An instrument used to identify and measure the concentrations of chemical elements on other planets. It exposes dust and rocks to a radioactive source that produces measurable alpha particles, protons, and X-rays, which in turn create measurable radioactive isotopes of the elements to be studied. NASA, which derived the spectrometer from a similar Russian instrument, attached it to the Sojourner rover on the Mars Pathfinder mission which landed on Mars in July 1997, and to the Mars rovers, Spirit and Opportunity, which landed in January 2004.