BepiColombo is a joint mission between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA), to will carry out a comprehensive exploration of planet Mercury. The mission was launched on 20 October 2018 from the European spaceport Kourou in French Guiana, and is currently on a eight-year-long cruise to Mercury. BepiColombo consists of two orbiters: the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (Mio). Following their release from the Mercury Transfer Module (MTM) one year from now, these orbiters will be put in orbit around the innermost planet of our Solar System in late 2026. Once in orbit, BepiColombo with its very comprehensive, interdisciplinary payload will perform measurements to increase our knowledge on the fundamental questions about Mercury’s evolution, composition, interior, magnetosphere, and exosphere. BepiColombo successfully completed the last of its 6 flybys of Mercury in January 2025, and will continue its cruise during the remainder of 2025 and much of 2026. Although the two BepiColombo orbiters are in a stacked configuration during the cruise, during which only some of the instruments can perform scientific observations, the mission has already produced some very valuable results, including striking observations of the planet using its three engineering monitoring cameras. We shall provide a summary of the mission status, a preview of the remaining plans for the mission up to and after arrival in orbit around Mercury, a broad overview of scientific results to date, and observations by the mission's monitoring cameras from the Mercury flybys.

How to cite: Jones, G. H. and Murakami, G.: BepiColombo: A Mission Update, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1594, https://doi.org/10.5194/epsc-dps2025-1594, 2025.

Introduction

Owing to its proximity to the Sun, the large eccentricity of its orbit (e=0.2), and the action of tidal forces, Mercury is locked in a 3:2 spin-orbit resonance: it turns exactly three times around its rotation axis during two revolutions around the Sun. However, this might not have always been the case. As identified by several studies [1,2], the geographical distribution of large impact basins on Mercury’s surface indicates that the planet might have been locked in a different spin-orbit resonance in past, with the most likely rotation states being the 1:1 and the 2:1 resonances [2]. A past transition between spin states, which resulted in a change of the insolation and surface temperature patterns, is also indicated by the relaxation states of the basins and by the hemispheric nature of some tectonic features [3,4].

The stability of spin-orbit resonances depends on several factors (Figure 1), the most important ones being the orbital eccentricity, the planet’s rheological parameters, and its prolateness. Mercury’s orbit is subjected to relatively fast eccentricity variations between e=0 and e=0.3 with a period of ~10⁶-10⁷ years and potential chaotic high-eccentricity excursions beyond e=0.4 resulting from its proximity to an overlap of secular resonances with Venus and Jupiter [5,6]. While the present-day 3:2 spin-orbit resonance is, in general, stable for a range of eccentricities [7], its stability might be diminished at very small or large eccentricites, especially if Mercury’s mantle was more dissipative or if the planet was less prolate earlier in the history.

Model and Methods

This study combines the modelling of Mercury’s interior structure and the analysis of stable spin states under the varying action of solar tides. The interior structure is determined by Bayesian inversion, using the Markov Chain Monte Carlo (MCMC) method [8] and estimating the probability distributions over several interior structure parameters (e.g., the core size, the concentration of light elements in the core, and the temperature profile) consistent with empirically constrained observables (e.g., the mean density, the moment of inertia of the planet, the relative moment of inertia of the mantle, and the tidal Love numbers).

Selected interior profiles are then endowed with different viscosities and used in simulations of the orbital evolution of Mercury and the other solar system planets. I investigate the changes in Mercury’s rotation rate (including transitions between spin-orbit resonances) arising due to the variations in eccentricity and, potentially, due to the changes in the planet’s shape, which is subject to viscoelastic relaxation. At the same time, I evaluate the tidal heat rate and identify periods when it might have presented a non-negligible contribution to the planet’s thermal evolution. The orbital evolution is calculated with the symplectic integrator Rebound [9], coupled with a tidal model developed previously [10] and based on the Darwin-Kaula expansion of evolution equation as presented in [11]. The tidal model accepts as an input the radially-dependent elastic and rheological parameters of Mercury and outputs the tidal contribution to the evolution rates of all orbital elements as well as the closest stable spin-orbit resonance and the associated tidal heating.

Both in the interior structure inversion and in the tidal-orbital model, Mercury is described as a viscoelastic differentiated body. Its mantle is characterised by the Andrade rheological model, the inner core by the Maxwell model, and the outer core is an inviscid fluid. The tidal response is calculated using the normal mode theory [12].

Preliminary conclusions

In concordance with previous studies of the planet’s interior structure [e.g., 13], the large tidal deformability, encoded in Mercury’s tidal Love number k₂, points at a low-viscosity zone in the lower mantle. The zone has viscosities in the range 10¹³-10¹⁸ Pa s, which might potentially account for an increased tidal dissipation. According to the preliminary results, Mercury can transition from the 3:2 resonance to the 1:1 resonance in about 1 Gyr from now if it keeps its present-day shape and if it is indeed highly dissipative (Figure 2). A coupled study of the orbital and tidally-induced rotational evolution can provide constraints on the rheology and thus the thermal state of Mercury that are consistent with the stability of the 3:2 spin-orbit resonance or any other hypothetical spin state in past [1,2]. It also shows that the eccentricity variations, combined with decreased mantle viscosity, can lead to transitions between different spin states without the need for impact-induced resonant unlocking.

References

[1] Wieczorek et al. (2012), Nature Geoscience, 5(1):18-21, doi: 10.1038/ngeo1350.

[2] Knibbe & van Westrenen (2016), Icarus, 281:1-18, doi: 10.1016/j.icarus.2016.08.036.

[3] Szczech et al. (2024), Geophysical Research Letters 51(22), doi: 10.1029/2024GL110748.

[4] Szczech et al. (2025), submitted to AGU Advances.

[5] Lithwick & Wu (2011), The Astrophysical Journal, 739(31), doi: 10.1088/0004-637X/739/1/31.

[6] Laskar & Gastineau (2009), Nature, 459(7248):817-819, doi: 10.1038/nature08096.

[7] Noyelles et al. (2014), Icarus, 241:26-44, doi: 10.1016/j.icarus.2014.05.045.

[8] Foreman-Mackey et al. (2013), Publications of the Astronomical Society of the Pacific 125(925), doi: 10.1086/670067.

[9] Rein& Liu (2012), Astronomy & Astrophysics 537(A128), doi: 10.1051/0004-6361/201118085.

[10] Walterová & Běhounková (2020), The Astrophysical Journal 900(1), doi: 10.3847/1538-4357/aba8a5.

[11] Boué & Efroimsky (2019), Celestial Mechanics and Dynamical Astronomy 131(7), doi: 10.1007/s10569-019-9908-2.

[12] Takeuchi & Saito (1972), Methods in Computational Physics, 11:217-295, doi: 10.1016/B978-0-12-460811-5.50010-6.

[13] Goossens et al. (2022), The Planetary Science Journal, 3(2): id.37, doi: 10.3847/PSJ/ac4bb8.

How to cite: Walterova, M.: Spin-orbit resonances and tidal heating of Mercury in light of its chaotic orbital evolution, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-299, https://doi.org/10.5194/epsc-dps2025-299, 2025.

Mercury, the innermost planet in the solar system, remains an enigma after the wealth of measurements collected by NASA's MESSENGER mission. The state of Mercury's iron-rich core and the dynamo action that generates Mercury’s relatively weak, axially aligned, north-south asymmetric internal magnetic field is not well understood. Here we investigate the dynamo action associated with one unique possibility of Mercury's core: a double-iron-snow (DIS) dynamo with an extended, stably-stratified basal iron snow zone. This DIS structure model is one possible scenario that fits most other geophysical constraints at Mercury, including the Moment of Inertia. Our three-dimensional numerical dynamo survey varying the convective forcing (Rayleigh number), relative electrical conductivity (magnetic Prandtl number), and Brunt-Väisälä frequency revealed that although a relatively weak surface magnetic field can be achieved within this set-up, the external magnetic field remains highly dipolar and north-south symmetric under most scenarios. We hypothesize that this symmetry preference is due to the magnetic anchoring effect of the basal iron snow zone and the electromagnetic screening effect of the top iron snow zone. Our results indicate that while the existence of a top iron-snow zone (or a stably stratified layer) can lead to a weak and more axisymmetric magnetic field, the existence of an extended basal iron-snow zone would prohibit the equatorial symmetry breaking in the magnetic field observed at Mercury. Thus, our dynamo modeling results argue against the existence of an extended basal iron snow zone inside Mercury's core at present.

How to cite: Cao, H., Wulff, P., Barik, A., and Soderlund, K.: Mercury’s enigmatic magnetic field: a dynamo case against an extended basal iron-snow layer in the liquid outer core? , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-449, https://doi.org/10.5194/epsc-dps2025-449, 2025.

Introduction:  Along with the discovery of Mercury’s dynamo-generated dipolar magnetic field (~200 nT at the equator), measurements from the MESSENGER spacecraft indicate the presence of crustal remanent magnetization [1, 2]. Mercury’s dipole field is anomalously weak (given its large core), which has elicited a variety of dynamo mechanisms (e.g., stable layers, iron-snow, double-diffusive convection, solar wind feedback, asymmetric heat flux, core growth) to explain this oddity [3]. Global measurements of crustal magnetization can give insights into the time-evolution of Mercury’s dynamo and interior.

MESSENGER measured the region spanning 40°-70° N and 120° E - 120° W, showing peak crustal field strengths of ~20 nT measured at ~20 km altitude, with no measurements made <30° N [1]. These crustal fields are co-located with ~3.9-3.7 Gyr old volcanic smooth plains [4, 5] and are inferred to be generated from thermoremanent magnetization (TRM) acquired in the presence of the ancient Hermean dynamo field.  Assuming uniform magnetization, paleointensities of 10¹-10⁴ nT are compatible with the measurements, with large uncertainty due to the poorly constrained TRM efficiency,  χ_TRM, and source thickness (<70 km) [1].

Here, we aim to answer the following: (1) How strong must the Mercury dynamo have been to account for the crustal magnetization observed by MESSENGER? To answer this, we determine the minimum field strength needed to stand off the solar wind and produce a steady field that could magnetize the crust. In answering this, we show that future BepiColombo low-altitude (<100 km) measurements of crustal fields and surface composition (especially latitudes <30° N) can further constrain the strength of the ancient dynamo.

Objectives and Methods: First, we estimate the solar wind and interplanetary magnetic field (IMF) conditions for Mercury ~3.9-3.7 Gyr ago. We next use magnetohydrodynamic (MHD) simulations of the ambient surface magnetic field conditions. We then use these MHD simulations to create surface magnetic field timeseries, which we couple with thermal cooling and magnetization models to match with MESSENGER measurements.

