The Surface Materials of Mars.

Allan H. Treiman
Lunar and Planetary Institute
Houston TX 77058-1113
treiman@lpi.usra.edu

For a Mars 2005 Sample Return mission, both site selection and sampling strategies must consider the nature of the surface and near surface materials of Mars. In simplest form, the surface of Mars consists of three fundamental units: dark, bright, and intermediate. Although named by their albedo in visible light, the units are also distinct in photogeology, thermal inertia, and radar reflection properties. The dark unit is interpreted to represent basaltic sands or outcrops, which are discussed elsewhere by H. McSween Jr.

The bright surface units are almost certainly dust. Bright materials is deposited by global dust storms and redistributed on timescales of years by normal winds. Bright regions also have low thermal inertias, consistent with very fine grain sizes. The chemical composition of the martian dust was analyzed by the Viking landers, and is basaltic, very similar to some of the martian meteorites. The dust contains % levels of sulfur. The mineralogy of the dust has been difficult to define. Variations in the VL XRF chemical analyses are consistent with varying proportions of titanomagnetite and of a magnesium sulfate. The VL magnetic properties experiment is consistent with titanomagnetite, and the VL Biology experiments are consistent with abundant ferroan smectite, but the 2.2 micrometer absorption charcteristic of clays has not been observed. IR and visible spectra are consistent with bulk and nanophase hematite and possibly ferric oxyhydroxides, and iron sulfates are possible. Absorptions from carbonate and sulfate minerals have been detected in IR spectra, but exact mineral identification has proved difficult. Carbonate absorptions are most consistent with a hydrous magnesium carbonate, but the sulfate absorptions are not diagnostic. Scapolite had been suggested as a possible surface mineral, but spectral and thermochemical data suggest that it is not a significant component of the dust. The dust contains % levels of water, based on distinct O-H absorption features, but its mineralogic siting is not known [1]. The martian meteorites contain low-temperature alteration minerals that may be of significance for the dust. Among the alteration minerals are smectite, illite, Ca- and Mg-carbonates, Ca- and Mg-sulfates, that can be mixed in reasonable proportions to replicate the chemical composition of the dust [2].

Intermediate albedo surfaces are widespread on Mars, and perhaps most prominent on Lunae Planum and Oxia in the circum-Chryse plateaus. The intermediate albedo unit is not a mixture of dust (bright) and basalt (dark), but a distinct unit with characteristic geologic and spectral properties. In Viking color, intermediate units appear "brown" to dark red, and are distinct in Phobos 2 ISM spectra: they are rich in water, contain little pyroxene, and contain more crystalline hematite than either bright or dark surfaces. The thermal inertias of intermediate albedo surfaces suggest of fine-grained materials cemented together. Geologically, intermediate units appear as plateaus and wrinkle-ridged surfaces, swept clear of dust and basaltic sand. Channels and chasms cut through some intermediate surfaces appear to expose a laterally extensive layer, ~500 m thick, of cemented soils. In the circum-Chryse region, this layer is exposed from Noctis Labyrinthus to the edge of Oxia, approximately 5000 km E-W and 1000 km N-S [3].

The mineralogy of the intermediate albedo surfaces is poorly known. Lunae Planum is interpreted as flood basalt province, but ISM spectra of the area show little pyroxene. The ISM spectra are consistent with hematite and a hydrous phase as the cements, and exposures of the cemented soils include both dark and light colored horizons. Another cementing phase, such as a hydrous iron oxide or sulfate is possible. The dust clods at the VL sites were apparently cemented by a magnesium sulfate, but this may not be relevant to the intermediate surfaces [3].

