Mars Pathfinder obtained multispectral, elemental, magnetic, and physical measurements of soil and dust at the Sagan Memorial Station during the course of its 83 sol mission. We describe initial results from these measurements, concentrating on multispectral and elemental data, and use these data, along with previous Viking, SNC meteorite, and telescopic results, to help constrain the origin and evolution of Martian soil and dust. We find that soils and dust can be divided into at least eight distinct spectral units, based on parameterization of Imager for Mars Pathfinder (IMP) 400 to 1000 nm multispectral images. The most distinctive spectral parameters for soils and dust are the reflectivity in the red, the red/blue reflectivity ratio, the near‐IR spectral slope, and the strength of the 800 to 1000 nm absorption feature. Most of the Pathfinder spectra are consistent with the presence of poorly crystalline or nanophase ferric oxide(s), sometimes mixed with small but varying degrees of well‐crystalline ferric and ferrous phases. Darker soil units appear to be coarser‐grained, compacted, and/or mixed with a larger amount of dark ferrous materials relative to bright soils. Nanophase goethite, akaganeite, schwertmannite, and maghemite are leading candidates for the origin of the absorption centered near 900 nm in IMP spectra. The ferrous component in the soil cannot be well‐constrained based on IMP data. Alpha proton X‐ray spectrometer (APXS) measurements of six soil units show little variability within the landing site and show remarkable overall similarity to the average Viking‐derived soil elemental composition. Differences exist between Viking and Pathfinder soils, however, including significantly higher S and Cl abundances and lower Si abundances in Viking soils and the lack of a correlation between Ti and Fe in Pathfinder soils. No significant linear correlations were observed between IMP spectral properties and APXS elemental chemistry. Attempts at constraining the mineralogy of soils and dust using normative calculations involving mixtures of smectites and silicate and oxide minerals did not yield physically acceptable solutions. We attempted to use the Pathfinder results to constrain a number of putative soil and dust formation scenarios, including palagonitization and acid‐fog weathering. While the Pathfinder soils cannot be chemically linked to the Pathfinder rocks by palagonitization, this study and McSween et al. [1999] suggest that palagonitic alteration of a Martian basaltic rock, plus mixture with a minor component of locally derived andesitic rock fragments, could be consistent with the observed soil APXS and IMP properties.
Abstract— Densities and porosities of meteorites are physical properties that can be used to infer characteristics of asteroid interiors. We report density and porosity measurements of 42 pieces of 30 ordinary chondrites and provide a quantification of the errors of the gas pycnometer method used in this study. Based on our measurements, we find that no significant correlation exists between porosity and petrologic grade, chemical group, sample mass, bulk and grain density, or shock level. To investigate variations in porosity and density between pieces of a meteorite, we examined stones from two showers, Holbrook and Pultusk. Examination of nine samples of Holbrook suggests relative homogeneity in porosity and density between pieces of this shower. Measurements of three samples of Pultusk show homogeneity in bulk density, in contrast to Wilkison and Robinson (2000), a study that reported significant variations in bulk density between 11 samples of Pultusk. Finally, examination of two friable ordinary chondrites, Bjurböle and Allegan, reveal variability in friability and porosity among pieces of the same fall. We suggest that friable ordinary chondrites may have formed in a regolith or fault zone of an asteroid.