Ancient solar wind, IMF conditions, and surface field timeseries. We use estimates of the ~0.5-0.7 Gyr-old Sun’s mass-loss and rotation rate to calculate solar wind speed, density, and IMF values at Mercury, assuming ancient Mercury had the same semi-major axis of 0.387 AU as today. We obtain solar wind speeds of 600–1000 km-s^-1, densities of 200–3,500 amu-cm^-3, and IMF strengths ≳300–600 nT, 1.5–3x stronger than Mercury’s modern dipole equatorial field [6].

To create the surface magnetic field timeseries for our magnetization models, we perform MHD simulations of the solar wind interacting with the Hermean dynamo field, varying the IMF and solar wind density for a fixed dipole field strength. We quantify the magnetic field history experienced at a given surface latitude by taking the surface magnetic field over all longitudes and mapping this magnetic field to a timeseries over the Hermean day (using longitude as a proxy for the time in the fixed surface point reference frame, given the axisymmetry of the dipole field [1]).

Crustal field calculations. From the MHD simulated surface magnetic field timeseries, we then estimate the TRM recorded in cooling volcanic deposits and compare the generated crustal fields to those measured by MESSENGER. We simulate cooling and TRM acquisition for three different structures: a cylindrical magmatic intrusion of diameter 2 km and length 8 km, a thin effusive volcanic disk of diameter 20 km and thickness 1 km, and a large cylindrical intrusion/lava flow of diameter 50 km and thickness 20 km. We then calculate the generated crustal fields from these sources with  χ_TRM values of various magnetic minerals. The main constraints on the candidate surface magnetic minerology are the MESSENGER measured average ~1.75 wt% Fe and likely extreme reducing conditions during formation like those for aubrite meteorites [7]. As such, we consider  χ_TRM values from aubrites [8] and Fe metal, FeNi alloys [1]. We note that magnetic carriers like suessite, schreibersite, and greigite could be stable in the reducing Hermean environment, but extensive TRM measurements are lacking [7]. Future BepiColombo measurements could provide further constraints on the surface magnetic minerology [9].

Results: We find that the Hermean dynamo field must have been >2,000 nT (10 stronger than at present) to magnetize the surface to MESSENGER measured anomalies at around 40° N latitude  [1]. (Fig. 1) We further find that crustal field measurements at low latitudes from BepiColombo could further constrain the ancient Hermean dynamo’s intensity. For example, the ancient dynamo intensity lower limit can be constrained to stronger values (possibly 20,000 nT) if BepiColombo measures similarly strong crustal fields (as did MESSENGER) at lower latitudes (e.g., 10° N).

Conclusions: In this study, we demonstrate that Mercury’s dynamo field at ~3.9-3.7 Gyr ago was very likely ≳2,000 nT to stand off the solar wind and magnetize the surface to the level inferred from MESSENGER data. The crustal magnetization may even have required a ~20,000 nT dynamo field if the Hermean surface magnetic mineralogy is dominated by Fe-metal or FeNi alloys.

Numerous dynamo models have been developed to explain Mercury’s modern planetary field. Our findings of a heightened, ancient dynamo field might point to different interior conditions ~3.9-3.7 Gyr ago, such that these stably stratified layers or Fe-snow zones were not present in that period. Furthermore, evidence for a stronger surface field might imply an enhanced heat flux or a larger inner core to power this ancient dynamo [10, 11]. Future low-altitude (<100 km) measurements of the lower latitude crustal field environment and surface mineralogy would help constrain the strength and time-evolution of the Hermean dynamo.

References:

[1]Johnson+2018,Mercury’s Internal Magnetic Field.[2]Hood+2018,JGR,2647-2666.[3]Heyner+2021,SSR,217.[4] Denevi+2013,JGR,891-907.[5]Wang+ 2021,GRL,e2021GL094503.[6]Vidotto+2021,LivingRevSolPhys,3.[7]Strauss+2016,JGR,2225-2238.[8]Rochette+2010,M&PS,405-427.[9]Rothery+2020,SSR 66.[10]Takahashi+2019,NatCommun,208.[11]Hauck+2018,Mercury’s Global Evolution,516-543.

Fig. 1: Crustal magnetic field parameter space for generating strongest MESSENGER-level magnetic field (B_Altitude) measurements (blue and red markers) for (top) 200 nT dipole field and (bottom) 2000 nT dipole field at 10° N latitude. The horizontal axis shows three magnetized volumes with different diameters (D) and heights (H) and the vertical axis is TRM efficiency, χ_TRM. For the proposed magnetic materials (iron-metal or similar composition to Aubrites), the ancient Hermean dynamo needed to be >2000 nT, possibly even 20,000 nT, to magnetize the strongest measured crustal anomalies.

How to cite: Narrett, I. S., Weiss, B. P., Steele, S. C., and Biersteker, J. B.: Mercury’s Ancient Crustal Magnetization: A Stronger Dynamo can be Confirmed by Future BepiColombo Measurements, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1118, https://doi.org/10.5194/epsc-dps2025-1118, 2025.

Despite its small size and thin mantle, volcanism was pervasive across the surface of Mercury. A crustal thickness of 35–38 km taken together with a core-mantle boundary depth of approximately 400 km indicates that ~10% of the silicate material within Mercury has experienced melting. This melt is directly linked to the current surface composition, as measured during the MESSENGER mission, with the standard assumptions that the crust was generated through partial melting processes of the mantle and experienced little to no fractionation. The surface composition of Mercury is unlike that of any other body in the Solar System – rich in magnesium and sulfur, as well as other moderately volatile and volatile (e.g., carbon) elements, but poor in iron and titanium, suggesting the surface is dominated by Mg-rich silicate minerals and sulfides. This implies that the mantle has a similarly low FeO content, and a high S content, around 7–11 wt.%. Here we focus on the space and time localization of Northern Smooth Plains (NSP) volcanism on Mercury and the associated active uplift, which post-dates NSP volcanism. We hypothesize an endogenic origin to NSP volcanism that can be explained by a transitional form of temperature-dependent rheology.  We considered a set of spherical shell convection calculations with free-slip top (2440 km) and bottom (2020 km), constant temperature boundary conditions (440 K at the top and 1600 K at the bottom), and constant material properties throughout the domain except for rheology which is a strong function of temperature. The calculations have no internal heating and no cooling core boundary condition.

The steady-state planform of the convection is illustrated by the isocontours of temperature in Figure 1. With a viscosity contrast of 20 (Fig. 1a), there is a pattern of (yellow) upwelling linear features and (blue) downwelling linear features in the low to mid latitudes that are interrupted a ring-shaped downwelling near 60–70° latitude in both the northern and southern hemispheres, with a plume at each pole. With a viscosity contrast of 10³ (Fig. 1b), we see a similar pattern of linear upwellings and downwellings at low and mid latitudes that merge into large upwelling plumes at the pole. Rather surprisingly, we find a transitional planform with a single upwelling hemisphere and a single downwelling hemisphere with a viscosity contrast of 10⁴ (Fig. 1c). The transitional phase occurs at the same viscosity contrast in these calculations as seen calculations with an Earth-sized core. Finally, with a viscosity contrast of 10⁵ (Fig. 1d) a stagnant lid forms with many small plume-lets (yellow) distributed evenly across the domain with downwelling sheets (blue) between them. The upwelling/downwelling hemisphere pattern (Fig. 1) is surprising because such a long-wavelength structure does not seem possible in a thin shell. We propose that activation energy and crustal rheology are key for the transitional upwelling/downwelling hemisphere form of convection (Fig 1c). The reduction in activation energy compared with olivine basalt is the result of sulfide and graphite within the silicate crust/lithosphere.

We propose this endogenic mechanism explains the observed concentrated region of volcanism expressed as the NSP. We demonstrate the relevance of these calculations to a cooling planet by adding a cooling core boundary condition and radiogenic Heat Producing Element (HPE) concentrations based on the surface abundances of K, Th, and U that were measured with the Gamma-Ray Spectrometer on MESSENGER. The volume and timing of melt generated using an iron free solidus will be compared with the NSP on Mercury. These time evolving calculations allow us to assess the observed post-volcanic uplift.

Figure 1: Isotherms from geodynamic models in a Mercury geometry thin silicate shell. Yellow denotes +100 °C above the mean mantle temperature; blue denotes     -100 °C below the mean mantle temperature. A. Roll pattern with viscosity contrast of 20; B. Roll pattern with viscosity contrast of 10³; C. Hemispheric up/down pattern with viscosity contrast of 10⁴; D. Many small plumes beneath a stagnant lid with viscosity contrast of 10⁵.

How to cite: King, S., Green, A., and Duncan, M.: Mercury's Last Gasp: A Volcanic Origin for the Northern Smooth Plains, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1090, https://doi.org/10.5194/epsc-dps2025-1090, 2025.

Explosive volcanism has significantly shaped Mercury’s surface, but questions remain about the formation mode and timing of eruptions. While compound vents formed over prolonged periods have been documented (Jozwiak et al., 2018; Pegg et al., 2021), the occurrence of single-pulse eruptions is still to be confirmed. On Earth, such eruptions produce a symmetric “bullseye” pattern in pyroclast distribution, with clast size decreasing with distance (Kilgour et al., 2019). Although Mercury lacks particle size data at this scale, analysis of MASCS spectra (Besse et al., 2020 ; Barraud et al., 2021) with deep learning techniques (Leon-Dasi et al., 2023 ; 2025) allows us to track spectral changes with distance from the vent source. We create isochrone maps that provide information on the changes in spectral properties as well as the rate at which they change. We find multiple evidence of symmetric patterns, supporting single-pulse eruptions, as well as patterns suggesting multi-pulse eruptions at single vents and interrupted eruptions from multiple vents.

This analysis provides evidence on the complex volcanic history of Mercury through time with different eruption mechanism within the same eruption style (i.e., effusive Vs. explosive). On-going analysis shall provide insights on the timeline of these eruption mechanisms, particularly if some were favored during the last stages of volcanism on Mercury. 

How to cite: Besse, S., Leon-Dasi, M., Barraud, O., and Doressoundiram, A.: Tracking Eruption Patterns with Deep Learning, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1234, https://doi.org/10.5194/epsc-dps2025-1234, 2025.

Introduction

Observations by the Mercury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft detected abundant volatiles on Mercury’s surface [1]. The K/Th ratios observed by Gamma-Ray Spectrometer (GRS), 6600 ± 2800, were higher than those on the other terrestrial planets [2, 3]. These observations led to several hypotheses regarding Mercury’s unique formation and evolutionary history [4-6]. However, these estimates were derived by integrating a whole GRS dataset, possibly biasing the global estimate. Indeed, due to MESSENGER’s elliptical orbit, GRS data were most effectively collected over Mercury’s northern high latitudes. To estimate a representative K/Th ratio over Mercury’s global surface, this study aims to determine the spatial variability of K and Th abundances. As a first step, we assumed geologically consistent spatial patterns of K and Th abundances and estimated the corresponding values.

