Martian crust

The composition of Martian surface materials can be assessed using laboratory analyses of Martian meteorites, in situ APXS analyses from Mars Pathfinder and the Mars Exploration Rovers, and orbital geochemistry analyzed by GRS and derived from TES spectra. 

Although the rocks in Meridiani Planum analyzed by the Opportunity rover are fascinating (see below), they are unusual. Therefore, we will focus on the rocks and soils in Gusev crater analyzed by the Spirit rover, which are spectrally similar to the bulk of the Martian crust. We will compare these compositions with those of Martian meteorites and Bounce Rock in Meridiani, which is similar to shergottites. We will also consider orbital geochemical data obtained by GRS. 

Alkalis versus silica diagram used for geochemical classification of volcanic rocks. Martian meteorites, Gusev crater rocks and soils analyzed by the Spirit rover, and Bounce Rock analyzed by the Opportunity rover generally plot in the field of basalts, as does the average Mars Odyssey GRS analysis. Compositions derived from TES spectra (Surface Types 1 and 2) and the Mars Pathfinder dust-free rock plot in the basaltic andesite and andesite fields. After McSween et at. (2009). 

Ratios of FeO (total iron expressed as FeO) to MgO versus silica in Mars rocks and soils. A diagonal line separates tholeiitic (TH) from calc-alkaline (CA) rocks tholeiitic magmas are anhydrous and calc-alkaline magmas are hydrous. All the Mars rocks plot in the tholeiitic field, as do GRS data (MgO was calculated from the Mg/Si ratio in Fig. 11.19). TES-derived data plot in the calc-alkaline field, but this is an artifact of alteration. After McSween et al. (2009). 

Unlike the Moon, Mars has clearly had water on its surface, at least in the distant past. The amount of water in the Martian interior, which was presumably delivered to the surface by volcanic outgassing, has been controversial. Wanke and Dreibus (1988) proposed that Mars 

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Illustration of the different ages and thicknesses of major parts of the Martian crust. 

How does the chemical composition of the Martian crust differ from that of the Moon ... 

A cold early climate and a thin CO2 atmosphere are consistent with the ubiquity of primary igneous minerals and the apparent absence of secondary minerals and copious carbonates on the surface. Aqueous eruptions introduced soluble chloride and sulfate salts into Martian soils, leaving clays and carbonates within the crust. If the Martian crust hosts a deep biosphere, aqueous eruptions could bring organisms or their chemical signature to the surface. The biotic component of any recent eruptions may be preserved in transient ice in cold traps on the surface. 

Singer, R. B. McSween Jr, H. Y. (1992) The composition of the Martian crust Evidence from remote sensing and SNC meteorites. In Resources of Near-Earth Space. (J. S. Lewis, ed. Univ. Arizona Press), in press. 

It is likely that the ancient crust of Mars is more mafic than the Earth s continental crust. Pervasive andesite may signal crustal fractionation, but the identity and significance of andesitic rocks is disputed. The martian crust is relatively more voluminous than the Earth s cmst, perhaps because it is not recycled. The cmst is characterized by high concentrations of incompatible hthophile elements, but fractionations are not as extreme as in terrestrial continental cmst, which has experienced repeated partial melting events over a protracted geologic history. 

Taylor G. J., Boynton W., Hamara D., Kerry K., Janes D., Keller J., Feldman W., Prettyman T., Reedy R., Bruckner J., Wanke H., Evans E., Starr R., Squyres S., Karunatillake S., Gasnault O., and Odyssey GRS Team (2003) Evolution of the martian crust evidence from preliminary potassium and thorium measurements by Mars Odyssey gamma-ray spectrometer. Lunar Planet Sci. XXXIV, 2004. The Lunar and Planetary Institute, Houston (CD-ROM). 

Valles Marineris—The giant Martian canyon, located in a place of numerous rifts and faults in the Martian crust. This canyon would stretch across the entire continental United States. 

Until recently it was thought that the Earth was unique amongst the terrestrial planets in possessing a chemically evolved, granitic, continental crust. We now know that, although the Martian crust is predominantly basaltic, felsic rocks are present in the Syrtis Major caldera on Mars (Christensen et al., 2005). This new discovery is not unexpected and raises interesting parallels between the surface of Mars and the Earth s earliest crust. 

Mass spectrometers were an integral component in the scientific payload of Viking Landers 1 and 2 and were designed to measure the composition and structure of Mars upper atmosphere. Carbon dioxide is the major constituent of the atmosphere, whilst the isotopic composition of C and O in the Martian atmosphere is similar to that of the terrestrial atmosphere. However, is enriched in Mars atmosphere by approximately a factor of 1.6. Certain meteorites have been identified as samples from the Martian crust. These SNC meteorites, including the orthopyroxenite ALH84001, on which claims for the evidence of life on Mars have been based, have produced a large quantity of isotopic evidence which has been used in conjunction with the Viking Lander data. Since meteorites are delivered to our doorstep. 

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