Bell et al. MARS SOILS AND DUST: PATHFINDER RESULTS 34
Mineralogic and Compositional Properties of Martian Soil and Dust: Results from Mars Pathfinder
J. F. Bell III1, H. Y. McSween Jr.2, J. A. Crisp3, R. V. Morris4, S. L. Murchie5,
N. T. Bridges3, J. R. Johnson6, D. T. Britt7, M.P. Golombek3, H. J. Moore8, A. Ghosh2,
J. L. Bishop9, R. C. Anderson3, J. Brückner10, T. Economou11, J. P. Greenwood2,
H. P. Gunnlaugsson12, R. M. Hargraves13, S. Hviid12, J. M. Knudsen12,
M. B. Madsen12, R. Reid7, R. Rieder10, and L. Soderblom6
1Department of Astronomy, Cornell University, Ithaca, New York
2Department of Geological Sciences, University of Tennessee, Knoxville
3Jet Propulsion Laboratory, California Institute of Technology, Pasadena
4Code SN, NASA Johnson Space Center, Houston, Texas
5Applied Physics Laboratory, Johns Hopkins University, Baltimore, Maryland
6U. S. Geological Survey, Flagstaff, Arizona
7Lunar and Planetary Laboratory, University of Arizona, Tucson
8U. S. Geological Survey, Menlo Park, California. Deceased Sept. 21, 1998.
9NASA Ames Research Center, Moffett Field, California
10Max-Planck-Institut für Chemie, Mainz, Germany
11Enrico Fermi Institute, University of Chicago, Illinois
12Niels Bohr Institute, University of Copenhagen, Denmark
13Department of Geosciences, Princeton University, Princeton, New Jersey
Submitted to JGR–Planets
April 2, 1999; Revised August 5, 1999
Text Pages: 49
Figures: 16 (in 34 parts) (see http://marswatch.tn.cornell.edu/soils.html)
Tables: 7
Address Correspondence to:
Jim Bell
Department of Astronomy
Cornell University
402 Space Sciences Building
Ithaca, NY 14853-6801
Phone: (607) 255-5911
Email:
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Bell et al. MARS SOILS AND DUST: PATHFINDER RESULTS 34
Abstract
The mineralogy, elemental composition, and physical properties of soils and dust on Mars can provide information on the style and timing of physical and chemical weathering processes during both past and present climatic regimes. Mars Pathfinder obtained multispectral, elemental, magnetic, and physical measurements of soil and dust at the landing site during the course of its 83 sol mission. We describe these measurements and the initial results gleaned from them, concentrating on the 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. Comparisons of IMP spectra of bright soil units to laboratory analog spectra and previous telescopic data indicates that 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 900 nm absorption seen in IMP spectra. Maghemite would also be consistent with the Pathfinder magnetic properties results, although they could also be interpreted in terms of FeTi spinels. The ferrous component in the soil cannot be well-constrained based on IMP data because of the 1000 nm wavelength limit, but candidates include high-Ca pyroxene and olivine. Compositionally, 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. Like the Viking soils, two principal geochemical components may be present in the Pathfinder soil APXS data,, but the inter-element correlations are not as strong. No significant linear correlations were observed between IMP spectral properties and APXS elemental chemistry at the APXS measurements sites, but this may partly result from the small sample size (6) and the relatively large uncertainties in the APXS compositional data. 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 the analyses of 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 elemental chemistry and multispectral properties. The available data on the acidification of tephra appear inconsistent with Pathfinder measurements, although this process could explain the origin of Viking soils. A fractionation process (aeolian? magnetic?) that leads to local concentrations of Ti-bearing phases at the Pathfinder site may provide a link to the Viking soil chemistry and a way to possibly resolve the non-unique constraints on soil origin. We conclude by discussing future in situ measurements of Martian fines that may help to constrain further their mineralogy and origin.
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Bell et al. MARS SOILS AND DUST: PATHFINDER RESULTS 34
Introduction
The chemistry and mineralogy of the Martian surface provide a window into physical and chemical weathering processes at work in the present and past Martian environment(s). The degree of crystallinity, cation content and oxidation state, grain size, pH, Eh, and abundances of water and organic matter in the soils and dust are the result of weathering and alteration processes acting on parent materials over geologic time scales. Careful observations of the chemistry, mineralogy, and morphology of weathered and altered surface materials provide constraints on the climatic or environmental conditions prevalent during their formation or subsequent modification. On Earth, numerous studies have demonstrated the link between soil physical and chemical properties and environmental conditions [e.g., reviews in Schwertmann and Taylor, 1989; Cornell and Schwertmann, 1996]; the working assumption is that similar inferences about environmental conditions on Mars, past and present, can be gleaned from measurements of the properties of soils found there today [e.g., Gooding, 1978; Burns, 1993; Burns and Fisher, 1993; Morris et al., 1993, 1995; Banin et al., 1997].