Method

In this study, we assumed four geological end-member units on Mercury, each of which has a representative elemental composition. Based on this assumption, we applied a linear mixing model, where each GRS observation reflects a linear mixing of end-member compositions within observational footprint [7].

First, we produced an end-member map (Fig.1) by defining four end-member units that reflect diversity in surface geological context: Northern Plains (NP), Caloris Plains (CP), High-Mg Region (HMR), and Intercrater Plains (IcP). NP and CP are smooth plains with low crater densities, likely formed by young volcanism [7-9]. The HMR unit was identified in areas with elevated Mg/Si ratios (>0.55) [10], likely ancient terrain formed by early volcanic or impact-related processes [11,12]. All remaining regions were designated as the IcP unit.

Fig.1. End-member unit map showing the spatial distribution of the four end-member units, NP (yellow), CP (light green), HMR (red), and IcP (light blue). The sub-nadir points of the GRS measurements used in this study (black points) were limited to the northern hemisphere.

The footprint areas for each GRS observation were calculated using spacecraft’s orbital and attitude data. For each footprint, we determined the fractional coverage of the four end-member units by comparing it with the end-member unit map. To parametarize the GRS’s detection efficiency of gamma-rays emitted from the surface, a weighting scheme was applied based on the distance and emission angle between the spacecraft and a given point on the surface.

We then selected GRS observations with similar mixing ratio of the four units and integrated them to generate 20 composite gamma-ray spectra. From each spectrum, we estimated K and Th abundances from the 1461 keV and 2615 keV gamma-ray peaks, respectively. Finally, we solved an inverse problem using the fitted K and Th values and their associated mixing ratios to estimate the elemental abundances for each end-member unit (Fig. 2).

Fig.2. K abundances (vertical axis) and mixing ratios of the four end-member units (horizontal) for 20 gamma-ray spectra (colored circles) and the estimated end-member compositions (colored pentagrams).

Results and Discussion

Our analysis successfully estimated the K and Th abundances of the four end-member (Table 1). The K abundance was highest in the NP, exceeding that of the CP by over a factor of four. Relatively uniform Th abundance was estimated except in the HMR, which showed more than twice compared to the other units. As a result, the K/Th ratio showed a wide spatial variation, with the highest K/Th ratio of ~14,000 observed in NP.

Table 1. The end-member K and Th abundances [ppm], and K/Th ratios, and absolute and relative surface areas occupying the northern hemisphere, A_u [km2] and A_N,rel[%].

Using the estimated end-member compositions and their spatial coverage, we derived average values for Mercury’s northern hemisphere: K and Th abundances of 620 ± 20 and 0.13 ± 0.01 ppm, respectively, and K/Th ratio of 4900 ± 600. The estimated K/Th ratio is lower than, but consistent with previous global estimates, within the error bar range [2,3], suggesting that our model reduces the observational bias caused by uneven spatial coverage of the GRS observations.

To investigate the influence of exogenous contaminations by cratering impacts, we compared K/Th ratios with crater density, a proxy for impact frequency experienced by present surface. No clear trend was observed: NP and CP, both with low crater densities, exhibited the highest and lowest K/Th ratios, respectively, while HMR and IP with higher crater densities had intermediate values. In contrast, a negative correlation was found between K abundance and average surface temperature across units. This is consistent with the “thermal redistribution” hypothesis, where volatile elements concentrate in colder regions through surface-exosphere interactions [3,14]. These results imply that thermal redistribution dominates current surface K and Th compositions rather than the surface contamination of impact-delivered materials.

Conclusions

The reported high surface K/Th ratio was considered a clue to Mercury’s evolutionary history. This study re-analyzed MESSENGER GRS data using a compositional mixing model. Significant variation among four geologically defined end-member units was observed, with the Northern Plains showing the highest potassium concentrations. The estimated global average K/Th ratio was slightly lower than previous estimates, indicating succesful removal of observational bias. While no clear correlation was found with crater density, K abundances showed a negative correlation with surface temperature, supporting the thermal redistribution hypothesis. Applying this approach to thermal models or future observations may further improve our understanding of volatile behavior on Mercury.

References [1] Evans et al. (2012) JGR: Planets, 117, E00L07.  [2] Peplowski et al. (2011) Science, 333(6051), 1850-1852. [3] Peplowski et al. (2012) JGR: Planets, 117, E00L04. [4] Walsh et al. (2011) Nature, 475(7355), 206–209. [5] McCubbin et al. (2012) Geophys. Res. Lett., 39(9), 2012GL051711. [6] Hyodo et al. (2021) Icarus, 354, 114064. [7] Hirata et al. (2024) JGR: Planets, 130, e2024JE008788. [8] Ostrach et al. (2015) Icarus, 250, 602–622. [9] Ernst et al. (2015) Icarus, 250, 413–429. [10] Denevi et al. (2013) JGR: Planets, 118(5), 891-907. [11] Nittler et al. (2020) Icarus, 345, 113716. [12] Mojzsis et a. (2018) EPSL, 482, 536–544. [13] Weider et al. (2015) EPSL, 416, 109–120. [14] Peplowski et al. (2014) Icarus, 228, 86-95.

How to cite: Hirata, K., Usui, T., Peplowski, P., and Suzuki, Y.: Updated estimates of Mercury's global K/Th ratio: An unmixing approach to surface composition analysis, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1395, https://doi.org/10.5194/epsc-dps2025-1395, 2025.

Introduction

The BepiColombo mission to Mercury [1] will arrive in late 2026 and carries the Mercury Imaging X-ray Spectrometer (MIXS) [2]. MIXS will recover elemental abundances from the upper tens of microns of Mercury’s surface through X-ray fluorescence (XRF), using solar coronal X-rays as the excitation source. MIXS will map elemental abundances at greater spatial resolution than has been achieved previously at Mercury through novel X-ray optics; MIXS also uses solid state DEPFET detector technology [3] to achieve greater spectral resolution (~140 eV full-width at half-maximum at Mn Kα) than previous instrumentation. MIXS comprises two channels, a high spatial-resolution telescope, MIXS-T, and a wide field-of-view collimator, MIXS-C, with identical detectors. The DEPFET detectors will allow for spectrally-resolved measurements of fluorescence peaks from closely separated elements, as well as analysis of fluorescence lines at lower energy than previously possible. For some elements, MIXS will make the first XRF measurements at Mercury (e.g. O, Na, K), whilst for others, they will be measured globally with more precise spatial resolution than ever before (e.g. S, Fe).

Methodology

As peaks from new elements become resolvable, some remain close to the resolution limit; in this case strong neighbouring peaks can overlap with the peak of interest. In addition to this, other artefacts (e.g. escape peaks, diffraction peaks) which are a part of the instrument response can restrict analyses. In order to better understand the impact on elemental sensitivity that such artifacts may have with data from Mercury, we describe the analysis of a collection of synthetic materials with compositions that probe cases where peak-overlap may occur [4]. We have targeted several examples where geochemically significant results are impacted: overlap between Na-Mg, Al-Si, K-Ca, as well as detection limits for S and Mn. For example, Na may be a key volatile in Mercury’s hollows e.g. [5] and as such, resolving the peak from Mercury’s strong Mg peak will be crucial to investigating these features.

We are analysing these samples with the MIXS Ground Reference Facility (GREF) [2,6], a large vacuum chamber which provides ground observations using the qualification model MIXS detector in a controlled environment. The samples are prepared as pressed powder pellets from analytical-grade powders, mixed with a binder to allow for exposure to high-vacuum. Each sample is analysed under the same illumination and observation conditions, with a detector and electronics chain which is identical to that of the two flight models. The spectra are background-subtracted and the peaks fitted to extract the relationship between detected line intensity and abundance. These relationships will highlight the detection limits for certain elements and, in other cases, the relationships between fluorescence peaks from different elements.

Preliminary Results

Figures 1 and 2 show some preliminary results from one of the series of samples which focuses on the sensitivity to Na in the presence of a Mg abundance which is similar to those expected within Mercury’s High Magnesium Terrane [7]. Figure 1 shows the calibrated spectra from each of those samples, which cover a Na₂O range of 0.1-8 wt%. The Na peak ceases to be resolvable as a peak below ~3 wt% Na₂O, however the results of the fitting, shown in Figure 2, suggest that MIXS detectors continue to have sensitivity at this level. For comparison, the Na₂O at Mercury is estimated at ~3-8 wt% Na₂O [7,8].

Next Steps

Our analyses will continue and, through fitting, will produce similar results for each of the other series of samples. This process will investigate escape peaks, diffraction peaks and enhancement effects via secondary fluorescence. The work will provide the basis for understanding the geochemical sensitivity that MIXS will achieve at Mercury, where detection limits will also be governed by count rate, correlated with solar flare state. At Mercury, the incident solar spectrum is highly variable and differs significantly in spectral shape from the laboratory X-ray source, this dataset will allow for better predictions of elemental sensitivity in data returned from Mercury. Interpretation of these experimental results will therefore inform refinements to the data analysis techniques that will be carried out on the MIXS data, thus providing crucial instrument information which will inform plans for observation campaigns of targets on Mercury’s surface. Maximising the returns of MIXS and BepiColombo will be key to driving our understanding of Mercury for decades to come.

Figure 1 – GREF X-ray spectra (around the peak of the Na K-series) from a series of samples targeting sensitivity to Na, increasing in Na abundance through samples Na1 to Na10.

Figure 2 – Fitted count rate to the Na peak from the spectra in Figure 1 after background subtraction.

References: [1] Benkhoff et al. (2021) Space Sci. Rev., 217:90(8). [2] Bunce et. al. (2020) Space Sci. Rev., 216:126(8). [3] Majewski et al. (2014) Exp. Astron., 37(3). [4] Lecaille et al. (2024) Geologica Belgica Luxemburga International Meeting., Liege. [5] Barraud et al. (2020) JGR Planets., 125(12) [6] Cartwright et al. (in prep.) [7] Peplowski and Stockstill-Cahill (2019) JGR Planets., 124(9). [8] Peplowski et al. (2014) Icarus., 228.

How to cite: Fox, A., Martindale, A., Barry, T., Lindsay, S., Bridges, J., Bunce, E., Cartwright, J., Charlier, B., Hall, G., Lecaille, M., Namur, O., and Tikkanen, T.: Experimental measurements of the elemental sensitivity of MIXS detectors in support of the BepiColombo mission, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1651, https://doi.org/10.5194/epsc-dps2025-1651, 2025.

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.

