Mars Pathfinder APXS analyses and interpretations

The chemical compositions of Martian rocks and soils examined with the in-situ mobile Alpha Proton X-ray Spectrometer (APXS) during the Mars Pathfinder 1997 lander mission were not previously fully determined. Preliminary chemical results included major element abundances determined by the incomplete calibration of the X-ray mode. The data collected from the alpha and proton detectors, capable of measuring some redundant elements to the X-ray mode and also providing the sample abundances of carbon and oxygen, were not previously analyzed due to significant atmospheric contributions to the spectra.; The calibrated APXS instrument is capable of measuring all major and minor concentrations of rock forming elements ranging from carbon through nickel in atomic number. Therefore, it is capable of constraining the petrology of the measured samples. In particular, the Pathfinder rock data are the first in-situ measurements and may be compared with the SNC meteorite abundances, which are thought to be from Mars, as well as with other previous remote sensing analyses.; The back-up instrument of the Pathfinder APXS flight instrument has been used to develop a technique for computing the elemental abundances of the samples at the Mars Pathfinder landing site using the three instrument modes. The calibration of the instrument has been completed under laboratory simulated Martian conditions. A method of atmospheric subtraction was developed for the alpha and proton modes, which has been tested using the laboratory instrument.; The Pathfinder abundances suggest that: (1) the rocks are covered with various amounts of soil; (2) the soil-free rock chemistry is supportive of the rocks being evolved SNC-like igneous tholeiitic basalt-andesites that are minimally altered; (3) the carbon and nitrogen contents are below detection limits for all samples; (4) the alpha mode oxygen abundance indicates that mineral-bound water, above the value for igneous rocks, is present in some rocks and is therefore indicative of some non-igneous alteration of them; and (5) the Pathfinder soils are similar to the Viking fines and may be comprised of mafic igneous material like the SNC meteorites and the depositions of volatiles from volcanic emissions, as previously suggested by Clark (1993) for the Viking soils.