Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.1302c&link_type=abstract
American Astronomical Society, DPS meeting #28, #13.02; Bulletin of the American Astronomical Society, Vol. 28, p.1116
Astronomy and Astrophysics
Astronomy
Scientific paper
Little is known about the composition of Mercury's crust. Some interesting results have been obtained recently using thermal infrared spectroscopy. However, a major problem for measurements in the thermal infrared region is correcting for atmospheric absorption. Sprague and coworkers (Sprague et al. 1994; Jeanloz et al., 1995; Sprague et al. 1995) used star spectra and atmospheric models to determine this correction. Their procedure has been criticized by Salisbury et al. (1995). We have developed a technique to obtain thermal infrared spectra of Mercury that is less dependent on atmospheric transmission models. We used a Fourier transform spectrometer to take high resolution (0.05 wavenumber) spectra of Mercury and the Sun during daytime. We then divided the Mercury spectrum by the solar spectrum. Atmospheric absorption effects cancelled in the ratio, to the extent that air masses were similar for the Mercury and Sun spectra. Since the solar spectrum closely approximates a featureless 5400K black body, we can attribute spectral features in the ratio spectrum to Mercury. We performed preliminary measurements in 1987, and more detailed measurements at higher resolution in 1995. The ratio spectra are nearly featureless, except for an emission feature at 11.2 microns that appears in spectra from January 18, 1995 at approximately 5% above background. It appears two days later at a reduced intensity, and has a different shape and appearance in spectra taken in March of 1987. This feature may be the "transparency peak" noted by Salisbury et al. (1995), inverted in emission because of thermal gradients in the surface layer of Mercury (Henderson and Jakoski, 1994). Support is acknowledged from the NASA Planetary Astronomy program and from the McMath-Pierce Solar Telescope of the National Solar Observatory References: Henderson, B.G., and Jakoski, B.M., (1994) JGR, 99, No. E-9, 19063-19073. Jeanloz, R. et al. (1995) Science, 268, 1455. Salisbury, J.W et al. (1995). Icarus, 115, 181. Sprague, A.L. et al. (1994) Icarus, 109, 156. Sprague, A.L. et al. (1995) Icarus, 118, 211.
Cooper Brendan
Morgan Thomas H.
Potter Andrew E.
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