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B. L. Cooper (B. Cooper and Associates), A. E. Potter, Jr. (Lunar and Planetary Institute), R. M. Killen (Southwest Research Institute), T.H. Morgan (NASA Headquarters)
We have developed a technique to obtain thermal IR spectra of Mercury in the 8-13 micron region in daylight [1]. We observed a smooth spectral curve for Mercury, with the exception of minor features near 9.6 microns and 12.7 microns. There was no evidence of a Christiansen feature down to the 8-micron limit of our spectra. This is in general agreement with Mercury spectra reported by [2], who found Christiansen peaks just at, or slightly below, 8 microns.
Features near 9.6 microns are due to incomplete compensation of the ozone absorption band. There appears to be a shallow dip in the spectrum around 12.7 microns. This may represent a transparency peak in emission. Features in the Mercury spectrum have been reported at this wavelength [2], but as a shallow elevation instead of a dip. Most acidic silicates have a feature at around 12.2 microns, whereas mafic minerals (such as hypersthene and olivine) show this feature at longer wavelengths--12.7 to 13.2 microns [3]. The 12.7 micron feature could be evidence for mafic material on the Mercurian surface. However, the lack of a Christiansen feature at 8.5 - 9.0 microns (typical of hypersthene or olivine) makes this doubtful.
The smoothness of the spectra suggests that the spectral contrast of the reststrahlen bands from the Mercury surface is below the signal-to-noise level of our data. The spectral contrast of reststrahlen features is greatly reduced in powdered minerals under vacuum, requiring S/N ratios of the order of 1000 to be detected [4]. Glass formation further reduces spectral contrast by disordering the crystal lattice. Consequently, our results imply that the surface of Mercury is covered by a fine powder, which is probably also glassy as a result of intense meteoroid bombardment [5].
References
[1] Cooper et al. (1998) EOS, 79, no. 24, W59 [abstract] [2] Sprague and Roush (1998) Icarus, 133, 174. [3] Salisbury et al. (1988) USGS Open-File Report 88-686, Reston, Virginia. [4] Salisbury et al. (1997) Icarus, 130, 125. [5] Cintala (1992) JGR, 97, 947.
The author(s) of this abstract have provided an email address for comments about the abstract: bcooper@ghgcorp.com