GLOBAL MINERALOGIC AND PETROLOGIC MAPPING OF MARS: TWO DECADES OF SPECTROSCOPIC OBSERVATIONS

One of the key observations in the exploration of Mars has been the global reconnaissance of the mineralogy and petrology of the surface materials. These data have provided the context for an understanding of the geologic history of Mars that is based on the igneous, sedimentary, and chemical alteration processes that have occurred. High-resolution mineral mapping began with the near-infrared observations from the ISM spectrometer on the Phobos mission, and has continued through the global mapping from the Mars Global Surveyor TES thermal IR spectrometer, the Odyssey THEMIS multi-spectral thermal infrared imager, and the Mars Express OMEGA and Mars Reconnaissance Orbiter CRISM near-IR imaging spectrometers. What these instruments have revealed is that Mars is fundamentally a volcanic planet that is dominated by basalt, but includes a broad range of compositions that varies from olivine-rich ultra-mafic basalts to andesites, dacites, and even granitoids. The minerals identified in these igneous terrains include low- and high-Ca pyroxenes, olivine, plagioclase feldspar and high-silica phases. These igneous rocks have varying plagioclase/pyroxene ratios, pyroxene composition, and high-silica component abundance, and occur in broad regional zones, suggesting spatial/temporal differences in formation processes that include differing degrees of fractional crystallization, assimilation, source region composition, and olivine fractionation. Superimposed on this volcanic background are rare, but significant, deposits of sedimentary and chemical precipitate rocks containing sulfates, crystalline iron oxides (hematite), and phyllosilicates. Together, these minerals indicate that water was present locally in significant abundance. Finally, carbonate minerals have been found, not in bedrock outcrops, but as a minor (2-3%) component of the ubiquitous martian dust. These carbonates most likely formed through interaction with the martian atmosphere. Taken together, this global mapping has discovered a large, and growing, suite of minerals on Mars, played a major role in the selection of landing sites, and formed the global context for interpreting martian geologic history.