A bit of pedologic pedagogy: the term "soil" has become commonly used in planetary science to describe the fine-grained, porous, uppermost layers of a regolith, despite the more purist viewpoint that the term applies only to terrestrial material formed by or in the presence of organic compounds [e.g., Johnson, 1968; Markewitz, 1997]. This narrow definition of "soil" has been adopted by the Soil Science Society of America [1984], who concede, however, that it is a combination of many influences, among them parent composition and climate, that leads to the formation of soil, which they define as "unconsolidated mineral matter that may differ chemically, physically, morphologically, or biologically from the material from which it is derived". In this paper, we adopt a similarly broad definition of "soil", except that the biological aspect is vanishingly small to nonexistent. Martian soil can also be thought of as the excited skin of the subaerial part of the Martian crust [after Nikiforoff, 1959; see also Retallack, 1998], a definition that also includes rock surfaces. In the grain-size scale used by soil scientists, clay is the size fraction <2 µm, silt is 2 to 50 µm, and sand is 50 µm to 2 mm. In our scheme we define "dust" as that finest-grained component of the soil that rather easily can become airborne [< 10 µm diameter ash or "fine dust" in the traditional soil science sense; Greeley and Iversen, 1985], and we define "sand" loosely as that slightly more granular component of the soil that is mobile but not usually airborne (including 5 to 50 µm diameter "coarse dust" or silt). While perhaps not conforming to traditional terrestrial pedologic nomenclature, these definitions serve us well in attempting to describe the properties and behavior of loose materials at the Martian surface.
The Mars Pathfinder (MPF) mission had a number of important science goals directly related to the study of soils and dust. Specifically, these included determination of the chemical composition, mineralogy, and physical properties (e.g., magnetism, grain size, stratigraphy) of soils and dust at the landing site; characterization of differences among the soils at the site and between these soils and those studied at the Viking landing sites; establishing whether there is a genetic relationship between soils and rocks at the site; and determining whether the observed composition and mineralogy of the soils can be used to constrain specific pedogenic weathering or alteration scenarios. Assessment of organic content and chemical reactivity of the surface was not a primary goal of the MPF mission, although some information on this subject could be provided by Alpha Proton X-Ray Spectrometer (APXS) alpha-channel measurements of carbon. More accurate constraints on organic and reactivity properties, as well as on a number of other important parameters related to soils, were provided by the Viking Landers [Toulmin et al., 1977; Clark et al., 1982; Banin et al., 1992], and the combination of Viking and MPF data provides a much more robust set of constraints on Mars soil and dust origin and evolution than either mission could provide alone.
In this paper we discuss the chemistry, mineralogy, morphology, and distribution of soils and dust measured at the MPF landing site. We first consider what was known or inferred about Martian soils and dust based on previous remote sensing and in situ Viking Lander investigations, providing the justification for the new measurements obtained during 1997 by Pathfinder. We provide a short discussion of data reduction and data processing activities associated with these measurements, but defer detailed discussions of these points to other papers. Results of Pathfinder soil elemental chemistry measurements and multispectral imaging studies are presented. The implications of these results, plus those from the MPF magnetic properties and soil mechanics experiments and Viking Lander multispectral imaging and elemental chemistry measurements, are used to discuss models of the origin and evolution of Martian dust and soil that are consistent with the available data. We conclude with some preliminary thoughts on which of these models might be most accurate, and a discussion of how future remote sensing and in situ measurements can help to resolve further the outstanding issues.
Previous Measurements and Interpretations
Most of what was known about the chemistry and mineralogy of Martian soils and dust prior to Mars Pathfinder came from telescopic and spacecraft remote sensing imaging and spectroscopy, and Viking Lander remote sensing imaging, magnetics, soil mechanics, and in situ elemental and chemical analyses.
