Comparison of The Christiansen Feature Position and Lunar Iron: Evidence for Space Weathering Effects

Diviner’s three channels near 8-microns characterize an infrared emissivity maximum called the Christiansen Feature (CF) which is has been established as compositional indicator in laboratory experiments. Laboratory measurements show that the CF is related to silicate polymerization and occurs at shorter wavelengths for feldspathic minerals and longer wavelengths for mafic minerals. Laboratory experiments regarding the effect of space weathering on this feature have not been conclusive. The Diviner Lunar Radiometer Experiment data begin to illustrate these effects. Because the hosts of iron in lunar materials are the mafic minerals pyroxene and olivine (with the rare ilmenite-rich maria being the chief exception), the CF wavelength position, roughly proportional to mafic mineral abundance, should correlate with iron abundance. Deviations from this correlation will be due to variations in mineral chemistry, olivine-pyroxene ratio, and possibly, space weathering effects. Using Clementine-derived global FeO estimates, calibrated to Lunar Prospector gamma-ray spectrometer iron data, we regressed iron on a global CF map with 12 km spatial resolution. The residuals of the difference between the FeO based CF prediction and the CF measurements show differences we attribute to rock and soil composition, but also, in the highlands, the effects of space weathering. The residual image (CF predicted from FeO, minus CF measured by Diviner) correlates reasonably well with optical maturity derived from visible and near-IR measurements. Using laboratory soils, optical maturity has been shown to be a reliable maturity indicator. Large fresh craters, including Tycho and Jackson, as well as a host of lesser craters, that show bright ray patterns but are not compositionally distinct from the background, show both CF and optical maturity anomalies. Specifically, fresh highland craters show shorter wavelength CF positions than mature highland background The origin of the CF dependence upon soil maturity is not known. Optical maturity is dominated by the abundance of nanophase iron that causes darkening, reddening, and loss of spectral contrast with space exposure. It is not clear how the CF, being a wavelength position parameter, could be affected by nanophase iron. However, soil maturity includes a host of other effects including prodigious production of glass, grain size evolution, and implantation of solar wind gases. At the 12km scale at which we are measuring there could also be macroscopic, but sub-pixel effects as well, such as variations in the rock-soil ratio. Further investigation of the Diviner data and supporting laboratory measurements will be used to understand this phenomenon.