DETERMINATION OF THE CHEMICAL-COMPOSITION OF MARTIAN SOIL AND ROCKS -THE ALPHA-PROTON X-RAY SPECTROMETER

The alpha proton X ray spectrometer (APXS) for the Mars Pathfinder mis sion is designed to provide a complete and detailed analysis of chemic al elements in Martian soil and rocks near the landing site. The APXS instrument is carried on the Pathfinder Microrover, which will provide transportation to places of interest on the Martian surface. It consi sts of a complex sensor head, mounted on a simple but sophisticated AP XS deployment mechanism (ADM) outside the warm electronics box (WEB) o f the Microrover, and the instrument electronics, mounted inside the W EB. The ADM permits the instrument sensor head to be placed against so il and rock samples in arbitrary positions, ranging from horizontal to vertical, in order to perform in situ analysis. The possibility to tr ansport the APXS to an arbitrary location, preselected on Earth, and t o perform in situ analysis there, constitutes one of the most exciting aspects of the Pathfinder mission. The principle of the APXS techniqu e is based on three interactions of alpha particles from a radioisotop e source with matter: simple Rutherford backscattering, production of protons from (alpha, p) reactions on light elements, and generation of characteristic X rays upon recombination of atomic shell vacancies Cr eated by ct bombardment. Measurement of the intensities and energy dis tributions of these three components yields information on the abundan ce of chemical elements in the sample. In terms of sensitivity and sel ectivity, data are partly redundant and partly complementary: alpha ba ckscattering is superior for light elements (C, O), while proton emiss ion is mainly sensitive to Na, Mg, Al, Si, S, and X ray emission is mo re sensitive to heavier elements (Na to Fe and beyond). A combination of all three measurements enables determination of all elements (with the exception of H and He) present at concentration levels above typic ally a fraction of 1%.