BepiColombo's 5th Flyby: MERTIS Observations of Mercury's Surface Variations – Preliminary results

On December 1st, 2024, ESA/JAXA BepiColombo has become the 3rd spacecraft to observe the surface of Mercury, after Mariner 10’s flybys in 1974-75 and MESSENGER orbiting Mercury between 2011 and 2015. BepiColombo’s arrival is planned for November, 2026 - an 8 years journey which includes six flybys to guide the spacecraft into the orbit. The 5th flyby geometry has offered an exceptional and first-time opportunity for the MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) to observe the planet’s surface through its Space Port. MERTIS is composed of a spectrometer (TIS) operating in the spectral range of 7-14 µm and a radiometer (TIR) with two channels at 8-14 µm and 7-40 µm [1]. The 5th flyby has marked the first time that Mercury’s surface has been observed in the spectrally resolved mid-IR range by a spacecraft [2]. This spectral range allows to better understand Mercury’s surface composition, as it is sensitive to Si–O stretching vibrations and allows identification of feldspars, pyroxenes, olivines, sulfides, volcanic glasses, silica-rich materials, potentially graphite-bearing or reduced materials. The intrinsic lack of diagnostic absorption features in Mercury’s VNIR spectra has historically (e,g, MESSENGER mission data) masked the planet’s true mineralogical diversity .

This work focuses on (1) mapping the surface variations observed within the 5th flyby coverage of the MERTIS instrument in the mid-IR range; (2) investigating correlations with previously reported surface features and geological units; (3) studying the global (within the 5th flyby coverage) effect of various surface- and temperature-dependent parameters on the MERTIS mid-IR signal variations.

Data calibration: The data acquisition and processing architecture of the MERTIS instrument is described in detail in [3]. The MERTIS instrument is designed to observe at Mercury through its planet port. The Space Port is intended as a calibration target with „no emission”, observing deep space [1]. The MERTIS planet port view was characterized in the laboratory during a radiometric calibration campaign under space-like thermal-vacuum conditions [3, 4]. Since launch, there have been observation campaigns for the Moon [5] and Venus [6]. The instrument had to be reprogrammed to observe the targets through the Space Port, which had not been intended for scientific observations. The current calibration of the 5th flyby data of Mercury is based on the calibration methods developed during the cruise alongside Venus and the Moon.

Preliminary results and discussion: Impact basins and complex craters serve as primary thermal and compositional benchmarks within the radiance dataset of the 5th flyby. This is most likely due to the strong temperature differences they exhibit compared to their surroundings, which are likely a result of differences in composition and/or degree of space-weathering. One example is the Tolstoj basin under study in [7]. A larger dataset of central peaks is currently under investigation [8]. The impact crater Bashō is another location where we observe anomalies in the mid-IR data. Similarly, MESSENGER visible images show that Bashō crater exhibits both dark and bright material. Other anomalous regions were observed where reduced visible radiance (in MESSENGER data) coincides with decreased thermal IR emission of MERTIS data [9, 10].

The Christiansen Feature (CF) is one of the most important spectral parameters in the mid-IR range. The CF position shifts with various factors such as silica content, glass content, grain size, temperature, and bulk composition, e.g., felsic materials show a CF at shorter wavelengths whereas mafic/ultramafic materials tend to have a CF at longer wavelengths.

The mean CF value of all MERTIS observation of the 5^th BepiColombo flyby lies approximately between 8.2 and 9.0 μm, which implies rather mafic assemblages, low silica content and abundant pyroxene/olivine-like materials[1] . Glassy volcanic material may also be present on the surface.

Next steps:

• Analysis of CF shift as a proxy to Mercury’s bulk composition
• Refinement of the CF retrieval from the flyby dataset
• Comparison with visible and NIR data of MESSENGER
• Laboratory work and comparison with analogue samples for more detail, see [11, 12, 13, 14].

Reference

[1] Hiesinger et al., Earth and Planetary Sci. L., 2008. [2] Hiesinger et al., LPSC, 2025. [3] D’Amore, M. et al, Infr. Remote Sens. Instr, 2018. [4] Walter, I., et al., Infrared Remote Sensing and Instr, XXI, 2013. [5] Barraud et al. SPIE 2024. [6] Helbert et al. Nature Comm. 2023. [7] D’Amore et al., LPSC 2025. [8] Pasckert et al., LPSC 2025. [9] Wohlfarth et al., LPSC 2026. [10] Tenthoff et al., EPSC 2026. [11] Maturilli et al., LPSC 2025. [12] Morlok et al., EPSC-DPS 2025. [13] Van den Neucker et al., LPSC 2025 [14] Verma et al., EPSC 2026