Thermal infrared spectroscopy to investigate the composition of mercury – The MERTIS instrument on BepiColombo Original Research Article

Telescopic measurements have shown that the surface composition of Mercury can be derived only indirectly from VIS-NIR-spectra: the spectral slopes and the absence of any feature in the range from 0.5 to 1 μm indicate feldspar as main component. [Hapke, B., Danielson, G.E., Jr., Klaasen, K., Wilson, L. Photometric observations of Mercury from Mariner 10. J. Geophys. Res. 80, 2431–2443, 1975; Hapke, B. Interpretations of optical observations of Mercury and the moon. Physics of the Earth and Planetary Interiors, vol. 15, no. 2–3, pp. 264–274, 1977.] suggested based on the analysis of albedo and color relationships on Mariner 10 images that very low contents of FeO (at about 3–6 wt%) and TiO2 (<1 wt%) could be present in the mercurian regolith. Feldspars are expected to compose the bulk of Mercury’s crust. In the NIR-range, however, pure feldspars have no specific spectral signature. In the Thermal Infrared (TIR), however, feldspars can be detected–and specified–by means of their diagnostic spectral signatures: Christiansen frequency, reststrahlen band, and transparency feature. In addition, pyroxenes and most other minerals can be detected and specified in this spectral range. Thermal infrared spectroscopy operating in the range between 7 and 14 μm will enable valuable mineral identification of feldspars and low-iron species that are expected to be prevailing on Mercury’s surface. Performing TIR measurements may make it possible to identify spectral features associated with the high radar backscattering efficiency of putative minerals and to differentiate between the proposed compositions (water ice, sulfur, and cold silicate glasses) for the high-latitude volatiles, something that cannot be done by ground based observing or near-infrared spectroscopy. In addition, TIR measurements will allow studying areas of geologic importance where composition can give significant new understanding to relative age of tectonic features. The MERTIS instrument, a TIR imaging spectrometer based on our suggestions, onboard of ESA’s BepiColombo mission will fulfil all the requirements needed to improve our knowledge about Mercury’s surface composition.