Laboratory spectral VNIR studies supporting VIRTIS’ nucleus surface composition analysis of 67P/CG and prospects for future observations

Abstract

The Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) onboard Rosetta provided 0.25-5.2 μm spectra of 67P/CG (Capaccioni et al., 2015) enabling a nucleus surface composition analysis. Thermally and photometrically corrected reflectance spectra display low geometric albedo, different red VIS and IR spectral slopes, and a complex broad absorption band centered at 3.2 μm. The low albedo and the structure of the ubiquitous 3.2 μm band suggest that the cometary surface is homogeneously enriched in refractory organic materials and darkening phases (Capaccioni et al., 2015). Detailed studies of laboratory analogue materials are necessary to disentangle different interfering factors that influence the appearance of reflectance spectra. Thus, such investigations are indispensable for reliable compositional analysis of remote sensing data. Here we present 0.25-5 μm laboratory reflectance spectra of well-characterized terrestrial hydrocarbon materials (solid oil bitumens, coals) and discuss their relevance as spectral analogues for 67P/CG refractory organics. In addition, the possible contribution of inorganic matter to spectral characteristics of 67P/CG should be understood based on laboratory reflectance measurements. Although a wide range of silicate compositions was found in cometary dust and anhydrous IDPs of presumably cometary origin, Mg-rich crystalline mafic minerals are dominant silicate components. A large fraction of silicate grains are Fe-free pyroxenes and olivines (forsterites) that are not found in terrestrial rocks but can be synthesized in order to provide a basis for laboratory studies and comparison with VIRTIS data. We report the results of the synthesis, analyses, and/or spectral reflectance measurements of Fe-free low-Ca pyroxenes (ortho- and clinoenstatites), forsterites, and a high-Ca pyroxene (diopside). Finally, fine-grained opaque refractory phases (e.g. iron sulfides, Fe-Ni alloys) are likely responsible for the low IR reflectance and low contrast of the 3.2 μm absorption band. Therefore, we present and discuss 0.25–5 μm reflectance spectra of pure Fe-sulfides (troilites, pyrrhotites) and their intimate mixtures with organic and inorganic analogue materials. Based on the reported laboratory data, we discuss the ability of iron sulfides to suppress absorption bands of other cometary refractory components and to affect the spectral slopes and reflectance values of the 67P/CG surface at different wavelengths from the near-UV to the IR. Summarizing these results, we discuss some prospects for future spectral studies.