MARS MINERALOGY: CONTEXT FROM SPACECRAFT AND METEORITES

The martian surface is dominated by minerals characteristic of mafic igneous rocks, and volcanic landforms are common. Global scale mineralogy has been mapped from orbit by the MGS spacecraft using thermal emission spectrometry (TES). Major surface units of the ancient crust consist of pyroxenes and plagioclase, with varying proportions of olivine and alteration minerals. Quartz is known from only one location. Because of spectral ambiguities, controversy exists about the interpretation of andesite (as seen at the Mars Pathfinder landing site) versus chemically weathered basalt. Materials spectrally interpreted to be weathered basalts are restricted to high latitudes where interactions with ice may have occurred. The martian regolith consists of a globally homogenized(?) dust having basaltic composition, with admixed local rock and salt components.

Martian (SNC) meteorites generally have young (<1.3 Ga) crystallization ages and represent later additions to the crust. They are basaltic or cumulate rocks composed of pyroxenes, olivine, plagioclase (or alkali feldspar), oxides, sulfides, phosphates, and silica. Rocks with similar spectra are not recognized in TES mapping, suggesting that SNCs were derived from equatorial volcanic terrains obscured by dust. Small amounts of secondary minerals (clay, carbonate, sulfate, halide) in martian meteorites formed by reaction with subsurface fluids.

Gusev Crater rocks analyzed by Spirit are picritic basalts, having compositions distinct from TES surface units but similar mineral compositions. These rocks expand the known compositional range of magmatic rocks in the martian crust. Surface rinds and veins confirm that some martian rocks have been altered by fluids. One Meridiani rock analyzed by Opportunity is similar in composition and mineralogy to shergottites. Sulfate outcrops at Meridiani suggest that salts found in soils at all landing sites could be an admixed global evaporite component rather than local cements.