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

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

Publication date: 15 February 2006

Authors: Warell, J. et al.

Journal: Icarus
Volume: 180
Issue: 2
Page: 281-291
Year: 2006

Copyright: Elsevier

We present infrared spectra of Mercury and the Moon in the wavelength range 0.7-5.3 mum obtained with the SpeX spectrograph at the NASA Infrared Telescope Facility. The spectra were acquired from pole and terminator locations of Mercury's surface 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 the same manner with identical calibration stars to minimize differences in the reduction process. The Copernicus spectra display the expected absorption features due to mafic minerals in the near infrared and show spectral features in the SiO combination/overtone vibrational band region above 4 mum. The spectra of Mercury from longitude 170° and north and south mid-latitudes display a 1-mum absorption band indicative of high-Ca clinopyroxene, while a spectrum from longitude 260° and northern mid-latitudes does not. The Mercury spectra show a broad feature of low emittance over the full 3 5 mum thermal infrared region, but no narrow features in this spectral range. The longitude 260° spectrum shows excess thermal emission around 5 mum attributable to the existence of a thermal gradient in the insolated dayside regolith. The thermal-IR spectra suggest a 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 near infrared wavelength range provides more discriminative information on the surface composition than the 2 4 mum region, where the solar reflected and thermally emitted radiation contribute approximately equally to the observed flux of these bodies.

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