Comparison of MERTIS Mercury Flyby Data with PSL Laboratory Emissivity Spectra of Natural and Synthetic Samples

Kurzfassung

The ESA’s BepiColombo mission to planet Mercury (launch October 2018) is on its way to reach the final working orbit around the planet (at the end of 2026). Since flying to Mercury from Earth is not possible following a simple and direct path, the spacecraft had to flyby 1-time Earth (2020), 2-times Venus (2020, and 2021) and already 4-times Mercury (October 2nd 2021, June 23rd 2022, June 20th 2023, and September 5th 2024). Due to constraints (from flight dynamics) on the spacecraft orientation during all those passes, the MERTIS spectrometer was not able to see the planet. MERTIS planet view is during cruise obstructed by the MTM (Mercury Transfer Module), so the space view (cold space calibration port) has always been used to observe targets (Moon, and 2-times Venus) during the perviousness flybys. MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) is an imaging spectrometer for the determination of the mineralogical composition of the Mercury surface using hyper-spectral data in the thermal infrared (7-14 μm) [1]. On December the 1st 2024 the BepiColombo spacecraft executed the 5th swing-by at Mercury (MSM#5) and for the first time ever, MERTIS acquired Mercury’s surface measurements in the MIR (7-14 microns) spectral range. The planet was in MERTIS’s FOV for approximately 36 mins and the closest approach was at 37268 km target distance. One of the main objectives of MERTIS is to provide a better understanding on Mercury’s surface mineralogy, by measuring its spectral emissivity and gaining insights on the planet’s geochemical composition and thermal properties [1]. To be able to interpret the acquired remote sensing spectral data within the observed region, analogue materials representative of the expected bulk mineralogy on Mercury are being selected and analyzed within the Planetary Spectroscopy Laboratory (PSL) of the German Aerospace Center (DLR), in Berlin. For this study, we selected a wide range of natural endmembers (minerals like plagioclase, forsterite, etc), rocks (Norite, Komatiite, Basalt), glasses (synthetic, Tektite, and Obsidian), carbon (grafite), synthetic sulfides (MgS, CaS, FeS, NaS, TiS, MnS, CrS) and their mixtures to reproduce the expected mineralogy on the surface of Mercury. All samples have grainsize <25μm. The samples are incrementally heated up to 450°C through induction heating system within the vacuum emissivity chamber of PS, to capture spectra in the same spectral range as MERTIS (7-14 microns) and at different temperature steps; 250, 300, 350, 400 and 450°C, hence simulating the different surface temperatures that we observe on Mercury depending on the sun incidence angles. By comparing the lab-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 MERTIS’s observed regions.