Abstract DGP2026-89

Thermal anomalies on Mercury’s surface observed by MERTIS during BepiColombo’s fifth flyby

K. Wohlfarth (1), M. Tenthoff (1), J. Knollenberg (2), T. Powell (3), H. Hiesinger (4), B. Greenhagen (3), O. Groussin (5), E. Kührt (2), C. Wöhler (1), S. Adeli (2), O. Barraud (2), K. Bauch (1), J. Benkhoff (5), J. Helbert (6), T. Heyer (5), G. Nishiyama (2), J. H. Pasckert (5), M. Reitze (5), N. Schmedemann (5), A. Van den Neucker (2), N. Verma (2), I. Weber (5), the MERTIS team

(1) AG Bildsignalverarbeitung, TU Dortmund, Germany (2) DLR Institute of Space Research, France (3) Johns Hopkins University Applied Physics Lab., USA (4) Institut für Planetologie, Universität Münster, Germany (5) Aix Marseille Univ, CNRS, CNES, Laboratoire d’Astrophysique de Marseille, France (6) ESA ESTEC, Netherlands

During BepiColombo’s fifth Mercury flyby on 1 December 2024, MERTIS captured the first spaceborne hyperspectral thermal-infrared (7–14 µm) data of Mercury [1–4]. These observations probe Mercury’s thermal environment under unique illumination. Results indicate a globally homogeneous, likely mafic surface, with localized thermal anomalies that break the Moon-like anticorrelation between visible reflectance and TIR radiance.

To interpret the flyby data, radiance was modeled with a fractal roughness thermal model [5] adapted to Mercury. Bolometric albedo (from MESSENGER), emissivity, and roughness were used, with the latter two optimized to match observations. Spectral emissivity maps were then compared to lunar references and MDIS color mosaics [6].

The joint inversion yields stable solutions indicating a rough surface (roughness ≈ 40°), somewhat rougher than typical lunar terrain (roughness ≈ 35°, [7]), and a low bolometric emissivity (≈0.75–0.80), consistent with earlier Mercury studies [8,9]. At Mercury’s high surface temperatures, the thermal emission peak shifts to shorter wavelengths where silicate emissivity decreases, reducing total radiance. The emissivity spectra show a pronounced Christiansen feature near 8.5 µm, characteristic of mafic silicates [10], which appears largely uniform at flyby resolution. Spatial variations mainly affect emissivity amplitude rather than spectral shape.

Two thermophysical regimes emerge. Normally, high visible–NIR reflectance, like fresh crater ejecta, coincides with low TIR radiance, consistent with lunar trends. Anomalous sites, all associated with low-reflectance material (LRM), break this pattern: some dark areas emit less thermal radiation, while some low-reflectance units show minimal infrared suppression.

Laboratory forsterite–graphite mixtures suggest a mechanism: Small amounts of graphite reduce visible reflectance and slightly lower emissivity in the MERTIS range, but increase emissivity in the 4–6 µm band where Mercury’s thermal emission peaks. Even trace graphite can therefore raise bolometric emissivity, lower equilibrium temperature, and reduce total radiance, with effects saturating at low abundances. Similar behavior may also arise from space weathering or grain size and porosity.

Mercury shows localized thermophysical behavior decoupled from albedo. Bolometric emissivity appears key to its energy balance, and future orbital MERTIS data are essential to test these patterns globally. Ongoing work will apply additional thermal models [11,12] and new DLR laboratory measurements to make these results more robust.

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