The Alpha Particle X-Ray Spectrometer is called APXS for short. When it is placed right next to a rock or soil surface, it uses two kinds of radiation to measure the amounts and types of chemical elements that are present.

Tech Specs

Main Job
Analyze chemical elements in Martian rock and "soil" (regolith).
On the turret at the end of Curiosity's robotic arm.
About the size of a cupcake.
Size of Sampled Area
Possibility of 13.9 microns/pixel
Focal Length
About 1.7 cm in diameter when the instrument is in contact with the sample.
Can operate day or night and takes about one-third of the time to process readings.

Alpha Particle X-Ray Spectrometer (APXS)

Alpha Particle X-Ray Spectrometer
Alpha Particle X-Ray Spectrometer: This picture shows the Alpha Particle X-Ray Spectrometer used on the Mars Exploration Rovers. The improved APXS instrument on the Mars Science Laboratory rover would be able to detect elemental composition more quickly and work both day and night. Credit: NASA/JPL-Caltech/Cornell/Max Planck Institut für Chemie/University of Guelph.
Download full image ›

The Alpha Particle X-Ray Spectrometer will measure the abundance of chemical elements in rocks and soils. Funded by the Canadian Space Agency, the APXS will be placed in contact with rock and soil samples on Mars and will expose the material to alpha particles and X-rays emitted during the radioactive decay of the element curium. X-rays are a type of electromagnetic radiation, like light and microwaves.

Alpha particles are helium nuclei, consisting of 2 protons and 2 neutrons. When X-rays and alpha particles interact with atoms in the surface material, they knock electrons out of their orbits, producing an energy release by emitting X-rays that can be measured with detectors. The X-ray energies enable scientists to identify all important rock-forming elements, from sodium to heavier elements.

The APXS will take measurements both day and night. Its sensor head is designed to be smaller than a soda can and will contain a highly sensitive X-ray detector in the middle of an array of curium sources. The longer the instrument is held in place on the surface of a rock or soil sample, the more clearly the signal from the sample can be determined. Most APXS measurements will take two to three hours to reveal all elements, including small amounts of trace elements. Ten minutes of operation will be sufficient for a quick look at major elements.

Portrait of APXS on Mars
Portrait of APXS on Mars: This image shows the Alpha Particle X-Ray Spectrometer (APXS) on NASA's Curiosity rover, with the Martian landscape in the background.
Download full image ›

As a contact instrument, the APXS is designed to work in concert with other payload elements on the instrument arm and in the body of the Mars Science Laboratory rover, such as the CheMin instrument and the Dust Removal Tool (brush). Scientists will use the APXS to help characterize and select rock and soil samples and then examine the interiors of the rocks following brushing. By analyzing the elemental composition of rocks and soils, scientists will seek to understand how the material formed and if it was later altered by wind, water, or ice. The APXS on NASA's two Mars Exploration Rovers has already provided evidence that water once played a major role in Mars' geologic past.

Two earlier missions to Mars carried previous versions of the Alpha Particle X-Ray Spectrometer. The first was the Alpha Proton X-Ray Spectrometer, launched to Mars on the Pathfinder mission in late 1996. The second was the APXS instrument, on board both the Mars Exploration Rovers that arrived on the red planet in January, 2004.

In addition to the Canadian Space Agency and NASA, major organizations involved in developing the APXS include the University of Guelph; MDA Space Missions; the University of California, San Diego; and Cornell University.