Spectral characteristics of basalts in the VNIR and MidIR: what we could learn integrating data from VIHI and MERTIS the spectrometers onboard BepiColombo

Kurzfassung

One of the primary objectives of the MESSENGER (NASA) mission, presently orbiting Mercury, and the following BepiColombo mission (ESA and JAXA joint project) is the determination of the surface mineralogy of the hermean planet. To this aim MASCS (Mercury Atmospheric and Surface Composition Spectrometer, on MESSENGER; McClintock & Lankton,2007), will analyze the reflectance in the 0.2 to 1.4 m wavelength range, whereas the BepiColombo spectrometers VIHI (Visible-near Infrared Hyperspectral Imager) will operate in a wider range from 0.4 to 2.0 m and MERTIS (Mercury Radiometer and Thermal infrared Imaging Spectrometer) will measure emissivity spectra from 7 to 14 m (Helbert et al.,2005; Sgavetti et al.,2007; Flamini et al.,2010). Mercury has the largest range in diurnal temperature in the solar system, varying from -173 to 430°C. This extreme temperature excursion is expected to affect the physical processes responsible for the spectral features of minerals and rocks. Recently Helbert&Maturilli (2009) highlighted that the spectra of a labradorite sample shows significant changes in spectral features in the mid-infrared with changing temperature. This result suggests that the surface temperature and the thermal history of an observed area must be taken into account in the interpretation of the spectra from an extreme planet such as Mercury. The new MESSENGER’s observations have documented that volcanism was important in shaping the surface of Mercury (Head et al.,2009). These new data have shown numerous volcanic vents, in particular around the interior of the edge of the Caloris basin, interpreted as expression of effusive volcanism. A possible shield volcano (100 km in diameter) was suggested, as well as pyroclastic eruptions represented by bright haloes around the vents, particularly distinctive in color data (Head et al.,2009). Moreover the MDIS (Multispectral Dual Imaging System) multispectral images revealed differences in color and thus, possibly, composition of Mercury’s crust indicative of lateral and vertical heterogeneities (Denevi et al.,2009). Here we describe the integrated approach aimed at the spectral characterization of basalts in VNIR reflectance and in the MidIR emissivity, trough accurately inter-calibrated data from different laboratories. We report the preliminary results of a study focused on two basaltic samples from an Etna lava flow collected in different vertical positions within the flow. These samples present similar bulk-rock compositions and mineral phases associations, but different textures, with different degrees of crystallinity, due to the different conditions of crystallization from the inner portion of the lava flow (that has a slower cooling) to the surface (faster cooling, with the formation of a glassy-microcrystal groundmass). Mineral composition has been analyzed by microprobe to relate the compositional characteristics to the absorption band parameters. The samples were spectrally characterized at different grain-sizes and the emissivity measured at three “hot” temperatures. The VNIR spectra of fine powders reveal a clear absorption band at 1 m compatible with the presence of pyroxene and olivine (Burns,1993) for the holocrystalline sample, on the contrary the hyalopilitic sample show a lower albedo and almost featureless spectra. The MidIR emissivity of fine powders show always a CF position at 7.7 m, and a similar absorption structure between the CF and 12m can be observed for both samples. These preliminary results indicate that an integrated approach can provide a better quantitative determination of the minerals and of their abundances, and will be extremely beneficial in the interpretation of our remote sensing data. The two instruments provide complementary information on composition and texture, which will allow, for instance, to separate different portion of a volcanic lava field with different rock texture and also to improve the interpretation of the surface composition.