BepiColombo's 5th Flyby: Early MERTIS Observations of Mercury's Surface Variations

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 spectrally resolved in the mid-IR range by a spacecraft [2]. This spectral range allows to better understand Mercury’s surface composition, as it is sensitive to elemental sulfur and sulfides [1], and iron-poor rock-forming minerals that are supposed to be common on Mercury [3].

Our understanding of the surface mineralogy and composition of Mercury is poorly constrained due to the lack of significant absorption features in the visible and near-infrared spectra acquired during the MESSENGER mission. This work focuses on (1) mapping the surface variations observed in the mid-IR range of the MERTIS instrument, within the coverage of the 5th flyby; (2) investigating the correlation with already reported surface features and geological units; (3) studying the global (within the 5th flyby coverage) effect of various surface- and temperature-dependent parameters in the MERTIS mid-IR signal variations.

The BepiColombo 5th flyby allowed observation of the surface for about 36 minutes from a distance of ~37268 km, resulting in a spatial resolution of 26-30 km/px (TIS). The TIS channel recorded 1,410,841 pixels on the surface of Mercury in full spatial resolution data acquisition mode with spectral 1x2 binning (no spatial binning). Deep space observations before and after intercepting with the planet were performed in order to provide a reference for a cold and “no emission” target. Here we see an increase in the measured signal shortly before intercepting with Mercury of less than one percent of the maximum radiance. This effect seems to be wavelength-dependent and reduces with increasing wavelength. This is a clear hint that our data are affected by straylight to a moderate extent. Observation through the space port is the main reason, which comes with some previously unknown parameters, since the space port was not initially planned for science observations. The team is currently investigating this effect.

Data calibration: The data acquisition and processing architecture of the MERTIS instrument is described in detail in [5]. The MERTIS instrument is designed to look at Mercury through its planet port. The space port is meant as a calibration target with „no emission”, observing deep space [1]. The planet port view on MERTIS was characterized in the laboratory in a radiometric calibration campaign under space-like thermal-vacuum conditions [5, 6]. Since launch, MERTIS has had Moon [7] and Venus [8] observation campaigns, for which the instrument had to be reprogrammed to observe the target through the space port, which had not been planned to be used for scientific observations. The current calibration of the 5th flyby data of Mercury is based on the calibration methods developed during the cruise along Venus and the Moon.

Mercury Surface Features: The acquired signal is strongly influenced by the illumination and viewing conditions (i.e., the incidence and emission angle). The surface can have strongly varying temperatures, e.g. impact craters with a very cold rim and a hot rim. Other parameters such as regolith, roughness, grain size, porosity, albedo and composition affect the acquired signal and need to be investigated. In order to better understand the influence of temperature and temperature variations, efforts were made to model the thermal behaviour of the surface and to understand the effect of surface roughness [9, 10].

Impact craters are the best distinguishable features on global scales in the datasets of the 5th flyby. This is most likely due to the strong temperature differences they exhibit compared to their surrounding areas, due to their geometry and/or composition differences. One example is the Tolstoj crater under study in [11]. A larger dataset of crater compositions is investigated in [12]. 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.

A considerable number of bright spots, very bright impact craters, a few hollows and faculae were observed by MASCS/VIRS on-board MESSENGER and are also covered by the 5th flyby MERTIS dataset, providing the opportunity of comparing the two datasets [13]. Laboratory work and comparison with analogue samples are necessary steps to enable us to interpret the composition of the surface of Mercury, for more detail see [14, 15, 16].

The goal of this work is to map the surface variations within the mid-IR MERTIS dataset of the 5th flyby, using the TIS and TIR channels on regional scales, and to provide a comparison with the previously reported surface features, such as bright and dark spots, low-reflectance material, volatile-rich deposits, smooth and rough terrains, impact craters, intercrater plains, hollows and faculae, as well as to search for anomalies which are not detectable in the VIS and NIR spectral range.

 

[1] Hiesinger et al., Earth and Planetary Sci. L., 2008. [2] Hiesinger et al., LPSC, 2025. [3] Izenberg et al., Icarus, 2014. [4] Denevi et al., Cambridge Uni. Press, 2018 [5] D’Amore, M. et al, Infr. Remote Sens. Instr, 2018. [6] Walter, I., et al., Infrared Remote Sensing and Instr, XXI, 2013. [7] Barraud et al. SPIE 2024. [8] Helbert et al. Nature Comm. 2023. [9] Tenthoff et al., LPSC 2025. [10] Powell et al., LPSC 2025 [11] D’Amore et al., LPSC 2025. [12] Pasckert et al., LPSC 2025. [13] Barraud et al., LPSC 2025. [14] Maturilli et al., LPSC 2025. [15] Van den Neucker et al., LPSC 2025. [16] Morlok et al., LPSC 2025.