Simulating Mercury’s environment - watchout for Magnesium and Space Weathering!

Summary:  Mercury’s surface studies are difficult from ground-based observations and from the limited number of missions that have explored the Hermean environment. Characterising further Mercury’s properties from laboratory measurements is a great and timely opportunity given BepiColombo’s arrival in late 2025. Through spectral measurements of Mercury analogues, our goal is to explore the mineralogical properties of Mercury’s surface and its dependency on orbital measurements and Mercury’s environment. Through dedicated measurements, we have explored the variability of spectral properties as a function of observation conditions and irradiation level. Our analysis shows that the variability of spectral properties with varying illumination conditions can be correlated to specific compositions, particularly the magnesium content. We notice that spectral slopes increase with higher magnesium content and higher phase angle. Similarly, we noticed that the spectral slopes increase with irradiations of samples and reach a plateau. This puts important constraints on the capability of orbital measurements to observe spectral variabilities that are linked to recent geological phenomena and specific mineralogical and elemental compositions.

High irradiation effects on reflectance spectroscopy: We used 20 keV He+ with fluences up to 5 x 1017 ions/cm2 to simulate ion irradiation reaching the surface. Terrestrial ultramafic lava already identified as good analogues for Mercury were used: a boninite, a basaltic komatiite and a komatiite. Spectra were acquired in the visible to mid-infrared (VMIR) wavelength range, between 0.4 and 16 μm. Spectral alterations induced by irradiation are observed in the visible to near-infrared (VNIR) with an exponential darkening, a reddening and a flattening of spectra. Above a certain irradiation dose (1 x 1017 ions/cm2 in our conditions), the darkening reaches a plateau and the reddening as well. In the mid-infrared (MIR) we observe a shift of Reststrahlen bands towards longer wavelengths (<0.42 μm). The Christiansen feature is shifted towards longer or shorter wavelengths according to the irradiation dose (<0.2). The spectral alteration is closely influenced by the composition. As Mercury's surface is compositionally heterogeneous, the degree of spectral alteration varies on the planet and putatively participates in the heterogeneous spectral properties of the surface.

Tracing magnesium variability: This study presents the UV-NIR spectroscopy of a Mercury simulant to understand the impact of observation geometry and temperature on the spectral properties of the planet’s surface. The simulant (a mixture of aubrites, albite, and synthetic sulfides) and its endmembers, are measured under six geometries that sample the viewing conditions of both missions. The samples are measured fresh and after heating to 450 deg during three cycles. This study finds that the observation geometry modifies the samples differently depending on the wavelength and composition. The analogue presents a brightening, reddening, and flattening with increasing phase angle, which is dominated by the incidence angle. The heated samples also present a brightening and reddening, with an eventual deepening of absorption bands. The spectral changes associated with observation geometry and heating are stronger with increasing Mg abundance. This result emphasizes the importance of an accurate photometric correction, which could benefit from an independent calibration across geochemical units for future Mercury studies

Continuing the preparation of BepiColombo arrival: These analyses were done with increasing complexity of the analogues with improved matching mineralogy and composition with the known properties of Mercury. Starting from komatiite and boninite terrestrial analogues, we later used the mixing of adequate meteorites and minerals to match the expected composition of Mercury. The following improvements are in the preparation of synthetic samples that better match the composition and crystalline structures, an activity currently ongoing and expected to provide samples for the Mercury community.

Acknowledgement: The authors thanks the Planetary Spectroscopy Laboratory (PSL) in Berlin and the IAS-CSNSM project (INGMAR) in Paris for providing access to their experimental setup.