The 0.7–5.3 μm IR spectra of Mercury and the Moon: Evidence for high-Ca clinopyroxene on Mercury

We present infrared spectra of Mercury and the Moon in the wavelength range 0.7--5.3 $\mu$m obtained with the SpeX spectrograph at the NASA Infrared Telescope Facility. The spectra were acquired from pole and terminator locations of Mercury'ssurface and of Mersenius C and the Copernicus central peak on the Moon. Spectra of both bodies were measured in close temporal succession and were reduced in thesame manner with identical calibration stars to minimize differences in the reduction process. The Copernicus spectra display the expected absorption features dueto mafic minerals in the near infrared and show spectral features in the SiO combination/overtone vibrational band region above 4 $\mu$m. The spectra of Mercuryfrom longitude 170$^\circ$ and north and south midlatitudes display a 1-$\mu$m absorption band indicative of high-Ca clinopyroxene, while a spectrum from longitude 260$^\circ$ and northern midlatitudes does not. The Mercury spectra show a broad feature of low emittance over the full 3--5 $\mu$m thermal infrared region, butno narrow features in this spectral range. The longitude 260$^\circ$ spectrum shows excess thermal emission around 5 $\mu$m attributable to the existence of a thermal gradient in the insolated dayside regolith. The thermal-IR spectra suggesta significant difference in the compositional and/or structural properties of Mercury and the Moon that may be due to grain size, absorption coefficient, or the magnitude of near-surface thermal gradients. The results indicate that the composition of Mercury's surface is heterogeneous on regional scales, and that the nearinfrared wavelength range provides more discriminative information on the surfacecomposition than the 2--4 $\mu$m region, where the solar reflected and thermallyemitted radiation contribute approximately equally to the observed flux of thesebodies.