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

Icarus 180 (2006) 281–291 /locate/icarus The 0.7–5.3 μm IR spectra of Mercury and the Moon: Evidence for high-Ca clinopyroxene on Mercury J. Warell a,? , A.L. Sprague b , J.P. Emery c , R.W.H. Kozlowski d , A. Long d a Institutionen f?r Astronomi och Rymdfysik, Uppsala Universitet, Box 515, SE-751 20 Uppsala, Sweden b Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721, USA c NASA Ames Research Center/SETI Institute, Moffet Field, CA 94035, USA d Physics Department, Susquehanna University, Selinsgrove, PA 17870, USA Received 5 April 2005; revised 4 August 2005 Available online 28 November 2005 Abstract We present infrared spectra of Mercury and the Moon in the wavelength range 0.7–5.3 μm 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 μm. The spectra of Mercury from longitude 170 ? and north and south mid-latitudes display a 1-μm 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 μm thermal infrared region, but no narrow features in this spectral range. The longitude 260 ? spectrum shows excess thermal emission around 5 μm 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