Most rock-forming minerals have distinctive absorptions in the thermal infrared portion of the spectrum (roughly 5-50 Êm). This useful property is the primary motivation for sending a series of thermal infrared instruments to Mars: The Thermal Emission Spectrometer on the Mars Global Surveyor, the Thermal Emission Imaging System on the Mars Odyssey, and the Miniature Thermal Emission Spectrometers on the two Mars Exploration Rover spacecraft. The massive volume of data returned from these instruments has provided the means to determine the global and local scale mineralogical compositions of Mars. A cornucopia of mineral identifications is an end result for the spacecraft data; however it is the start of a series of questions about the geologic history of Mars and its development over time. Two broad questions that the Martian surface mineralogy addresses are: 1) What is the timing and extent of igneous processes on a planet that does not have plate tectonics? and 2) To what extent has liquid water interacted with the Martian surface and subsurface?
The Martian surface displays a striking hemispheric dichotomy in surface composition. Basalts or basaltic andesites similar in bulk composition to those commonly found on Earth dominate the southern highlands. A more silica rich surface that closely matches typical terrestrial basaltic andesites or andesites is present everywhere on the planet, though the highest concentrations are found within the younger northern lowlands.
Though this dichotomy may be explained by two different igneous compositions, the high-silica glass in the more silicic composition could instead be a secondary mineral related to an alteration process rather than volcanism. Regardless of the source of the glass, Martian meteorite compositions are not typical of much of Martian igneous compositions. Unaltered compositions are present in the Martian soils, dust, and rock, including extensive exposures of olivine without any associated sign of chemical alteration. While trace carbonates are present in the dust and the planetfs reddish hue itself are indicative of alteration, there are few definitive indications of large amounts of chemical weathering. There are also few indications of extensive water-rock interactions with the possible exceptions of hematite exposures in Sinus Meridiani, Aram Chaos, and Valles Marineris.
There is still much to glean from these datasets about the variety of surface compositions on Mars. A picture is emerging that Mars is perhaps more interesting than expected from an igneous perspective and perhaps its climate history has been cold and stagnant. This depiction will be continuously tested as new measurements and data pour in from existing and future Martian spacecraft.