Particle Induced X Ray Emission and Mars Exploration Rovers

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Alpha Particle Induced X Ray Emission

  • APXS uses a variety of radiation techniques to determine atomic nuclei mass, including:

    • Alpha particle
    • Protons
    • X-Ray

  • Radioactive Currium is used as the alpha particle source
Photo of Mars Pathfinder
with APXS highlighted in red

Alpha Particle

*Used only by Mars Pathfinder, subsequent Mar Exploration Rovers did not use

Wavelength = K/E
where E is the energy emitted & K is a constant = 1.9865 x 10-16 nm J
  • The number and energy of X ray emitted are measured


  • Rock sample from Barnacle Bill in Ares Vallis on Mars
  • Unexpectedly high in silica and potassium; low in magnesium compared to martian soils and martian meteorites.
  • Analysis indicates that sample rocks are andesitic (volcanic)
X-Ray spectrum from Barnacle Bill on Mars exhibiting a
diverse chemical composition  (Reider 1997). 


R. Rieder, T. Economou, H. Wänke, A. Turkevich, J. Crisp, J. Brückner, G. Dreibus, H. Y. McSween Jr. "The Chemical Composition of Martian Soil and Rocks Returned by the Mobile Alpha Proton X-ray Spectrometer: Preliminary Results from the X-ray Mode."  Science (1997).  278:1711-1774