The Moon: Mid-infrared (7.5- to 11.4-μm) spectroscopy of selected regions

Abstract

Groundbased observations of midinfrared (7.5- to 11.4-μm) spectral radiance from five locations on the Moon are reported. We have obtained medium resolution spectra from three locations in Copernicus crater and two locations in mare basalt. Two locations in the crater walls of Copernicus show similar spectra which in turn differ from spectra from the eastermost central peak. Mineralogic interpretation of the features in the central peak spectra is consistent with the composition of the locations as determined by near-infrared reflectance (0.67- to 2.5-μm) spectroscopy. In addition to emission minima, which we attribute to olivine, there are unidentified features in the spectra which may provide additional compositional information. An emissivity maximum at shorter wavelengths than the reststrahlen bands is identified as the Christiansen feature in the Copernicus spectra. The wavelength of maximum emission shifts to longer wavelengths with increasing mafic composition as predicted by laboratory spectroscopy. The mare locations observed are of different albedos and our spectra indicate that the locations are of slightly different composition. We see a broad emission minimum centered at 8.5 μm in the Kepler ejecta spectrum but not in that from the mare east of Euler. Spectra from both mare locations have a triplet of minima at 8.1, 8.4, and 8.75 μm which are not identified. Spectra from both mare locations exhibit local maxima just longward of 9 μm and a sharp drop in emissivity at wavelengths longer than 11 μm.