Composition of Lunar Crater Central Peaks: Integrating Thermal IR and Near-IR Spectral Measurements

Details Composition of Lunar Crater Central Peaks: Integrating Thermal IR and Near-IR Spectral Measurements Composition of Lunar Crater Central Peaks: Integrating Thermal IR and Near-IR Spectral Measurements

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13d1693s&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P13D-1693

Other

[1026] Geochemistry / Composition Of The Moon, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

This is an ongoing study of lunar complex craters between -30:30 degrees latitude. The main objective for this study is to examine the central peaks of these craters using thermal emission spectra from the Diviner Lunar Radiometer on board the Lunar Reconnaissance Orbiter. Diviner has 3 spectral filters near 8 microns (7.81, 8.28, 8.55) which are used to derive the wavelength location of the Christiansen Feature (CF). The CF occurs as an emissivity maximum located just shortward of the fundamental molecular vibration bands, making it sensitive to the bulk composition of lunar soils. Bulk composition derived from Diviner will be compared with more specific mineralogy identified using VIS-NIR spectra from Kaguya lunar orbiter's Spectral Profiler instrument. Based on several lines of evidence and the crystallization sequence of the early lunar magma ocean, the lunar crust is believed to become more mafic with depth. Previous studies of complex craters attempt to sample composition sourced from a range of depths in the crust using near-IR spectroscopy of central peaks. These studies have reported the presence of mafic minerals in most central peaks (e.g. Pieters 1986; Tompkins & Pieters, 1999). Quantitative radiative transfer modeling of Clementine multispectral imaging located some mafic and ultramafic peaks (Cahill and Lucey, 2009). The standout crater in these works has been the large relatively fresh crater Copernicus, where the central peaks were interpreted to be extremely olivine-rich. Conversely, CF position indicates the presence of about 50% feldspar, consistent with prior suggestions that Copernicus exposes troctolite. We have also found convincing evidence of substantially more mafic olivine exposures in the walls of Copernicus. Because Diviner is sensitive to the ratio of feldspar to mafic mineral abundances, documenting the CF of central peaks should be a sensitive test of the hypothesis that the lunar crust is increasingly mafic with depth. We have been analyzing the CF in a wide variety of central peaks, and thus far have found no peaks with CF positions consistent with ultramafic compositions. All these locations must contain substantial feldspar - roughly at least equal in abundance to the mafic component. We will integrate the results of radiative transfer models of central peaks using the new NIR data from Kaguya with the CF results to arrive at improved solutions. Our approach is to emphasize the strengths of the NIR methods by using the model results to quantitatively characterize the mafic mineralogy of a location. We'll then use this result to refine the estimate of feldspar to mafic ratio. In addition to composition, other physical properties have been shown to shift the wavelength location of the CF, such as the degree of space weathering and anisothermality. We're in the process of understanding the underlying physics of these effects and quantifying their magnitude, however there may be a need for more complex modeling to determine how they behave when compounded together on a highly inhomogeneous central peak.

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