How to cite: Adeli, S., Helbert, J., Maturilli, A., D'Amore, M., Barraud, O., Säuberlich, T., Knollenberg, J., Ulmer, B., Bauch, K., Domac, A., Wöhler, C., and Hiesinger, H. and the MERTIS Team: BepiColombo's 5th Flyby: Early MERTIS Observations of Mercury's Surface Variations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1825, https://doi.org/10.5194/epsc-dps2025-1825, 2025.

Introduction

On December 1^st, 2024, the ESA/JAXA spacecraft BepiColombo performed its fifth Mercury swing-by (MSB#5), during which the onboard Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) [1,2] observed Mercury’s surface for the first time [3]. MERTIS consists of a radiometer (TIR, two channels, 7-14 µm and 7-40 µm) and an imaging spectrometer (TIS, 80 channels, 7-14 µm). A thermal model is required to retrieve emissivity spectra from the TIS data and to address the anisotropic thermal emission caused by surface roughness. Possible approaches are simple semi-analytical roughness models fit to an emission phase function (EPF) [4,5] or fully numerical models. For this work, we use our numerical thermophysical roughness model [6], which has been validated for the lunar surface with Gaofen-4 [7], Diviner [8], and MERTIS [6] data.  We derive essential parameters to tailor the model to Mercury and retrieve spectral emissivities. Performing Principal Component Analysis (PCA) and clustering allows us to gain first insights into Mercury’s emissivity variations.

The planet baffle of MERTIS was blocked during MSB#5 due to BepiColombo’s stacked cruise configuration. The observations were instead taken through the space baffle. While the planet baffle was extensively calibrated before launch [9], the space baffle was initially not intended for target observations. The effects of the baffle complicate data calibration. Hence the results presented here should be considered preliminary. The team is currently investigating and improving the calibration [3].

Thermal Model

Many airless planetary bodies (e.g., Mercury, asteroids, and the Moon) are covered by a rough and highly insulating regolith. Different surface orientations on small spatial scales (mm-cm) lead to significant temperature variations on those scales. Consequently, the observed thermal emission is a superposition of multiple Planck functions. The thermal emission produced by a rough surface can significantly deviate from that of a smooth surface (corresponding to a single Planck function).

We model this subpixel roughness with fractal surfaces [6], including self-scattering and self-heating between the facets. Computing the model for every TIS pixel is computationally expensive. Instead, we evaluate the model for a uniform grid in the parameter space and interpolate the results according to the MSB#5 geometry. We estimate three main model parameters:

• The bolometric directional-hemispherical albedo A_dh, which controls the reflected solar irradiance.
• The bolometric hemispherical emissivity ε_h, which controls the total emitted thermal radiance.
• The surface roughness θ, which alters the directional thermal emission characteristic.

Model Parameter Estimation

A theoretical analysis of laboratory data suggests that the bolometric emissivity of Mercury’s surface is lower than that of the Moon, which is often assumed to be ε_h = 0.95 [11]. Figure 1 shows modeled radiances for different global values of the directional-hemispherical albedo compared to the measured radiances. Even when A_dh is set to values outside the plausible range, the model does not match the data. By reducing the bolometric emissivity to ε_h = 0.75 , we can reproduce the measured radiances (see Figure 2).

Figure 1. Radiances at 8.6 µm with ε_h = 0.95

Figure 2. Radiances at 8.6 µm with  ε_h = 0.75

We derived individual A_dh values for each MERTIS measurement from MDIS data [10] to incorporate spatial albedo variations. Averaging the values yields 0.07.

Figure 3 shows the same profile as Figure 1 and 2 for different surface roughness parameter values. Near the subsolar point, a rough model behaves like a smooth model, and the parameter has no effect. However, a smooth model cannot fit the data for increasing latitudes due to thermal beaming [12]. We find that a roughness between 30° and 35° is required to explain the data.

Figure 3. Radiances at 8.6 µm for different roughness values.

Figure 4 shows the radiances gridded to a perspective projection. While the model agrees with the data in the latitudinal direction, there is an offset in the longitudinal direction. However, the highest temperature of a slowly rotating body like Mercury should be centered on the subsolar point and decrease in all directions with increasing incidence angle. This offset is likely caused by the unusual observation conditions. We used a wavelength-independent and pixel-position-dependent fourth-order polynomial fit to correct the radiance scaling without affecting the shape of the spectra.

Figure 4. Radiances at 8.6 µm. The black star marks the subsolar point. The dashed area is investigated in Figure 5.

High-Level Emissivity Analysis

Selecting only frames where all 100 TIS pixels saw Mercury allows us to use Fourier filtering to suppress the striping artefacts. To reduce the dimensionality of the data and visualize spatial variations, we applied a PCA to the retrieved spectral emissivities. We only use the first two components PC1 and PC2. Figure 5 shows a false color image with PC1 in red, PC2 in green and a constant value of 0.5 in blue. Further, we used a self-organizing feature map (SOM) to cluster the features PC1 and PC2 into 9 clusters.

Figure 5. Comparison between maps derived from MDIS [10,13] and MERTIS data. The Caloris basin is visible in the PCA image and cluster index map. Several show up in the emissivity-derived maps. The low reflectance material around Tolstoj vanishes, and the Tolstoj interior is assigned to the same cluster as part of the Caloris interior.

References

[1] Hiesinger, H., et al. 2010, Planet Space Sci. 58 [2] Hiesinger, H., et al. 2020, Space Sci. Rev., 216, 147 [3] Adeli, S., et al., EPSC 2025. [4] Jhoti, E., et al. (2023), TherMoPS IV, 48 [5] Powell, T., et al. 2025, LPSC 2025 [6] Wohlfarth, K., et al., 2023, Astronomy & Astrophysics, 674, A69. [7] Wu, Y., et al. 2021, Geophys. Res. Lett., 48 [8] Bandfield, J. L., et al. 2015, Icarus, 248, 357 [9] D’Amore, M. et al. 2018, Infr. Remote Sens. Instr, 26 897-904 [10] Becker, K. J., et al. Lunar Planet. Sci. Conf. 43. No. 2654. 2012. [11] Wohlfarth, K. 2025, 10.5281/zenodo.14727529 [12] Delbo, M. et al. 2015,AsteroidsIV,107 [13] B. W. Denevi et al., LPSC2016, 1264 (2016).

How to cite: Tenthoff, M., Wohlfarth, K., Wöhler, C., Schmedemann, N., Bauch, K. E., Barraud, O., Verma, N., Powell, T. M., Greenhagen, B. T., Knollenberg, J., D‘Amore, M., Helbert, J., Adeli, S., and Hiesinger, H. and the MERTIS Team: MERTIS Thermal Modeling: Mapping Emissivity Variations from BepiColombo’s Mercury Swingby 5, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1029, https://doi.org/10.5194/epsc-dps2025-1029, 2025.

Introduction: 

Nanophase iron (npFe) is known to be the cause of increasing optical maturity (spectral reddening, an overall darkening, and decreasing prominence of spectral features) on the Moon and S-type asteroids [e.g., 1, 2], but what happens when Fe is in short supply? How do highly reduced surfaces behave in a harsh space weathering environment? MESSENGER observations of Mercury found no features due to Fe²⁺ in visible-to-near-infrared (VNIR) spectra  of the surface [e.g., 3], and  other instruments confirmed that the surface contained less than 1% FeO [e.g., 4, 5]. If npFe is an unlikely space weathering product on reduced bodies [6], could carbon play a role in the optical and microstructural maturation of these regoliths?

Carbon is hypothesized to be present as a darkening agent on the surface of Mercury and on carbonaceous asteroids [e.g., 7, 8]. On Mercury, this carbon may be exogenic material introduced by impactors [9] or endogenic material excavated from a theorized graphite floatation crust [7]. Carbon may exist in concentrations of up to 5 wt. % both within Mercury’s low reflectance material and throughout carbonaceous asteroids [10, 11]. To better understand the role carbon may play in space weathering on Mercury and C-type asteroids, we utilized pulsed laser irradiation to simulate aspects of micrometeorite bombardment on powders of low- to no-iron silicates mixed with C-bearing opaques.

We present VNIR reflectance spectra from 0.3-2.5 µm, transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS) analyses of two select samples from our sample suite.

Samples and Methods: 

Sample Suite. This abstract focuses on two analogs from our sample suite of twelve: (1) San Carlos olivine (SCO, < 75 µm), to use as a testbed/proof of concept for its frequent use in space weathering experiments and (2) a mixture of 95 wt. % synthetic enstatite (ENST+CB, < 80 µm) and 5 wt. % of carbon black (< 1 µm). The enstatite was produced by MatExcel and contains 0.03 wt. % Fe.

Simulated Space Weathering. Two lasers of differing pulse widths (6 ns and 100 ns) were used to simulate aspects of micrometeorite bombardment on our samples using the Washington University Laser Space Weathering Laboratory [12]. Each sample was weathered at ~10^-7 torr for the amount of time it takes for San Carlos olivine to reach VNIR optical maturity [7] with this experimental setup.

Results:

VNIR Spectral Results. VNIR spectra from 0.3-2.5 µm are presented in Figure 1 for our SCO and ENST+CB samples. The pure enstatite sample (ENST) is shown for comparison and to illustrate the lack of 1-µm feature due to Fe²⁺. As expected, the SCO spectrum darkens (~76% decrease @ 1.05 µm), reddens (~2100% increase over 1.75-2.5 µm), and experiences reduction in prominence of the 1-µm feature. The ENST+CB spectrum darkens (~44% decrease at 0.56 µm) and reddens (~198% increase over 0.4-0.55 µm) as well, though this reddening is more complex. The ENST+CB slope in this region flipped from negative (blue) to positive (red), thereby experiencing a major change in spectral behavior.

TEM and EDS results. Both TEM and EDS data of the SCO sample show all expected, classical space weathering products. This includes a melt layer ranging from ~0.06-1.78 µm in thickness, metallic npFe of varying sizes (~8-230 nm), and concentrations, vesicles, and regions of olivine recrystallization. EDS and selected area electron diffraction (SAED) patterns show that the npFe present is likely metallic in nature, but that some of the npFe could also include Ni (Figure 2). Analyses of the ENST+CB sample revealed a thin melt layer (~0.03-0.24 µm) with vesicles of varying shapes and sizes dispersed in the thickest portions of the melt. No nanophase opaques are seen. We note that the melt is depleted in Si and enriched in Mg when compared to the bulk. Finally, both carbon black aggregates are noted to have trapped silicate melt splashes as a direct result of the weathering process (Figure 3).

Acknowledgments: FIB preparation and TEM/EDS analysis were performed with and supported by the NASA Facility for Astromaterials Research at Johnson Space Center under NFAR_2024_0028. This work was also directly supported by NASA's Solar System Exploration Research Virtual Institute cooperative agreement notice 80NSSC19M0214 for the Center for Lunar and Asteroid Surface Science (CLASS).

