The Alpha Particle X-Ray Spectrometer (APXS) will measure the abundance of chemical elements in rocks and soils. The APXS will be placed in contact with rock and soil samples, exposing them to alpha particles and X-rays emitted by the radioactive decay of the curium sources. The X-ray counts and energies measured by the APXS detector will enable scientists to identify all important rock-forming elements, from sodium to heavier elements. This is an improved design compared to the APXS instruments on Spirit and Opportunity, requiring a shorter amount of time to take a measurement.