NASA description of the scientific objective of Pathfinder prior to the mission

"The Mars Pathfinder project is one of the first NASA Discovery class missions. It will land a single vehicle with a microrover (Sojourner) and several instruments on the surface of Mars in 1997. Sojourner's mobility provides the capability of "ground truthing" a landing area over hundreds of square meters on Mars. Pathfinder will be investigating the surface of Mars with three additional science instruments (a stereoscopic imager with spectral filters on an extendible mast (IMP)), an Alpha Proton X-Ray Spectrometer (APXS), and an Atmospheric Structure Instrument/Meteorology package (ASI/MET)"

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Alpha Proton X-ray Spectrometer

This elemental composition instrument consists of alpha particle sources and detectors for back-scattered alpha particles, protons and X-Rays. The APX Spectrometer will determine elemental chemistry of surface materials for most major elements except hydrogen.

The analytical process is based on three interactions of alpha particles with matter: elastic scattering of alpha particles by nuclei, alpha-proton nuclear reactions with certain light elements, and excitation of the atomic structure of atoms by alpha particles, leading to the emission of characteristic X-Rays.

The approach used is to expose material to a radioactive source that produces alpha particles with a known energy, and to acquire energy spectra of the alpha particles, protons and X-Rays returned from the sample. Such an instrument can identify and determine the amounts of most chemical elements.

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