Mid-infrared reflectance spectroscopy of synthetic glass analogs for mercury surface studies

Morlok A, Renggli C, Charlier B, Reitze MP, Klemme S, Namur O, Sohn M, Martin D, Weber I, Stojic AN, Hiesinger H, Joy KH, Wogelius R, Tollan P, Carli C, Bauch KE, Helbert J

Abstract

We have synthesized and analyzed by mid-infrared reflectance spectroscopy silicate glasses that are representative for the glasses on the surface of Mercury. The glasscompositions are based on high-pressure laboratory experiments and the resulting compositions of the glass phase. The spectra are of interest for investigating thesurface of Mercury using the MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) instrument on board of the ESA/JAXA BepiColombo mission.Both powdered fractions and polished blocks have been analyzed. Powdered size fractions of 0–25 μm, 25-63 μm, 63-125 μm, and 125–250 μm were measured inreflectance in the thermal infrared (2–20 μm). Spectra for powdered bulk glasses (1.6 wt% - 19.0 wt% MgO) show a single, dominating Reststrahlenband (RB,9.3–9.8 μm), a Christiansen Feature (CF; 7.6 μm - 8.1 μm), and a size dependent Transparency Feature (TF; 11.6–11.9 μm). Micro-FTIR analyses of polished blocks ofglasses (3.4–26.5 wt% MgO) have characteristic bands at 7.8–8.2 μm (CF), and 9.3–9.9 μm (RB). Only few olivine crystalline features were observed.Spectral features correlate with compositional characteristics, e.g. SiO2 content or SCFM (SiO2/(SiO2 + CaO + FeO + MgO) index. The strongest correlationbetween band features CF and the strong RB are with Mg/Si. No simple mixture of glass spectra from this study is able to reproduce the entire ground based spectrumof the surface of Mercury. However, Mg-rich glasses reproduce identified features at 8.5 μm, 9.9 μm and 12.4 μm.