Mercury's Surface as Seen by MERTIS: Early Insights from Laboratory Analyses on FeO-Free Materials in Comparison to BepiColombo MSB#5 Data

Kurzfassung

The MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) of the BepiColombo mission reached a major milestone on December 1, 2024, by successfully acquiring its first surface measurements in the MIR (7-14 microns) spectral range over a duration of ~30 mins and closest approach of 37268 km, during a high-altitude swing-by (MSB#5) of the spacecraft at Mercury (Fig.1). It was the first flyby with a favorable altitude for MERTIS to operate and observe Mercury. One of the main objectives of MERTIS is to provide a better understanding on Mercury’s surface mineralogy, by measuring its spectral emissivity in the MIR range and gaining insights on the planet’s geochemical composition and thermal properties [1]. By covering a broader wavelength range and focusing on thermal infrared emissions, MERTIS is expected to reveal details about Mercury’s surface composition and mineralogy that were beyond the reach of MESSENGER's instrumentation. Up to now, MESSENGER revealed that Mercury has low bulk FeO, low Ti, and high S abundances, as well as abundant exotic sulfides (e.g., CaS, MgS), which aligns with the highly reducing conditions under which Mercury have formed [2]. Spectral data acquired by the MASCS spectrometer on MESSENGER only revealed the spectral characteristics of Mercury’s surface in the 0.3-1.45 microns wavelength range, in which the spectra are mostly flat and featureless, reconfirming that Mercury’s surface is poor in iron-containing mafic minerals [3]. MESSENGER data, however, offer insightful indications on potential analog materials that can be used to comprehend the MERTIS MSB#5 data. FeO-free analogue materials, representative of the expected mineralogy on Mercury, were selected and are being analyzed within the Planetary Spectroscopy Laboratory (PSL) of the German Aerospace Center (DLR) in Berlin, with the goal of ultimately using them to interpret the acquired MERTIS remote sensing spectral data. To be able to achieve this, the measured samples were incrementally heated up to 450°C through induction within the high-temperature emissivity chamber of the PSL. The emissivity chamber, attached to a Bruker Vertex 80V FTIR spectrometer and capable of operating under vacuum, allowed us to capture spectra in the same spectral range as MERTIS (7-14 microns) at different temperature steps: 250, 300, 350, 400 and 450°C. The collected spectra at these selected temperature steps are comparable to Mercury’s expected surface temperature. By comparing the laboratory-measured emissivity spectra of the analogue materials with the data acquired during MSB#5, we aim to get a better understanding of Mercury’s surface mineralogy within the observed region.