Multispectral observations of Phobos by the VSK (Videospectrometric) TV cameras and KRFM (Combined Radiometer and Photometer for Mars) UV‐visible spectrometer on Phobos 2 have provided new determinations of the satellite's spectral reflectance properties, at greater spatial and spectral resolutions and over a greater geographic range than have previously been available. Images of the ratio of visible and NIR reflectances covering the longitude range 30°–250°W were constructed from 0.40–0.56 μm and 0.78–1.1 μm VSK images. Eight‐channel 0.3–0.6 μm spectra obtained by the KRFM instrument package were used to provide greater spectral resolution of parts of these images. The data were calibrated using instrumental parameters measured on‐ground and in‐flight, and the calibrations were refined and tested using previous spectral measurements of Phobos and telescopic spectra of Mars, which is visible in the background in both data sets. The average color ratio of Phobos was found to be ∼0.97±0.14, consistent with previously obtained measurements. However, the surface is heterogeneous, with at least four recognizable spectral units whose absolute color ratios were determined to within ∼10%: a “red” unit with a color ratio of 0.7–0.8, a “reddish gray” unit with a color ratio of 0.8–1.0, a “bluish gray” unit with a color ratio of 1.0–1.1, and a “blue” unit with a color ratio of 1.1–1.4. The “red” unit occurs in the interiors of several dark‐floored craters and as adjacent patches. The “blue” unit composes the interior of Stickney, as well as a lobate deposit superposed on the crater's rim and extending to the southwest. The “blue” lobe is surrounded by a broad “bluish gray” aureole that breaks up into patchy outliers in its distal portions. Intervening surfaces are “reddish gray.” The “red,” “reddish gray,” and “bluish gray” units were sampled by the KRFM spectrometer, and the “bluish gray” unit was found to have a distinct 0.3–0.6 μm spectrum. The spatial distributions of the color ratio units and their reflectance systematics are inconsistent with Mars shine or particle size differences alone being responsible for color variations, but lateral optical or compositional heterogeneity is supported by the units' different UV‐visible spectra. The redder and bluer color units are interpreted to have been excavated by impacts, from an optically and/or compositionally heterogeneous interior overlain by a “reddish gray” surficial layer. The location of the “blue” lobe emanating from Stickney correlates with the location of one of the morphologic classes of grooves, as predicted by ejecta reimpact models of groove origin. The large color ratio of “blue” material is inconsistent with a carbonaceous chondrite composition but is comparable to that of an assemblage of mafic minerals like that forming black chondrites. Qualitative and quantitative comparison of the color ratio and UV‐visible spectral properties of “bluish gray” material with those of meteorites indicates that black chondrites are this material's closest spectral analog. The UV‐visible spectra of “reddish gray” and “red” materials most resemble spectra of black chondrites but are also comparable to spectra of some carbonaceous chondrites.
We have performed spectrophotometry of the Near-Earth Asteroid Rendezvous flyby target 253 Mathilde using five filters from 1.25 to 3.35 μm. We present a synthesized spectrum of ground-based data for 253 Mathilde from 0.3 to 3.35 μm combining our data with those of R. P. Binzelet al.(1996,Icarus119, 447–449). We find 253 Mathilde to have a spectrum consistent with C-class asteroids in the near-IR, though without the 3-μm water-of-hydration feature commonly (but not always) seen on asteroids of this class. We compare Mathilde with plausible meteorite analogs, and find that the surface of this asteroid is not consistent with a surface composition of common carbonaceous chondritic material. We suggest some alternative analogs on the basis of comparison of meteorite data with a synthesis of ground-based observations of Mathilde.
Abstract Evidence from Arrokoth and comets strongly suggests a very low density for this and similar small Kuiper belt objects. Plausible compositions imply high porosities, in excess of 70%, and low compaction crush strengths. If so, impact craters on Arrokoth (especially Sky, its largest) formed largely by compaction of pore space and material displacement. This is consistent with geological evidence from New Horizons imaging. High porosity reduces cratering efficiency in the gravity regime whereas compaction moves it toward crush strength scaling and increased efficiency. Compaction also guarantees that most impactor kinetic energy is taken up as waste heat near the impact point, with momentum transferred to the rest of the body by elastic waves only. Monte Carlo simulations of Sky‐forming conditions indicate that the momentum imparted likely separated Arrokoth's two lobes, but displacement was limited by dissipation at the neck between them. Unusual strength properties are not required to preserve Arrokoth's bilobate configuration.