 References: [1] Pieters & Noble (2016) JGRP, 121, 1865-1884. [2] Cudnik (2023) Springer, 943-949. [3] Warell et al. (2006) Icarus, 180, 281-291. [4] Evans et al. (2012), JGRP, 117(E12). [5] Weider et al. (2014), Icarus, 235, 170-186. [6] Nobel & Pieters (2001). [7] Vander Kaaden & McCubbin (2015) JGRP, 120, 195-209. [8] Vilas and Hendrix (2020) AGU 2020. [9] Bruck Syal et al. (2015) Nat. Geo., 8, 352-356. [10] Klima et al. (2018), GRL, 45, 2945-2953. [11] Glavin et al. (2018), Prim. Met. and Ast., 205-271. [12] Gillis-Davis (2022) EPSC 2022, #1193.

Figure 1. SCO (top), ENST (middle), and ENST+CB (bottom) spectra before and after irradiation. Note the different scales for reflectance on the y-axis for each plot. Spectra were taken by Dr. Takahiro Hiroi at RELAB.

Figure 2. TEM image of SCO alongside elemental maps of Ni and Fe over the same region. The spatial correlation of Fe and Ni detections are indicative of possible npFe-Ni alloys.

Figure 3. BF STEM images of both carbon black aggregates present above the enstatite bulk with elemental maps for Mg and C obtained via EDS. Each color bar displays normalized counts for each map to obtain better image contrast. A) Aggregate displaying Mg enrichment. B) Aggregate with distinct melt spherule embedded within, evidenced by the bright Mg-rich deposit and corresponding C-depleted region.

How to cite: Shackelford, A., Donaldson Hanna, K., Gillis-Davis, J., Rahman, Z., Cymes, B., Christoffersen, R., and Keller, L.: Space Weathering Effects on Mercurian Surface Analogs: Insights from Coordinated Spectral, Microstructural, and Chemical Analyses, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1078, https://doi.org/10.5194/epsc-dps2025-1078, 2025.

As the smallest and least explored terrestrial planet, Mercury’s surface composition, partially characterised following the successful NASA MESSENGER mission (2011-2015) (e.g. [1]), represents a geochemical conundrum. As the closest planet to the Sun, Mercury has experienced high temperatures in its past, which should have stripped away any/all volatiles and leaving behind stable refractory elements. However, MESSENGER data revealed unexpected surface compositional variability, with evidence for distinct geologic terranes alongside evidence for unusual regions enriched in moderately volatile elements [2-4]. There were also clues that suggested formation under reducing conditions in the form of low iron concentrations coupled with high sulphur and carbon contents (e.g., [5]), alongside evidence for explosive volcanism [6].  The ESA/JAXA BepiColombo mission, scheduled to arrive at Mercury in 2026, features a suite of instrumentation that has the potential to obtain improved compositional data for the surface. Specifically, the Mercury Imaging X-ray Spectrometer (MIXS), designed and built at the University of Leicester (UoL), has the capability to acquire higher resolution surface elemental composition data (max. achievable resolution ~ 10 km vs. ~ 200 km non-imaging field of view for MESSENGER XRS [1]). MIXS uses X-Ray Fluorescence (XRF) with solar-coronal X-rays and charged particles as the excitation source, which subsequently interact with the Mercurian regolith, releasing X-rays characteristic of elemental compositions [7]. Importantly, the MIXS instrument features new technology including optics and a novel depleted p-channel field effect transistor (DEPFET) detector [7]. To gain the highest science output, ground-truthing the spectra gathered by MIXS with spectra gathered in the lab will allow for the best interpretation of Mercurian terranes and features.

We are using the MIXS Ground Reference Facility (GREF) based at UoL, which is equipped with flight-like MIXS instrumentation including a flight spare DEPFET detector and an X-ray source, where the facility can be run under vacuum and cooled to mimic the space environment [8]. With GREF, we are applying XRF to Mercurian analogues including meteorites (e.g., aubrites, enstatite chondrites), terrestrial samples (e.g., boninites, komatiites) alongside synthesised materials that share properties with Mercury (e.g., the Mercury-Y blind sample [9]), or that can be used to establish measurement sensitivity (e.g., [10]), to establish a sample library to allow for direct comparison with incoming mission data from MIXS. In particular, we are looking to explore the volatile enrichments on the surface, while also establishing the effects of topography and space weathering. Our preliminary results confirm the higher resolution capabilities of MIXS compared to MESSENGER [11] and successful forward modelling of appropriate compositions including aubrite Northwest Africa (NWA) 14185 (Fig. 1). We are continuing to analyse a range of materials, to help establish a sample library for comparison with MIXS data while maximising the science-returns of the BepiColombo mission to understand Mercury’s history.

[1] Solomon S. C., Nittler, L. R., Anderson B. J. (2018) Mercury: The View after MESSENGER (Book). [2] Peplowski P. N. & Stockstill-Cahill, K. (2019) JGR:Planets 124:9:2414-2429. [3] Vander Kaaden K. E., & McCubbin F.M. (2016) GCA 173:246-263. [4] Weider S. Z. et al., (2015) EPSL 416:109-120. [5] Namur O. & Charlier B. (2017) Nature Geoscience 1:9-13. [6] Pegg D. L. et al., (2021) Icarus 365:114510. [7] Bunce E. J. et al., (2020) Space Science Reviews 216:8:126. [8] Cartwright J. A., et al., (In Prep) RASTI. [9] Barraud O. et al., (2025) EPSC. [10] Fox A. J. D. et al., (2025) EPSC. [11] Nittler L. R. et al., (2011) Science 333:6051:1847-1850. [12] Keil K. (2010) Geochemistry 70:4:295-317.

How to cite: Cartwright, J., Lindsay, S., Barry, T., Hall, G., Fox, A., Martindale, A., and Bunce, E.: Exploring Mercury’s Surface Compositions through the Mercury Imaging X-ray Spectrometer (MIXS) Ground Reference Facility (GREF), EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1742, https://doi.org/10.5194/epsc-dps2025-1742, 2025.

Introduction. The ESA-JAXA BepiColombo mission consists of two scientific spacecraft that, after arrival at Mercury, will be placed in separate polar orbits [1, 2]. The Mercury Planetary Orbiter (MPO) will focus on Mercury’s surface while the Mercury Magnetospheric Orbiter (MMO-Mio) will focus on Mercury’s environment and interactions with the Sun. The instruments of most relevance to studying Mercury’s surface are the BepiColombo Laser Altimeter (BELA), the Mercury Radiometer and Thermal Imaging Spectrometer (MERTIS), the Mercury Gamma-Ray and Neutron Spectrometer (MGNS), the Mercury Imaging X-ray Spectrometer (MIXS), and the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory System (SIMBIO-SYS) [3]. Additionally, BepiColombo will study the surface-environment interactions of Mercury with the Hermean Exosphere by Ultraviolet Spectroscopy (PHEBUS) and the Search for Exosphere Refilling and Emitted Neutral Abundances (SERENA) instruments, as well as MIXS [4]. Laboratory studies of Mercury analogs under simulated conditions are crucial for interpreting future remote-sensing data. The Mercury-Y Lab project was launched at the Mercury Laboratory Workshop (MLW) 2024, held at the Institute of Planetary Research of the German Aerospace Center (DLR) in Berlin (09/2024). The project consists of creation of a unique blind analog sample named ‘Mercury-Y’, distributed to MPO research partners, and laboratory analysis of the blind analog under conditions similar to the future observations of the BepiColombo MPO instruments, using representative instrumentation where possible. The primary objective of the various laboratory teams will be to determine the mineralogical and elemental composition alongside the physical properties of the blind sample. Here, we present the protocol for creating Mercury-Y, the laboratories and instrumental teams involved, the objectives of the project and the first results.. 

Blind sample. The Mercury-Y blind sample was designed to replicate the regolith of Mercury, leveraging our current understanding of Mercury’s surface composition derived from NASA MESSENGER observations [e.g., 5] and petrological studies [e.g., 6]. It was developed in collaboration with the Planetary Spectroscopy Laboratory (PSL) at the Institute of Planetary Research, DLR Berlin, and the Infrared and Raman for Interplanetary Spectroscopy (IRIS) laboratory at the Institute for Planetology at the University of Münster, following extensive discussions on the experimental constraints of each laboratory during and following MLW2024. Mercury-Y comprises  a mixture of terrestrial and synthetic components and was delivered to participating teams as a powder. Its bulk chemical composition, including trace and volatile elements, has been characterized at the Service d’Analyse des Roches et des Mineraux (SARM, CRPG, Nancy), and will form the basis of comparison for the project. The composition of the sample will not be revealed to the teams until the completion of the project.

Experimental procedure. The experimental procedure implemented for this project consists of performing laboratory measurements similar to BepiColombo experiments. Visible to near-infrared bi-directional reflectance is measured to simulate SIMBIO-SYS and BELA observations. Thermal infrared emissivity is measured at surface temperatures up to 700 K and under environments simulating near‐surface conditions of airless bodies to simulate MERTIS observations. X-ray fluorescence experiments are performed which mimic the interaction between MIXS, the solar flux and the Mercury surface using the MIXS qualification model detector within a ground reference facility (GREF). Particle back scattering and sputtered ions are measured in-situ after ion bombardment to simulate future observations by SERENA. Finally, high-temperature evaporation processes are simulated and analyzed at Mercury surface conditions, supporting the anticipated measurements from the BepiColombo SERENA and PHEBUS instruments.

Preliminary results. The primary objective of the measurements is to determine the blind sample's mineralogical and chemical composition, alongside its physical properties. The first reflectance measurements in the SIMBIO-SYS domain do not show significant absorption by mafic minerals such as olivine or pyroxene (Fig 1a). A strong drop in reflectance is observed at shorter wavelengths, with a blue slope in the visible and near-infrared (reflectance decreases as the wavelength increases). At 400°C, the emissivity measurements in the domain of MERTIS exhibits several spectral features, including multiple vibrational bands (Reststrahlen bands) and a highly contrasted emission maximum (Christiansen feature) just before 8 µm (Fig 1b). Preliminary X-ray fluorescence measurements with MIXS-GREF show the presence of Na, Mg, Al, Si, K, Ca and Fe (Fig 2). In comparison to other terrestrial silicate materials analysed previously on MIXS-GREF, Mercury-Y is enriched in Na and Mg, whilst being depleted in Al and Fe. We observed K to be significantly enriched compared to measured Mercury abundances, while S is entirely absent [5].

Figure 1: (a) Visible to near-infrared hemispherical reflectance (i=30°) measured at University of Helsinki in the wavelength domain of SIMBIO-SYS. (b) Emissivity at 400°C measured at PSL-DLR, Berlin in the wavelength domain of MERTIS.

