Midinfrared spectra of Mercury


  • Bonnie Cooper,

  • Andrew Potter,

  • Rosemary Killen,

  • Thomas Morgan


Observations of Mercury in the mid infrared (8–12.5 μm region) obtained over a variety of longitudes using the Fourier Transform Spectrometer (FTS) on the McMath-Pierce 1.5 m solar telescope at Kitt Peak reveal low-contrast spectra, except in cases where atmospheric noise or difficulties in guiding the telescope caused spurious signals. Although there are features that are suggestive of emissivity maxima (Christiansen features), their proximity to the telluric absorption band, and the noise in the spectra, preclude their unambiguous interpretation. We see little evidence of reststrahlen bands in our data; however, there are strong indications of transparency features. A shallow emissivity minimum occurs at 12 μm in the spectra centered at 80°, 256°, and 266° Hermean longitudes. A minimum occurs at 12.5 μm in the spectra centered at 15°, and a doublet minimum, with one trough at 12.2 μm and a second trough at 12.4–12.6 μm, is seen in the spectra centered at 229°. These features indicate the presence of a fine powder on the Mercurian surface, and their low spectral contrast suggests a significant percentage of agglutinitic material. On the basis of the position of the transparency features, we conclude that our spectra are indicative of intermediate, mafic, and ultramafic rock types. Further specificity is not warranted by the data.