Mercury: Mid-infrared (3-13.5 μm) observations show heterogeneous composition, presence of intermdiate and basic soil types, and pyroxene

The Aerospace Corporation's broadband array spectrograph system (BASS) mounted on the NASA infrared telescope facility (IRTF) on Mauna Kea, Hawaii was used to obtain spectral measurements of Mercury's thermal emission on 1998 March 21 (45-85° longitude), and on 1998 May 12 (68-108° longitude). The spectra show heterogeneous composition on Mercury's surface between longitudes 45-85° and about 68-108°. These observations include measurements from 3 to 6 μm, a spectral region not previously covered by mid-infrared spectroscopy. Excellent quality data were obtained in the atmospheric windows between 3-4.2 and 4.6-5.5 μm. These wavelength regions exhibit high emissivity characteristic of a regolith with strong thermal gradients maintained in a vacuum environment with spectra dominated by grain sizes of ∼30 μm. Emission peaks are present at 3.5 and 5 μm in the 45-85° longitude data. The 5 μm peak has been tentatively attributed to clinopyroxene. Data were also obtained in the 7.5-13.5 μm spectral region. Spectra obtained during both observing periods show well-defined emissivity maxima (EM) in the spectral vicinity (between 7.7 and 9.2 μm) of the Christiansen frequency of silicate soils. The location of the EM for longitudes 45-85° (7.9 μm) is consistent with a surface composition of intermediate SiO₂ content. The overall spectral shape is similar to that obtained previously at the same location with different instrumentation. In the region 68-108° longitude, three EM are observed at 7.8, 8.2, and 9.2 μm, indicating the presence of distinctly different surface composition from the other location. Comparisons of these data to other mid-infrared spectra of Mercury's surface and asteroids, and of the different instrumentation used in observations are included.