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

Abstract

Abstract. 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 matehals. 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.

A model for formation of dust, soil, and rock coatings on Mars: Physical and chemical processes on the Martian surface

“…The character of the sulfate‐bearing Santorini soil (182) spectrum includes the stronger curvature (0.6–0.67–0.75 μm) of the disturbed soils shown in Figure 9. Bridges et al [2001] analyzed the spectral properties of rocks and soils measured by the IMP and compared spectral ratios with Si, Fe, Cl and S abundance of MPF soils and found multiple trends that are best explained by homogeneity in the soil units. Comparison of trends in many elements with S for soils at the MPF and Viking sites [ McSween and Keil , 2000; Morris et al , 2000] shows variations in the soil chemistry at the MPF site and between the MPF and Viking sites.…”

“…Aeolian mixing of these two alteration materials produces a nonhomogenous but regionally similar, fine‐grained material. This explains the subtle differences in dust/soil composition measured at the Viking and Pathfinder lander sites [ Clark et al , 1982; Bell et al , 2000; Bridges et al , 2001; Foley et al , 2001].…”

“…The Imager for Mars Pathfinder (IMP) has produced numerous spectra of specific rock and soil units near the lander [ Smith et al , 1997]. These spectra have been recalibrated and compared with both chemistry and viewing geometry in a recent study [ Bridges et al , 2001]. Spectral analyses of laboratory soil analog materials and several soil units measured by the IMP showed that there are specific differences in the soils observed by Pathfinder that are attributed to differences in mineralogy and composition of the soils [e.g., Bishop et al , 1998d; Bell et al , 2000; Morris et al , 2000].…”

“…Some soil units have a stronger shoulder feature near 0.6 μm and some exhibit a weak, broad band near 0.9 μm. Variations in the chemical abundances of these soils indicate a good correlation of wt.% SO 3 with the red/blue reflectance ratios [ Bridges et al , 2001]. This suggests that the ferric minerals responsible for the reflectance maximum near 0.75 μm are correlated with sulfates in the soils.…”

Generation and performance of automated jarosite mineral detectors for visible/near-infrared spectrometers at Mars

“…This conservative approach would minimize time allocation to the investigation of misidentified targets. Although dust contaminates the majority of VNIR spectra, relatively dust-free rock surfaces have been identified at the Mars Pathfinder (Bell et al, 2000;Bridges et al, 2001) and Gusev crater sites . Because VNIR point spectra are relatively computationally inexpensive to acquire, onboard processing of all spectra could potentially be accomplished to rapidly identify target minerals in the subset of dustfree or dust-poor rocks.…”