Bulk mineralogy of Lunar crater central peaks via thermal infrared spectra from the Diviner Lunar Radiometer - A study of the Moon's crustal composition at depth
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The central peaks of lunar impact craters are thought to be composed of uplifted material originating from varying depths of the crustal column. The crystallization sequence of the early lunar magma ocean resulted in an anorthositic upper crust that becomes progressively more mafic as it approaches the olivine-rich mantle. Emissivity spectra centered around 8 microns from the Lunar Reconnaissance Orbiter (LRO) Diviner Radiometer are used to derive the wavelength location of the Christiansen Feature (CF), which is sensitive to bulk silicate mineralogy. A survey of CF values has been performed for the central peaks of 135 complex craters, providing global and regional observations of the heterogeneity of crustal compositions. Crustal thickness models give context to the pre-impact depth of the central peak material and its proximity to the crust-mantle boundary. The bulk composition results were also compared to central peak FeO abundance and optical maturity parameters (both derived from Clementine Ultraviolet/Visible camera data) to better understand spectral behavior in the 8 µm region due to varying surface properties. This study has identified 6 craters with potentially ultramafic regions on their central peaks. More common occurrences of mafic material, found in a wide variety of crater central peaks, show a silicate composition with a mafic component similar to mare basalt. The compositional distribution and variety of the 135 craters is similar to that of the rest of the Moon, which implies that uplifted material found in the central peaks is not significantly different from the range of compositions found on the lunar surface. Bulk composition of the central peak material does not appear to be correlated with its crustal depth of origin, suggesting both lateral and vertical heterogeneity in crustal composition rather than a gradual transition from felsic to mafic composition. It is likely that the Moon's extensive cratering history has continually overturned the original crust, erasing the stratigraphic sequence caused by cooling of the global magma ocean.