Telescopic observations have revealed that the visible to near-IR spectral properties of airborne dust are nearly indistinguishable from those of the classical bright regions [e.g., McCord et al., 1977; Wolff et al., 1997]. The major visible to near-IR spectroscopic characteristics of this highly mobile unit are a low reflectivity in the blue (radiance factor or I/F < 0.05 at 400 nm), a high reflectance in the red (reflectivity > 0.30 at 750 nm), a relatively featureless spectrum between blue and red wavelengths, and weak indications of absorption in the short-wave near-IR (800 to 1000 nm) [Singer, 1982; Bell et al., 1990]. These data have been interpreted to indicate the presence of small amounts (perhaps 2-4 wt.%) of a well-crystalline iron oxide like hematite occurring in a matrix of more poorly crystalline (perhaps nanophase) ferric oxides and other, spectrally neutral, aluminosilicates [e.g., Morris et al., 1989, 1993, 1997; Bell et al., 1993; Morris and Golden, 1998]. These interpretations support the idea that a rather homogeneous and fine-grained weathering product is an important and probably extensive component of bright local soils everywhere on the planet.
Telescopic observations of the low albedo or classical "dark" region of Mars have revealed important differences in spectroscopic properties compared to bright regions, likely related to differences in composition and/or physical properties. The major visible to near-IR spectroscopic characteristics of dark regions are a low reflectivity in the blue (I/F < 0.05 at 400 nm--nearly identical to the blue reflectivity of the bright regions), a 750 nm reflectivity of < 0.15 (and thus a red/blue reflectance ratio of 2 to 3 times lower than the bright regions), a relatively featureless spectrum between blue and red wavelengths, and indications of weak absorption in the near-IR longward of 900 nm. These data have been interpreted to indicate the presence of low calcium pyroxene in these surface regions, which, because of their red color, must also be either heavily oxidized or mixed with the same type of poorly crystalline ferric material(s) as the bright regions [Adams and McCord, 1969;Singer et al., 1979; Bell et al., 1990]. More recent Phobos-2 ISM data and HST observations reveal that at higher spatial resolution, stronger and more complex near-IR absorption features are evident in many classical dark regions, indicating the likely presence of two-pyroxene basalts in some regions, as well as the presence of more crystalline ferric-bearing alteration products in other regions [Mustard et al., 1993, 1997; Mustard and Sunshine, 1995; Bell et al., 1997]. Evidence for differences in physical properties causing at least some of the observed spectral differences between bright and dark regions comes primarily from Viking Orbiter thermal inertia measurements that showed a strong inverse correlation between rock abundance and broadband albedo [Christensen, 1986; Christensen and Moore, 1992], suggesting that the presence of blocks and/or coarser-grained soils may explain at least part of the lower reflectivity of dark regions compared to bright regions.
Telescopic and spacecraft evidence for other, spectrally heterogeneous and mobile soil and dust units has also been obtained from Viking Orbiter [Soderblom et al., 1978;McCord et al., 1982], Phobos-2 [Murchie et al., 1993, 1999a; Mustard et al., 1993], groundbased [Bell, 1992; Merényi et al., 1996], HST [Bell et al., 1997; Bell and Morris, 1999], and Mars Global Surveyor Thermal Emission Spectrometer (TES) [Christensen et al., 1999; Lane et al., 1999] multispectral imaging and spectroscopic observations. Of most relevance to this study is the evidence for localized occurrences of crystalline ferric phases (in addition to hematite) that could potentially provide more diagnostic information on weathering history than the nanophase soil components. In nearly all previous remote sensing studies, increasing spatial resolution has led to increased detection of spectral heterogeneity of the Martian surface. At the Viking Lander sites, an examination of small-scale variations in rock and soil color provided evidence that surface coatings or rinds and abrasion/spallation can strongly influence local soil spectral properties [Sharp and Malin, 1984; Adams et al., 1986; Guinness et al., 1987]. Adams et al. [1986] and Guinness et al. [1987] concluded that although the soils at the Viking Lander sites have a component with a greater degree of ferric iron crystallinity than the nanophase global aeolian dust, they could find no evidence that the bulk of the soil has been derived from the weathering of what are assumed to be mafic dark gray rocks at the sites. Because of the limited spectral range and sampling of the Viking Lander cameras, however, it was not possible to identify uniquely the mineralogy of either the more crystalline soil units or the supposedly mafic rocks based on their observed color difference [cf. Arvidson et al., 1989b].