Near‐infrared spectral reflectance data are presented for systematic variations in weight percent of two component mixtures of ferromagnesian and iron oxide minerals. Mixtures were chosen for application to the study of the dark materials on Mars, but the results are equally applicable to mafic and ultramafic assemblages elsewhere in the solar system. Olivine spectral features are greatly reduced in contrast by admixture of other phases but remain distinctive even for low olivine contents. Clinopyroxene and orthopyroxene mixtures show resolved pyroxene absorptions near 2 μm. The addition of limonite greatly modifies pyroxene and olivine reflectance but does not fully eliminate distinctive spectral characteristics. Using only spectral data in the 1‐μm region, it is difficult to differentiate orthopyroxene and limonite (goethite) in a mixture. Additional spectral coverage or other evidence may be required for a unique interpretation. All composite mineral absorptions observed in this study are either weaker than or intermediate in strength to the end‐member absorptions and have bandwidths greater than or equal to those for the end‐members. In general, spectral properties in an intimate mixture combine in a complex, nonadditive manner, with features demonstrating a regular but usually nonlinear variation as a function of end‐member phase proportions.

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Abstract. Rocks at the Mars Pathfinder site are probably locally derived. Textures on rock surfaces may indicate volcanic, sedimentary, or impact-generated rocks, but aeolian abration and dust coatings prevent unambiguous interpretation. Multispectral imaging has resolved four spectral classes of rocks: gray and red, which occur on different surfaces of the same rocks; pink, which is probably soil crusts; and maroon, which occurs as large boulders, mostly in the far field. Rocks are assigned to two spectral trends based on the position of peak reflectance: the primary spectral trend contains gray, red, and pink rocks; maroon rocks constitute the secondary spectral trend. The spatial pattern of spectral variations observed is oriented along the prevailing wind direction. The primary spectral trend arises from thin ferric coatings of aeolian dust on darker rocks. The secondary spectral trend is apparently due to coating by a different mineral, probably maghemite or ferrihydrite. A chronology based on rock spectra suggests that rounded maroon boulders constitute the oldest petrologic unit (a flood deposit), succeeded by smaller cobbles possibly deposited by impact, and followed by aeolian erosion and deposition. Nearly linear chemical trends in alpha proton X-ray spectrometer rock compositions are interpreted as mixing lines between rock and adhering dust, a conclusion supported by a correlation between sulfur abundance and red/blue spectral ratio. Extrapolations of regression lines to zero sulfur give the composition of a presumed igneous rock. The chemistry and normative mineralogy of the sulfurfree rock resemble common terrestrial volcanic rocks, and its classification corresponds to andesite. Igneous rocks of this composition my occur with clastic sedimentary rocks or impact melts and breccias. However, the spectral mottling expected on conglomerates or breccias is not observed in any APXS-analyzed rocks. Interpretation of the rocks as andesites is complicated by absence of a "1 gm" pyroxene absorption band. Plausible explanations include impact glass, band masking by magnetite, or presence of calcium-and iron-rich pyroxenes and olivine which push the absorption band minimum past the imager's spectral range. The inferred andesitic composition is most sinfilar to terrestrial anorogenic icelandites, formed by fractionation of tholeiitic basaltic magmas. Early melting of a relatively primitive Martian mantle could produce an appropriate parent magma, supporting the ancient age of Patlff•nder rocks inferred from their incorporation in Hesperian flood deposits. Although rocks of andesitic composition at the Patlff•nder site may represent samples of ancient Martian crust, inferences drawn about a necessary role for water or plate tectonics in their petrogenesis are probably unwarranted.

