Ground-Based IR Spectroscopy of Mercury for Composition

Remote sensing measurements of the Moon and Mercury in the thermal infrared have been plagued with problems due to atmospheric absorption and other effects. Controversial results have been discussed by Nash et al. and by Salisbury et al.. We have developed a technique to obtain thermal infrared spectra of Mercury together with solar and sky spectra in the 8-13 micrometer region suitable for determining the presence or absence of mineralogical features. High-resolution (0.05 cm(exp -1) between data points) spectra were obtained during daylight using the Fourier Transform Spectrograph (FTS) at the McMath-Pierce Solar Telescope Facility at Kitt Peak National Observatory, Arizona. The reduced data were independent of any model of the transmission of the Earth's atmosphere and rest on a calibration using sky spectra along the same line-of-sight through the atmosphere as the actual Mercury raw spectrum. This method can be applied at all solar elongation angles including small elongation angles (5 deg or less). The basic sequence and the technique are robust.