The Importance of Mars Samples in Constraining the Geological and Geophysical Processes on Mars and the Nature of its Crust, Mantle, and Core.

Abstract

In situ compositional and mineralogical measurements on the Martian surface, combined with analyses of Martian meteorites, indicate that most igneous rocks are lavas and volcaniclastic rocks of basaltic composition and cumulates of ultramafic composition [1]. Alkaline rocks are common in Early Hesperian terranes and tholeiitic rocks dominate younger Amazonian martian meteorites [1]. Very uncommon feldspathic rocks represent the ultimate fractionation products, while granitoid rocks have not been identified [1]. The impact-driven delivery mechanism for the Martian meteorites [2] biases in favor of more competent samples – young, igneous rocks [e.g., 3] – and against rocks that are more representative of the Martian crust [e.g., 4]. Comparisons of rock types found among the meteorites to those documented by landed missions demonstrates this bias unequivocally [1]; furthermore, of the over 100 martian lithologies represented by the martian meteorites, only one (NWA 7034 and pairs) is a regolith breccia [e.g., 1, 5]. While the meteorites provide important insights into the nature of the silicate portion of Mars, including the origin of mantle components with differing geochemical characteristics [e.g., 6], they do not provide information on the composition of the original crust Mars, nor the nature of the mantle sources from which rocks at the Martian surface have been derived (e.g., igneous rocks at Gusev and Gale craters). Thus, there is much to be learned from the study of carefully selected samples from the martian surface.