The emissivity of a fine-grained labradorite sample at typical Mercury dayside temperatures

Analyzing the surface composition of Mercury's regolith from remote-sensing measurements is a challenging task. In support of the National Aeronautics and Space Agency's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission and especially in preparation for the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) instrument on the BepiColombo mission of the European Space Agency and the Japan Aerospace Exploration Agency, we are developing a Planetary Emissivity Laboratory at Deutsches Zentrum für Luft-und Rahmfahrt (DLR) in Berlin. Here we present the first measurements of labradorite as a Mercury surface analog obtained with a test setup at a temperature of 420 °C, appropriate for the dayside of Mercury. Because of limitations on the current test apparatus, we present here data only for mid-infrared wavelengths. The spectra show strong indications of significant changes in spectral features in the mid-infrared with changing temperature. This result introduces an additional complication to the analysis of spectra obtained from the surface of Mercury, as the surface temperature and the thermal history of an observed area must be taken into account in the interpretation of the spectra. This inference implies that the latitude, and possibly also the time of day, must be considered when analyzing a spectrum. In particular, the surface at the equator might exhibit different spectral characteristics than the surface at high latitudes even for the same composition. These observations, while still preliminary, underscore strongly the need for temperature-dependent measurements of emissivity of candidate planetary surface materials.