Emissivity of Powdered Silicates in TIR Spectral Range (7-14 µm) Under Simulated Daytime Surface Conditions of Mercury and Their Detection from the Orbit

Kurzfassung

Introduction: The mid infrared (MIR) spectral region is especially sensitive to the abundance of Si-O, unlike the visible-near infrared spectral region. Though the geochemical suite on the NASA MESSENGER spacecraft to Mercury revealed compositionally diverse crustal materials [1], the spectrometer suite (MASCS; VIS-IR) could not reveal the silicate mineralogy of crustal materials due to the Fe2+-poor nature of the silicate minerals on the surface of Mercury. On October 20, 2018, ESA/JAXA’s BepiColombo mission was successfully launched to Mercury. MERTIS (Mercury Radiometer and Thermal Imaging Spectrometer) onboard BepiColombo will be the first thermal infrared (TIR) hyperspectral imager (7 – 14 μm) and radiometer (7 – 40 μm) to orbit Mercury mapping global spectral emissivity and surface temperature of Mercury at a spatial resolution of ~500 m/pixel and ~2 km/pixel respectively [2]. MERTIS will therefore provide spatially resolved information on mineralogy of various geological terrains including hollows and pyroclastic deposits, rock abundance, grain size, thermal inertia, and surface temperature [3]. Studying the thermal emissivity measurements of silicates at Mercury surface temperatures up to 450°C and under vacuum will help us to create the standard spectral library for MERTIS data analysis. Sample selection and preparation: Over a decade, the Planetary Spectroscopy Laboratory in the Department of Planetary Laboratories at the Institute for Planetary Research, DLR, Berlin has been undertaking huge efforts in collecting natural silicate endmembers from various sources in preparation to MERTIS data science [4]. These silicates are suggested by groundbased observations of Mercury and indirect mineralogy derived from NASA MESSENGER geochemistry suite [e.g., 1,5] and they include; a) olivine: forsterite, b) pyroxenes: enstatite, diopside, c) plagioclase feldspar: hypersthene, anorthite, labradorite, andesine, oligoclase, orthoclase, and d) nepheline. Here we present the emissivity of these silicates (at grain size of <25μm) at 7-14 μm as a function of temperature under vacuum conditions. Facility and Methods: A Bruker Vertex 80V instrument with MCT HgCdTe detector (cooled by liquid nitrogen) and KBr beamsplitter is used at PSL to measure the thermal infrared (TIR) emission spectra of the samples. This spectrometer is attached to an external chamber where the samples are placed in steel cups which are heated up to Mercury’s peak daytime temperatures via induction technique under vacuum (Fig. 1). Each sample is heated from 100° to 500°C (step 100°C) at medium vacuum (0.7 hPa) and then cooled down in vacuum. Radiance from the heated samples is collected by a gold (Au) coated 90° off-axis parabolic mirror and reflected into the spectrometer. It samples the thermal emission spectra of the silicates at wavelength intervals of 7-14 μm at spectral resolution of 4 cm-1 (Fig. 1). The spectra are calibrated following the standard PSL calibration procedure. A blackbody target with a known emissivity spectrum is measured at the same geometry and temperature as the samples. Emissivity is derived by dividing the sample signal by the blackbody signal correcting for the emissivity spectrum of the calibration target. Figure 2 shows the resulting emissivity measurements at temperatures of 100°C, 200°C, 300°C, 400°C, and 500° C