Mercury Surface in the Planetary Emissivity Laboratory (PEL): Preparing for MERTIS on BepiColombo

Abstract

MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer) is one of the scientific payloads of the ESA deep space mission BepiColombo. MERTIS is an imaging spectrometer obtaining hyper-spectral data in the thermal IR (TIR) wavelength range (7-14 µm) with a medium spatial resolution to determine the mineralogical composition of the Mercury’s surface. Sharing the same optical path a pushbroom micro-radiometer is integrated allowing measurements of the Mercury surface temperature and obtaining the thermal inertia of the regolith. MERTIS will measure the thermal radiation emitted from the hot Mercury surface, and by means of internal calibration target measurements, those data will be calibrated to surface emissivity spectra. The interpretation of surface emissivity spectra can be successfully carried out only with the support of a wide database of emissivity measurements on analogue materials. At the Planetary Emission Laboratory (PEL) of DLR we measure emissivity of powdered or bulk materials in vacuum, at typical Mercury diurnal temperature [1, 2]. The Bruker VERTEX 80V Fourier Transform Infrared-Spectrometer (FTIR) can be operated under vacuum to remove atmospheric features from the spectra. An external evacuable chamber is used to measure emissivity: it contains a motor-driven carousel for up to 12 samples in the same working session, thermal sensors for each sample and a webcam for monitoring the activities inside the chamber. Induction heating system allows heating up the samples (in stainless steel cups) from 50°C to higher than 800°C. Bruker A513 accessory is used to obtain bi-conical reflectance with variable incidence angle i and emission angle e between 13° and 85° at room temperature, under purge or vacuum conditions, in the 1 to 100 µm spectral range [3]. A Bruker IFS 88 FTIR spectrometer is operated with a Harrick SeagullTM variable-angle reflection accessory to measure bi-conical reflectance at room temperature, under purging conditions in the extended spectral range from 0.4 to 55 µm, for incidence=emission angles from 5° to 85° [3]. Reflectance measurements in the TIR wavelength range are used for comparison (mostly on band positions) with emissivity spectra under Mercury conditions. Reflectance spectra of thermally processed Mercury analogues in the visible and near infrared region [3] have been used for the interpretation of spectra collected from MASCS spectrometer on NASA MESSENGER mission [4].