Thermal infrared study of central uplifts of Martian impact craters

Determining the mineralogy of a planetary surface and subsurface is important for understanding crustal evolution and alteration processes that occurred. Impact craters are natural probes into the subsurface. Particularly, the central uplift of an impact crater exposes rocks from the deep subsurface. We present a global spectral study of low-albedo central uplifts from Martian impact craters with diameter larger than 40 kilometers. For the initial part of this study, we focused on analysis of mineralogical composition using Mars Global Surveyor Thermal Emission Spectrometer (TES) data; follow-up work will included analysis of other data sets, including CRISM. There are 81 craters that have high-quality TES data coverage of the central uplift; however we note that the number of spectra covering each central uplift is relatively low, which results in a lower signal to noise level. Considering the small numbers of TES spectra, mineral abundances were not derived for each individual uplift. Rather, the atmospherically corrected spectra derived from each uplift were grouped into 5 classes based on spectral similarity using spectral angle calculations and hierarchical clustering techniques. A linear least squares fitting algorithm was then used to derive the mineral abundance from the average spectrum of each group. Six groups of major minerals which are above the detection limit for TES data are derived for analysis: feldspar, olivine, sulfate, LCP: low-Ca pyroxene (orthopyroxene and pigeonite), HCP: high-Ca clinopyroxene (augite and diopside), high-silica: combined modeled abundance of phyllosilicates, zeolites, opals and glasses. Most craters are located in Noachian or Hesperian terrains, consisting primarily of feldspar and LCP. Class1 is characterized by highest abundance of feldspar, locating in mid- to high-latitude regions. Class2 is characterized by highest abundance of sulfate among all classes. Class3 consists highest abundance of high-silica and lowest feldspar. Class4 is characterized by higher olivine and pyroxene than other classes. Class5 contains the highest abundance of HCP. There is no obvious geographic association of these 5 classes. Comparison with crater diameter shows that only class 5 has a consistent diameter range (40-60 km, which is the small end of the diameter range in our dataset). Detailed investigation of all 81 uplifts using TES, MOLA, THEMIS, CRISM and HiRISE data, as well as comparison to surrounding regolith compositions may help to interpret the bulk composition, distribution and alteration of Martian subsurface or surface materials.