The martian meteorites may provide important clues to the nature of cemented (intermediate albedo) surfaces on Mars. Almost all the martian meteorites contain secondary or alteration materials that were produced on Mars (conclusively pre-terrestrial in origin). These alteration materials come in three distinct assemblages. First, the shergottite meteorites contain very small quantities of a Ca-Mg salt assemblage, rich in Ca- and Mg- carbonates and sulfates. Also present are an Mg-phosphate, an aluminous (illitic) clay, and a poorly crystalline aluminosilicate containing significant S and Cl. The nakhlite meteorites all contain relatively abundant alteration materials (1-2% by volume) dominated by ferroan, low-Al smectite clays, which are rich in S, Cl, and P. Grains of ferrihydrite, hematite, Ca-sulfate, and Ca-Fe-Mn carbonates are also present. Finally, the ALH84001 meteorite contains a Mg-Fe carbonate assemblage. It is dominated by ellipsoids of anhydrous carbonate minerals, which are zoned from Ca-rich cores to Mg-Fe mantles to pure Mg rims. Other minerals present include pyrite, magnetite, sphalerite, and a poorly defined iron sulfate. Of these alteration types, the clay-iron-mineral assemblage in the nakhlites is closest to the iron mineralogy implied by reflection spectra, although there is no spectroscopic evidence for clays on Mars. However, the assemblage in Nakhla is hydrothermal in origin, and thus a poor analog for widespread surface cementation. The Ca-Mg salt assemblage resembles the salt assemblage implied by reflection spectra, in containing sulfate and carbonate minerals. Texturally, some of the carbonate minerals could have been hydrous originally. At this point, however, none of the alteration assemblages is a convincing match for the assemblages observed spectroscopically [2,4].

From a sample return perspective, surface materials are among the easiest and most certain of collection. A sample of dust should be collected, at least as a contingency sample. The physical characteristics of the dust are crucial inputs to models of Mars' current climate; measuring these properties directly would greatly refine the models. If the dust represents the products of global erosion, its chemical composition and mineralogy will be critical to understanding the composition of Mars' crust (perhaps comparable to shales on Earth), and early planetary differentiation. If the dust represents singular volcanic events, its mineralogy and composition will define those events. Because dust is highly mobile in the present climate, it seems unlikely that drill cores through dust deposits will yield anything beyond more dust and cemented dust. Samples of cemented surfaces will provide direct evidence of the action of solvents, most likely water, in redistributing salts in the regolith. This evidence will be critical in unraveling Mars' ancient climate, and the mobility and stability of water at and near its surface.

References:
[1] Clark B. et al. (1982) JGR 87, 10059.
Clark R. et al. (1990) JGR 95, 14463.
Murchie S. et al. (1993) Icarus 105, 454.
Bell J. III (1992) Icarus 100, 575.
Bell J. III et al. (1994) Icarus 111, 106.
Calvin W. et al. (1994) JGR 99, 14659.
Burns R. (1984) LPSC XXV, 203.
Blaney D. and McCord T. (1995) JGR 100, 14433.
Bell J. III et al. (1995) JGR 100, 5297.
Bishop J. and Pieters C. (1995) JGR 100, 5369.
Bishop J. et al. (1995) Icarus 117, 101.
Roush T. (1996) JGR 101, 2215.
Bell J. III (in press) in Martian Spectroscopy.

[2] Gooding J. (1992) Icarus 99, 28.

[3] Clark B. and van Hart D. (1981) Icarus 45, 370.
Presley M. and Arvidson R. (1988) Icarus 75, 499.
Christensen P. and Moore H. (1992) 1135 in Mars.
Treiman A. et al. (1995) JGR 100, 26339.
Treiman A. (1996) JGR 101, submitted.

[4] Treiman A. et al. (1993) Meteoritics 28, 86.
Mittlefehldt D. (1994) Meteoritics 29, 214.
Treiman A. (1995) Meteoritics 30, 294.

Vugraphs:

VG. 1: Title: Surface Materials on Mars

VG 2: Three Types of Surface Units

VG 3: Mineralogy of Dust - Viking

VG 4: Color Thin Section View of the Rusty Alteration Materials in Lafayette.

VG 5: Sample Collection

VG 6: Advertisement: Planetary Surface Instrument Workshop, 1995

VG 7: Mineralogy of Dust - Spectra

VG 8: Intermediate Albedo Surface - Duricrust or Hardpan

VG 9: Intermediate Albedo Spectra

VG 10: Martian Meteorite Alterations I

VG 11: Martian Meteorite Alterations II

VG 12: Meteorite Clues to Surfaces

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