Next steps. The teams are invited to cross-check the measurements in order to refine each other's results. This analysis stage is crucial to optimize the scientific results obtained by each lab/instrumental team. In a second phase, we will distribute a transformed/altered Mercury-Y sample, created to mimic space-weathering processing (e.g. thermal cycles, irradiation) and will run similar laboratory experiments. This further analysis will allow the teams to investigate the effects of space weathering on BepiColombo measurements, and to determine how these processes need to be considered when attempting to characterise Mercury’s surface composition.

Figure 2: (a) Mercury-Y pellets produced and used by the MIXS team for X-ray measurements. The lighter pellet is composed of the Mercury Y sample only while the darker pellet contains PVA-based binder (b) X-ray fluorescence spectrum recorded with the MIXS Ground Reference Facility (10 kV source, i=30°) from a Mercury-Y pressed pellet with a PVA-based binder. Note the Rh peaks are scattered from the X-ray source and do not represent fluorescence from the sample itself.

References: [1] Benkhoff, J., et al. (2021). Space Sci Rev 217(8), 90. [2] Murakami, G., et al. (2020) Space Sci Rev 216, 113. [3] Rothery, D. A., et al. (2020). Space Sci Rev 216, 1-46. [4] Milillo et al 2020, SSR special issue [5] Peplowski, P. N., & Stockstill‐Cahill, K. (2019). JGR: Planets, 124(9), 2414-2429. [6] Namur, O., and Charlier, B. (2017). Nature Geoscience, 10(1), 9-13.

How to cite: Barraud, O., Weber, I., Maturilli, A., Donaldson Hanna, K., Martindale, A., Carli, C., Cartier, C., Van den Neucker, A., Benkhoff, J., Flahaut, J., Domingue, D., Doressoundiram, A., Renggli, C., Penttilä, A., Adeli, S., and Jones, G. and the Mercury-Y team: Mercury-Y: A preparatory laboratory study for BepiColombo mission, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1747, https://doi.org/10.5194/epsc-dps2025-1747, 2025.

Introduction

Aubrites are a rare group of highly reduced achondrites. They are characterized by very low amounts of FeO in silicates and the presence of sulphides composed of typically lithophile elements, indicating a formation under extremely reducing conditions [1]. Their highly reduced nature and mineralogy suggest that aubrites represent promising analogues of Mercury’s surface rocks [2] based on MESSENGER mission results [3]. Mercury’s surface is affected by space weathering processes that influence its spectral properties increasing the VIS spectral slope (spectral reddening) and suppressing the absorption features in the VIS-NIR spectral range, due to the presence of iron nanoparticles (npFe⁰) [4]. For a more consistent comparison between the spectral properties of Mercury’s surface and of its potential analogues, the effect of space weathering should be considered. So far, space weathering simulations through ion irradiation on natural highly reduced materials have been conducted on one enstatite chondrite [5]. In this study we investigate ion irradiation spectral effects in the visible-middle infrared region on four aubrites: Norton County, Rantila, Peña Blanca Spring and NWA 14185.

Methods

The aubrite samples were crushed into powder (≤100 μm) and pressed to create pellets for the ion irradiation experiments [e.g. 6,7,8]. To simulate the solar wind irradiation affecting Mercury’s surface, we performed two separate ion irradiation experiments using He⁺ at 20 keV and C⁺ at 30 keV. He⁺ was chosen because helium represents an important component of the solar wind ions. C⁺ was chosen to simulate the effects of another solar wind species, whose irradiation effects on meteorites have been investigated only in a few studies [9].

The irradiation experiments were conducted under vacuum (P ~ 10^-7 mbar) using the SIDONIE electromagnetic isotope separator at the IJCLab, Université Paris-Saclay, Orsay, France. The whole sample surface was irradiated at every fluence step (1 x 10¹⁶, 3 x 10¹⁶, 6 x10¹⁶ and 1 x 10¹⁷ ions/cm²).

The visible (VIS) and near-infrared (NIR) reflectance spectroscopy measurements were conducted in-situ, in vacuum and at room temperature, through the INGMAR (IrradiatioN de Glaces et Météorites Analysées par Réflectance VIS-IR) setup, of IAS (Institut d'Astrophysique Spatiale)- IJCLab, Université Paris-Saclay, Orsay, France. VIS bidirectional reflectance spectra were acquired using a halogen visible source and a Maya2000Pro (Ocean Optics) spectrometer. NIR bidirectional reflectance spectra were acquired using a Fourier Transform spectrometer (Tensor37 Bruker). Middle-infrared (MIR) reflectance spectra were taken ex-situ on fresh samples and on samples after the final irradiation dose at SMIS (Spectroscopy and Microscopy in the Infrared using Synchrotron) beamline of the Synchrotron SOLEIL, France, using an Agilent Cary 670/620 micro-spectrometer equipped with MCT detector.

Preliminary results and discussion

Spectral effects of ion irradiation show a common general trend for all the investigated samples, and it is possible to observe some differences in the spectral effects produced by the two different ions. The main effects in the VIS-NIR range are a general spectral darkening and reddening, progressive with the increasing of irradiation doses. The extent of darkening and reddening varies among the samples, for both He⁺ and C⁺ irradiations. A possible explanation is that the samples present some differences in their mineralogy and mineral chemistry. For example, Rantila aubrite presents higher FeO contents (up to 0.3 wt%) in silicates than the other samples (<0.02 wt%) which causes a visible absorption band at ~0.9 μm, while the other samples are almost featureless. Rantila spectra also show a small absorption feature around 0.5 µm attributable to a spin-forbidden Fe²⁺ absorption in olivine [10], which is present in larger amount in this sample than in the others. These absorption features flatten with increasing irradiation doses, for both He⁺ and C⁺. Some absorption features are visible in the 1.9-2.1 μm region due to a minor terrestrial aqueous alteration of the samples. These features do not seem to be affected by irradiation.

In the MIR range we observe a shift in the position of the Christiansen feature (CF) and the Reststrahlen bands (RBs) between the spectra of fresh and irradiated samples. Also, the reflectance intensity of the RBs decreases, and they become less sharp.

Ongoing activities and perspectives

The data analysis is still ongoing, and additional investigations are planned on rock fragments of these aubrites to compare the effects of He⁺ and C⁺ irradiation on powders (pellets) and rocks. Studying the spectral properties of these highly reduced meteorites could support the future investigation of Mercury with the ESA's BepiColombo mission [11]. In particular, the knowledge of the spectral properties of reduced meteorites, in correlation with their mineralogy, could be useful in the interpretation of the future data from the Visible and near-Infrared Hyperspectral Imager (VIHI)/Spectrometer and Imaging for MPO BepiColombo Integrated Observatory SYStem (SIMBIO- SYS) [12] and the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) [13] which will map the surface in the 0.4-2 μm and 7–14 μm, respectively.

Acknowledgements

This research is supported by the Italian Space Agency (ASI) under ASI-INAF agreement 2017-47-H.0 (Bepicolombo SIMBIO-SYS) and financed by the Space It Up project funded by the Italian Space Agency, ASI, and the Ministry of University and Research, MUR, under contract n. 2024-5-E.0 – CUP n. I53D24000060005. It has been conducted during and with the support of the Italian national inter-university PhD programme in Space Science and Technology, University of Trento.

References:

[1] Keil K (2010) Chem Erde 70 : 295-317

[2] Wilbur Z E et al. (2022) Meteorit. Planet. Sci. 57: 1387-1420

[3] Nittler L R & Weider S Z (2019) Elements 15: 33-38

[4] Domingue D et al. (2014) Space Sci Rev 181

[5] Vernazza P et al. (2009) Icarus 202 : 477-486

[6] Brunetto R et al. (2014) Icarus 237: 278–292

[7] Lantz C et al. (2017) Icarus 285 : 43–57

[8] Caminiti E et al. (2024) Icarus 420

[9] Fulvio D et al. (2012) A&A 537

[10] Carli C et al. (2018) Meteorit. Planet. Sci. 53: 2228–2242

[11] Benkhoff J et al. (2021) Space Sci Rev 217

[12] Cremonese G et al. (2020) Space Sci Rev 216

[13] Hiesinger H et al. (2020) Space Sci Rev 216

How to cite: Landi, A. I., Brunetto, R., Lantz, C., Carli, C., Capaccioni, F., and Pratesi, G.: Ion irradiation of aubrites: visible to mid-infrared spectroscopic effects of space weathering simulation on Mercury analogues, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-631, https://doi.org/10.5194/epsc-dps2025-631, 2025.

Introduction

Hollows are peculiar surface features that were highlighted by the MESSENGER mission [e.g. 1]. They are small cavities showing flat-floored and rimless depressions often clustered in fields and relatively bright. They are also associated with a peculiar cyan color within the RGB of the enhanced color mosaic [2]. This color variation mainly indicates a higher reflectance and a bluer slope within the 400-1000nm wavelength range [3]. In literature, few cases have been deeply spectroscopically investigated at the best spatial resolution possible, and only for the hollow field in the Dominici crater was reported a potential absorption around 600-700nm [e.g. 4,5,6] with a subsequent inflection around 1000nm, which could indicate another absorption [5,6]. The putative absorption has been attributed to some sulfides (e.g. CaS, [4]) or with some chloride (e.g. NaCl, [6]). Whereas [5] argues to the possibility of attributing that absorption to rock forming minerals considering transitional elements, different from iron, on mafic silicates.

Here we investigated four different hollow fields widespread on the floor of Eminescu, Hopper, Sander and Warhol Craters, which shows a relatively high 600nm Band Depth parameters [7] and a high reflectance in correspondence of the hollow fields.

Data and Analytical Approach

MDIS 8 color mosaics have been produced with the same process described in [8] at different spatial resolutions taking care of the original image resolution from circa 385m/px up to 1600m/px. Global BDR, LOI and, preliminary, 8 color enhanced color mosaics, together with the best spatial resolution NAC and WAC images for each crater were used for morphological mapping. We used the ad-hoc color mosaics to stretch the enhanced color combination and better highlight the variability within the hollow fields and with respect to the surrounding terrains. Moreover, we investigate the spectral variability within the hollow fields of each crater, defining specific trends with respect to the different surrounding materials.

Preliminary Results

First, we analyzed the two different cases of Warhol and Hopper craters, where we have relatively high (385m/px) and low (1600m/px) color mosaics. We showed that the hollows are principally present on the crater floor, and partially on the exposed bedrock of the central pick, with a higher concentration of hollows around the central pick or on contact with the crater wall (in Figure1 an example for Warhol crater). Crater floors are mainly characterized by smooth material representing plains formed due to the refilling after the impact.

