The Alpha Particle X-Ray Spectrometer (APXS) measures the abundance of chemical elements in the rocks and soil on Mars.
Roughly the size and shape of a Rubik's cube, the APXS's sensor can gather data day and night. It takes two to three hours to analyze a sample to determine its constituent elements, including trace elements. A quick-look analysis can be completed in about ten minutes. The APXS, which sits on the end of the rover's robotic arm, is placed close to a sample and bombards it with alpha particles (charged Helium nuclei) and X-rays to study the properties of the energy emitted from the sample in response. The APXS instrument on Curiosity is an updated version of the spectrometers that were successfully used on the Mars Exploration Rover (MER) and Mars Pathfinder missions.