Because of the lack of direct mineralogic data available for Mars, spectral remote sensing techniques and, in particular, earth‐based reflectance spectroscopy remain the primary source of this information. Presented here are laboratory observations which further constrain the mineralogy and origin of the high‐albedo soils and dust. Earth‐based observations show the reflectance spectra of classical bright regions to be fairly uniform, with a strong Fe3+ → O2− charge transfer absorption edge extending from the near UV through the visible. This absorption is relatively smooth, unlike those observed for crystalline ferric oxides which have superimposed Fe3+ crystal field bands. Dilution of ferric oxide with a spectrally more neutral medium (montmorillonite) weakens the charge transfer and crystal field absorptions together and does not serve to make the ferric oxide spectrum more Mars‐like (smoother). Nontronite (ferric‐iron‐bearing smectite clay) was also investigated for spectral agreement with telescopic observations. Pure nontronite has Fe3+ absorptions analogous to but different from ferric oxides. As in the previous case, the absorptions are very distinct and inappropriate for Mars. Admixture of neither montmorillonite nor ferric oxide serves to improve the nontronite spectrum. The conclusion is that nontronite is not a major component of martian soils, although the presence of other iron‐poor clays cannot be totally excluded based on currently available observational data. The best spectral analogs known for high‐albedo soil and dust are a specific type of palagonite from Hawaii: X‐ray amorphous weathering products of mafic volcanic glass. The indication is therefore that ferric iron on Mars is likely to occur in poorly defined crystallo‐graphic sites such as found in these amorphous materials. These materials form slowly under semiarid conditions at ambient temperature. Even low‐temperature hydrothermal alteration of glass might provide enough ion mobility to favor some formation of crystalline clays over amorphous gels and may therefore not be the primary mechanism responsible for soil formation on Mars. The amorphous Hawaiian soils exist metastably for thousands of years on Earth. Similar materials on Mars would be expected to survive considerably longer under the present cold and dry climatic conditions.

Abstract. Mars Pathfinder successfully landed at Ares Vailis on July 4, 1997, deployed and navigated a small rover about 100 m clockwise around the lander, and collected data from three science instruments and ten technology experiments. The mission operated for three months and returned 2.3 Gbits of data, including over 16,500 lander and 550 rover images, 16 chemical analyses of rocks and soil, and 8.5 million individual temperature, pressure and wind measurements. Pathfinder is the best known location on Mars, having been clearly identified with respect to other features on the surface by correlating five prominent horizon features and two small craters in lander images with those in high-resolution orbiter images and in inertial space from two-way ranging and Doppler tracking. Tracking of the lander has fixed the spin pole of Mars, determined the precession rate since Viking 20 years ago, and indicates a polar moment of inertia, which constrains a central metallic core to be between 1300 and -2000 km in radius. Dark rocks appear to be high in silica and geochemically similar to anorogenic andesites; lighter rocks are richer in sulfur and lower in silica, consistent with being coated with various amounts of dust. Rover and lander images show rocks with a variety of morphologies, fabrics and textures, suggesting a variety of rock types are present. Rounded pebbles and cobbles on the surface as well as rounded bumps and pits on some rocks indicate these rocks may be conglomerates (although other explanations are also possible), which almost definitely require liquid water to form and a warmer and wetter past. Airborne dust is composed of composite silicate particles with a small fraction of a highly magnetic mineral, interpreted to be most likely maghemite; explanations suggest iron was dissolved from crustal materials during an active hydrologic cycle with maghemite freeze dried onto silicate dust grains. Remote sensing data at a scale of a kilometer or greater and an Earth analog correctly predicted a rocky plain safe for landing and roving with a variety of rocks deposited by catstrophic floods, which are relatively dust free. The surface appears to have changed little since it formed billions of years ago, with the exception that eolian activity may have deflated the surface by -3-7 cm, sculpted wind tails, collected sand into dunes, and eroded ventifacts (fluted and grooved rocks). Pathfinder found a dusty lower atmosphere, early morning water ice clouds, and morning near-surface air temperatures that changed abruptly with time and height. Small scale vortices, interpreted to be dust devils, were observed repeatedly in the afternoon by the meteorology instruments and have been imaged.