The Enhanced color mosaics of the selected images show a widespread distribution of bright and cyan deposits feeding almost all the crater floor of Warhol and Hopper (see figure1b for Warhol). If we stretched the enhanced color mosaics zooming on regions enriched on hollow fields, we highlighted a color variation emphasizing the fields with respect to the surrounding materials (e.g. Figure1c). We then investigate the spectral variation within different regions of interest (R.O.I.) on the hollow fields (see figure1b) showing that the spectral features are always consistent one to each other (Figure1d), with the presence of the putative band and the inflation towards the infrared. The main variation seems to be the relative reflectance values from 0.10 up to 0.15 at 560nm.

Moreover, if we zoom on specific R.O.I.s we can see how the stretched enhanced color mosaics match with the pixels showing morphological evidence of the hollows. As an example, here, we consider the area (figure2a) consistent with the brown R.O.I. on figure1b. Considering a specific transect that moves from the central area of an isolated hollow/small hollows field and the surrounding halo we can highlight a reduction of the absorptions where the morphological evidence indicating the hollow disappears. Thus suggesting a variation of composition with respect to the material deposited outside the hollows. The change in spectral slope seems to be the last to modify towards reddening and darkening spectra (Figure2b). Isolating the absorption band around 600-700nm shows how spectrally the main contribution of the material characterizing the composition of the hollows seems to involve the 630nm filter (Figure2c), which disappears on spectra outside the hollows, where the apparent absorption is mainly attributable to the 750nm filter.

Implications

The spectral variability within different regions of interest we investigated on Warhol crater, as well as in Hopper crater, and in the ongoing investigation on Eminescu and Sander craters, seems to highlight as the isolated hollows and hollow fields are characterized by the 600-700nm absorption and an inflation towards 1000nm. Stretching the enhanced color mosaics on hollow fields permits to better constrain the regions characterized by these features which match with the morphological counterpart of the hollows. Hollows spectral properties seem to suggest that they are compositionally different from the surrounding, as already shown for Dominici crater, taking also into account that the surrounding terrains on those craters look like to have different spectral properties. Moreover, it seems that a net change is present on the absorptions moving from inside to outside of a specific hollow/hollows field indicating that the halo could have spectral properties different from the hollow itself. This could highlight that the peculiar spectral properties of the hollows could be to assign to the material that characterized the region inside the hollows than those attributed to the halo, maybe revealing the evidence of the rock forming minerals instead of secondary alteration phases (e.g. sulfides or chlorides).

The spatial resolution will play an important role in understanding the possible nature of the material present in the hollows and to understand the hollows formation. Hollows can be considered as one of the most interesting targets for the BepiColombo mission, where we could combine the higher spectral range and the high spatial resolution from the SIMBIO-SYS instrument.

Acknowledgment

This research is funded from the Italian Space Agency (ASI) within SIMBIOS-SYS project under ASI-INAF agreement 2024-18-HH.0.

 References

[1] Blewett et al.(2011) Science 333. [2] Denevi et al.(2011) Science 333.  [3] Blewett et al.(2013) JGR 118. [4] Vilas et al.(2016) GRL 43.[5] Lucchetti et al.(2018) JGR 123. [6] Emran & Stack (2025) Icarus 435. [7] Klima et al.(2018) GRL, 45. [8] Zambon et al.(2022) JGR 127.

How to cite: Carli, C., Zambon, F., Carminati, F., Giacomini, L., Galiano, A., Massironi, M., Capaccioni, F., Filacchione, G., and Palumbo, P.: Comparison of extended hollow fields in Warhol, Hopper, Sander and Eminescu craters, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1417, https://doi.org/10.5194/epsc-dps2025-1417, 2025.

The formation of hollows on Mercury—small, irregular, depressions often surrounded by brighter halos—has been thought to be driven by the preferential loss of volatile components due to intense solar radiation, micrometeoroid bombardment, and thermal cycling, leaving behind a refractory, spectrally distinct layer. One such process is the decomposition of regolith sulfides under solar wind ion irradiation.

Previous ion irradiation experimental studies on sulfides such as NiS, CuS, CoS, FeS, and MoS have demonstrated the formation of metallic surface layers through cation chemical segregation and preferential sulfur loss [1-5]. Laboratory observations of Fe surface-enhancement with subsequent visual darkening for ion and laser irradiated troilite (FeS)  and recently, pentlandite [(Fe,Ni)₉S₈], thereby informed the hypothesis that similar process could occur with Mercury-relevant sulfides [e.g., 6, 7].  Instead of FeS, however, Mercury appears depleted in Fe at the surface, and sulfides are expected to contain mostly Mn, Ti, Cr, Mg, and Ca, based on the correlation with sulfur on the planet’s surface as inferred from MESSENGER’s X-Ray Spectrometer (XRS) and Gamma-Ray Spectrometer (GRS)  data [8-10].

To test if sulfides relevant to Mercury darken under ion irradiation, we focus on MgS and CaS, two sulfides that also were proposed as likely hollow-forming material based on visible-to-near-infrared spectral analysis [11]. Unlike their transition-metal counterparts, these sulfides exhibit irradiation-hardening behavior, resisting the formation of a metallic top layer when exposed to solar wind-speed protons and helium ions [12]. This suggests that their response to space weathering differs fundamentally from that of previously studied sulfides, which also impacts how global exospheric models incorporate sputtered sulfur. Notably, our data reveal that MgS and CaS undergo significant brightening under irradiation in not only the visible-to-near-infrared (VNIR), but also the thermal infrared (TIR) spectral range. This optical alteration for stoichiometrically sputtered sulfide species aligns with the observed reflectance properties of hollows’ bright halos on Mercury.

We therefore propose that irradiation-hard sulfides, such as MgS and CaS, could be responsible for the bright spectral signatures surrounding hollows. However, rather than forming metal-rich coatings, these compounds develop radiation-resistant, chemically stable surfaces that enhance reflectance while maintaining their compositional and structural integrity under Mercury’s extreme solar radiation conditions. This mechanism offers a new perspective on hollow bright material formation, emphasizing the role of non-transition-metal sulfides in shaping the planet’s surface evolution.

[1] Feng and Chen (1974), J. Phys. C, 7(5), L75

[2] Coyle et al. (1980), J. Electron Spectrosc. Relat. Phenom., 20(2), 169–182

[3] Loeffler et al. (2008), Icarus, 195(2), 622–629

[4] Christoph et al. (2022), J. Geophys. Res. Planets, 127(5)

[5] Chaves et al. (2025), Meteorit. Planet. Sci., in process

[6] Blewett et al. (2011), Science, 333(6051), 1856–1859

[7] Blewett et al. (2013), J. Geophys. Res. Planets, 118(5), 1013–1032

[8] Nittler et al. (2011), Science, 333(6051), 1847–1850

[9] Weider et al. (2012), J. Geophys. Res. Planets, 117(10), 1–15

[10] Weider et al. (2015), Earth Planet. Sci. Lett., 416, 109–120

[11] Barraud, Besse and Doressoundiram (2023), Sci. Adv., 9(12)

[12] Jäggi et al. (2024), 55 Lunar Planet. Sci. Conf., Abstract #1306

How to cite: Jäggi, N., Barraud, O., and Dukes, C. A.: Radiation-Hard Sulfides as Mercury Hollow Bright Material, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-878, https://doi.org/10.5194/epsc-dps2025-878, 2025.

This study introduces an automated method for extracting and tracking geomorphic features on Mercury by leveraging high-resolution Digital Terrain Models (DTMs) generated from MESSENGER’s Mercury Laser Altimeter (MLA) data (Cavanaugh et al., 2007). The DTMs are produced using a robust RANSAC-based region-growing technique, ensuring detailed and accurate terrain representation of Mercurian landscapes (Arghavanian, et al., 2024). Feature identification begins with principal curvature analysis of the DTMs to detect key geomorphic structures such as valleys and ridges (Besl, 1988), (Richards, 2013). To enhance classification accuracy and minimize false positives, particularly in complex volcanic terrains the algorithm integrates morphological parameters (Byrne, et al., 2013), (Du, et al., 2020), including solidity and width-to-length ratios. Cross-sectional profiles are then extracted along local directions of maximum curvature, utilizing an adaptive section length (or ‘bandwidth’) to optimize feature validation. Each cross section undergoes thorough analysis to confirm geomorphic interpretations and refine feature delineation (Arghavanian& Leloglu, 2024). Application of this algorithm to volcanic regions on Mercury demonstrates its efficiency and reliability in planetary geomorphic feature classification, contributing to a deeper understanding of Mercurian surface processes.

References:

Arghavanian A., Stenzel O., Hilchenbach M., 2024, Smooth Hermean surface extraction by region growing from Messenger Laser Altimeter Data, EPSC 2024 conference, Berlin, Germany.

Arghavanian A. & Leloğlu U. M., 2024. Extraction and classification of channels from LiDAR in plains by channel tracking. Environmental Modelling and Software, Volume 171, p. 105838.

Besl P. J., 1988. Surfaces in Range Image Understanding. 1th ed. New York, USA.: Springer.

Byrne P. K., Klimczak C., Williams D. A., Hurwitz D. M., Solomon S. C., James W. Head, Preusker F., Oberst J., 2013, An assemblage of lava flow features on Mercury, Journal Of Geophysical Research: Planets, VOL. 118, 1303–1322, doi:10.1002/jgre.20052.

Cavanaugh J.F. et al. (2007) ‘The Mercury Laser Altimeter Instrument for the MESSENGER Mission’, Space Science Reviews, 131(1), pp. 451–479. Available at: doi.org/10.1007/s11214-007-9273-4.

Du J., Wieczorek M. A., Fa W., 2020, Thickness of lava flows within the northern smooth plains on Mercury as estimated by partially buried craters, Geophysical Research Letters, 10.1029/2020GL090578

Richards, J. A., 2013. Remote Sensing Digital Image Analysis. 5th ed. Canberra, Australia: springer.

How to cite: Arghavanian, A., Stenzel, O., and Hilchenbach, M.: Hermean curvature-based geomorphic feature classification using Laser Altimetry data, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1695, https://doi.org/10.5194/epsc-dps2025-1695, 2025.

Solar wind ions that impact Mercury contribute to alteration of its surface and erosion via sputtering [1]. The exact contribution of sputtering to the weathering of Mercury’s surface is, however, still unclear. To provide updated estimates of sputtering as a source for Mercury’s exosphere, we use a combination of two simulation approaches: the Amitis hybrid code and the SDTrimSP-3D ion-surface interaction model [2,3]. In doing so, we first simulate the global interaction of Mercury’s magnetic field with the solar wind to derive energy-resolved precipitation maps (see Figures 1 and 2). These precipitation fluxes are then combined with sputter yields for a regolith-covered surface to calculate sputter flux maps and global sputter source rates. In this work, we specifically consider the sputtering of the refractory elements Ca and Mg, which have been observed by both NASA’s MESSENGER mission and ESA and JAXA’s BepiColombo [4-7]. Due to their strong bonds at the surface, only sputtering and micrometeoroid impact vaporization are candidate processes for releasing these atoms into the exosphere.

