Characteristics of the Pathfinder APXS sites: Implications for the composition of Martian rocks and soils

An integrated study of spectral, elemental abundance, and image data from the Pathfinder alpha proton X-ray spectrometer (APXS) measurement sites has led to a better understanding of the nature of Martian surface materials. This rigorous analysis provides a new level of detail that forms the basis for the results reported here and that can be used by future scientists trying to understand rocks and soils on Mars. Each APXS site has been precisely located by analyzing stereo Imager for Mars Pathfinder (IMP) and rover camera images to determine the position of the APXS during deployment. Rover and IMP images are used to assess local geology, namely, the presence of pebbles, clods, and aeolian features in soils and surface textures and dust concentrations on rocks. IMP spectra using the latest calibrations for radiance and reflectance are analyzed at a number of different illumination geometries for each APXS site. The three-dimensional orientation of the sites has been determined and then used to compute the true sunlight (incidence) and viewing (emission) angles. Where suitable photometric coverage is available, reflectances are fit to the Hapke-Irvine function. This is then used to adjust spectral reflectances to a normalized illumination geometry common to all rocks and soils. Comparing all the data sets shows a positive correlation among red/blue reflectance ratio, SO₃ content, and dust abundance on rocks, indicating that rocks are coated with varying amounts of sulfur-rich, red dust. Other elements, such as silicon and iron, are poor tracers for dust because their abundances within soil are similar to those within rock. Rock APXS targets that are bluer, poorer in sulfur, and have minimal dust coverage face toward the northeast, the direction in which winds may be capable of efficiently removing loose material under current Martian conditions. The IMP spectral properties of the soils show poor correlations with APXS elemental abundances. On the basis of currently released APXS data, bulk soils at the Pathfinder landing site are richer in silica and sulfur than the Viking landing sites, suggesting a different composition. The APXS soils with the most apparent pebbles are the poorest in SiO₂, indicating that either the pebbles are more mafic than the APXS rocks or the pebble-free soil component is inherently enriched in SiO₂ compared to Viking soils. The mixture of materials at the APXS sites is a reflection of the overall complexity of Martian surface materials, a detailed understanding of which should drive the choice of future instruments and missions.