that each element peaks at its own specific energy levels.