Laboratory studies of Mercury analogue materials: optical spectroscopy and space weathering simulation experiments

Abstract

MERTIS (Mercury Thermal Infrared Imaging Spectrometer) is a part of ESA's BepiColombo mission payload and will map Mercury's surface from 7 to 14 µm with high spatial resolution. To support MERTIS and for cross-calibration with other instruments onboard BepiColombo (SYMBIO-SYS) and MESSENGER (MASCS) spacecraft, we compiled a list of Mercury analogue materials (Helbert et al., 2007, Adv. Space. Res. 40, p. 272-279). The list currently includes plagioclase and potassium feldspars, low Ca and high Ca pyroxenes, forsterite, elemental sulfur and an Apollo 16 lunar highland soil. For the spectral studies we prepared size fractions of ¡25, 25-63, 63-125, and 125-250 µm (except for the lunar soil). The samples were characterized in terms of chemical composition, mineralogy, and grain size distribution. Here we report the results of spectral reflectance measurements of these analogues from 0.5 to 18 µm. We compare the TIR reflectance spectra of the samples to their emissivity spectra from Berlin Emissivity Database (Maturilli et al., 2008, Planet. Space. Sci. 56, p. 420-425) to evaluate deviations from Kirchhoff's law for biconical reflectance. In addition, we report on a space weathering simulation experiment on plagioclase, and discuss future laboratory studies needed to support space missions to Mercury.