Figure 1: Due to Mercury’s magnetic field, ions impact the surface only at specific locations.

Our simulations consider a range of solar-wind upstream parameters, as the variability of Mercury’s space environment affects the flux of ions to the surface significantly. First, we consider average solar wind conditions and find that sputter rates of Ca and Mg are 1-2 orders of magnitude too low to explain MESSENGER observations. This is in line with micrometeoroid models that predict the seasonal dependence of their exospheric sources, and the dawn-centered emission well [8]. Our study suggests that the sputter yields in previous models have been overestimated, and it supports that micrometeoroid impact vaporization dominates refractory exospheres at most times.

Figure 2: Compared to average precipitation conditions (left, dynamic pressure of 9 nPa), the ion flux to the surface under strong CME impacts is vastly increased (right, 428 nPa).

However, under extreme space weather conditions during the impacts of coronal mass ejections (CMEs), we see a significant change of the exospheric regime. The solar wind pressure has been observed to increase by up to around a factor of 50 compared to regular conditions [9]. This has been reported to lead to compression and even vanishing of Mercury’s dayside magnetosphere [10]. As the planet’s dayside surface becomes exposed directly to the solar wind, our simulations show that surface precipitation and sputtering increase by as much as two orders of magnitude (see Figure 2). Under such conditions, sputtering exceeds the source rates for Ca and Mg exospheres from micrometeoroids. We thus infer that a temporary reconfiguration of Mercury’s exosphere occurs during strong CME impacts, especially when the solar wind dynamic pressure reaches more than 100 nPa. This is further underlined by Direct Simulation Monte Carlo (DSMC) simulations of the spatial distribution of atoms in Mercury’s exosphere [11]. Compared to a micrometeoroid-dominated dawn-centered exosphere under average SW conditions [12], we demonstrate the distribution of refractory atoms becomes centered on the dayside due to the vast sputtering increase. The upcoming BepiColombo mission will provide plenty of opportunities to observe such events to better understand how Mercury’s exosphere, surface, and space environment are connected.

References

[1]  P. Wurz, et al., Space Science Reviews 218.3 (2022), 10.

[2]  S. Fatemi, et al., Journal of Physics: Conference Series 837.1 (2017).

[3]  U. Von Toussaint, et al., Physica Scripta,  2017.T170 (2017), 014056.

[4]  M.H. Burger, et al. Icarus 238 (2014), 51.

[5]  A.W. Merkel, et al., Icarus 281 (2017), 46.

[6]  R. Robidel, et al., Journal of Geophysical Research: Planets 128.12 (2023), e2023JE007808.

[7]  Y. Suzuki, et al. Journal of Geophysical Research: Planets 129.10 (2024), e2024JE008524.

[8]  P. Pokorný et al., The Astrophysical Journal 863.1 (2018), 31.

[9]  R. M. Winslow et al., The Astrophysical Journal 889.2 (2020), 184.

[10]  J.A. Slavin, et al., Journal of Geophysical Research: Space Physics 124.8 (2019), 6613.

[11]  S.R. Carberry Mogan, et al.,  Journal of Geophysical Research: Planets 127.11 (2022), e2022JE007294.

[12]  J.-Y. Chaufray, et al., Icarus 384 (2022), 115081.

How to cite: Szabo, P. S., Poppe, A. R., Carberry Mogan, S. R., Fatemi, S., Mutzke, A., Huang, J., Sun, W., and Zhao, J.: Sputtering Contributions to Mercury’s Exosphere During Average and Extreme Solar Wind Conditions, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-458, https://doi.org/10.5194/epsc-dps2025-458, 2025.

Meteoroid bombardment of Mercury’s surface causes seasonal variability in its calcium (Ca) exosphere, as observed by the MESSENGER mission [1, 2]. The observed high-energy Ca source exhibits a dawn enhancement and a distinct dawn–dusk asymmetry, consistent with the directionality of meteoroid impacts. The most likely mechanism generating Mercury’s Ca exosphere involve a combination of processes, including the release of both atomic and molecular surface materials [3].

In this study, we use the exosphere generation model developed at the Institute for Space Astrophysics and Planetology (IAPS) [6] to simulate the 3D spatial distribution of atomic Ca and Ca-bearing molecules produced by the Meteoroid Impact Vaporization (MIV) process. The model includes two distinct populations: a high-energy Ca component, originating from shock-induced, non-equilibrium dissociative ionization and neutralization of Ca+ during the vapor cloud expansion, and a lower energy Ca component, generated through the photo-dissociation of Ca-bearing molecules released by MIV [4,5].

Model parameters, such as the photolysis lifetime of the molecules and the relative abundances of atomic and molecular Ca components involved in the different released processes, play a crucial role in shaping Mercury’s Ca exosphere, but they are still not well constrained by observations.

In order to better understand the mechanisms governing the release and distribution of the Ca exosphere, we compare model simulations with data acquired by the PHEBUS spectrometer (Probing of Hermean Exosphere By Ultraviolet Spectroscopy) onboard ESA-JAXA’s BepiColombo spacecraft, currently in the cruise phase towards its destination. During the first three Mercury flybys, performed in 2021, 2022 and 2023, PHEBUS was able to observe the Ca emissions near closest approach using the two visible channels dedicated to the Ca measurements [7]. The three flybys had similar trajectories: the spacecraft approached from the planet’s nightside, crossing the shadow and reaching the closest approach about 200 km above the surface, then moved to the planet’s dayside. However, during the third flyby, the closest approach was slightly further at 235 km of altitude, and PHEBUS’s lines of sight was pointing southward, in the opposite direction with respect to the first two flybys where it was pointing northward (Fig. 1).

We reconstructed the geometry of the observations for each flyby and simulated the Ca exosphere under those conditions. Then, we derived Ca altitude profiles along the PHEBUS line of sight and compared them with the observational ones. Preliminary results suggest that a high-energy Ca component with a temperature of approximately 50,000 K is consistent with the observed intensities, while the contribution from a low-energy component seems to be negligible. The absence of lower energy component in the PHEBUS Ca observations is consistent with the MESSENGER data. This does not necessarily imply that this component, expected by the surface release modeling after micro-meteoroid impact, does not exist. It could be due to limitations in PHEBUS’s observations.

It should be noted that the constrained set of model parameters offers the opportunity to also investigate the seasonal variability of the Ca source rate and to reconstruct the exospheric seasonal Ca profile along Mercury’s orbit. Our theoretical calculations (Fig. 2) show good agreement with MESSENGER/UVVS data, supporting the interpretation of seasonal effects driven by meteoroid flux variations. During nominal mission, PHEBUS will collect extensive data along Mercury’s orbit, that will be complemented by SERENA-Strofio in situ measurements, so we’ll be monitoring seasonal variations too and these data could be used to compare with the model. This study allows evaluation of the consistency between the modeled and observed Ca distributions, and helps constrain the physical parameters governing exospheric source processes. Our results advance our understanding of the MIV process on Mercury, providing a valuable tool for interpreting data and guiding observational strategies for the ESA/JAXA BepiColombo mission, which will insert into Mercury’s orbit at the end of 2026.

Figure 1: Geometry of PHEBUS observations during the first three flybys of Mercury: spacecraft arrived on the planet's nightside, crossed its shadow and then moved to its dayside

Figure 2: Simulation of seasonal Ca content in Mercury's exosphere due to the meteoroid flux (red line) compared with the observations (blue line) [2] along the orbit, and, hence, True Anomaly Angle (TAA)

References

[1] Burger, M.H., Killen, R.M., McClintock, W.E., Vervack, R.J., Merkel, A.W., Sprague, A.L., Sarantos, M., 2012. “Modeling MESSENGER observations of calcium in Mercury’s exosphere”. J. Geophys J. Res. 117

 [2] Burger, M.H., Killen, R.M., McClintock, W.E., Merkel, A.W., Vervack, R.J., Cassidy, T.A., and Sarantos, M., 2014. “Seasonal variations in Mercury’s dayside calcium exosphere”, Icarus 238, 51–58

[3] Killen, R.M., and Hahn, J.M., 2015. “Impact vaporization as a possible source of Mercury’s calcium exosphere”, Icarus 250, 230-237

[4] Killen, R.M., 2016. “Pathways for energization of Ca in Mercury’s exosphere”, Icarus 268, 32–36.

[5] Moroni, M., Mura, A., Milillo, A., Plainaki, C., Mangano, V., et al., 2023. “Micro-meteoroids impact vaporization as source for Ca and CaO exosphere along Mercury's orbit”, Icarus, 401, 115616

[6] Mura, A., Milillo, A., Orsini, S., and Massetti, S., 2007. “Numerical and analytical model of Mercury’s exosphere: dependence on surface and external conditions”, Planet. Space Sci. 55, 1569–1583

[7] Robidel, R., Quémerais, E., Chaufray, J. Y., Koutroumpa, D., Leblanc, F., Reberac, A., et al., 2023. “Mercury's exosphere as seen by BepiColombo/PHEBUS visible channels during the first two flybys”, Journal of Geophysical Research: Planets, 128(12)

How to cite: Moroni, M., Robidel, R., Milillo, A., Mura, A., Quemerais, E., Mangano, V., Aronica, A., Brin, A., De Angelis, E., Di Bartolomeo, P. P., Kazakov, A., Massetti, S., Orsini, S., Plainaki, C., Richards, G., Rispoli, R., Sordini, R., and Stumpo, M.:  Modelling of Mercury’s Ca exosphere observed by PHEBUS during the First Three Flybys, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-304, https://doi.org/10.5194/epsc-dps2025-304, 2025.

Mercury’s escaping atmosphere is known to form an extended, comet-like sodium tail that extends as far as 1500 Mercury radii (R_M) from Mercury (Baumgardner et al. 2008, Schmidt et al. 2010). The tail forms due to radiation pressure, a consequence of the resonant scattering of sunlight, the primary sodium emission mechanism at Mercury. Resonant scattering is the process by which photons at wavelengths corresponding to resonant transitions in an atom are absorbed and quickly reemitted. Because incident photons originate from the direction of the Sun and are reemitted isotropically, there is a change in the total momentum of the photons. This momentum is imparted tot he atoms, resulting in a net force directed radially away from the Sun.

The magnitude of radiation acceleration is proportional to the photon scattering coefficient, the so-called g-value, the product of the incident photon flux at the resonant wavelength and the scattering probability per atom (Chamberlain 1961). The Na g-value varies by over an order of magnitude during a Hermean year due Mercury’s changing distance from the Sun and variations in the solar flux at the Doppler-shifted resonance wavelength caused by deep Fraunhofer absorption lines in the solar spectrum. Figure 1 shows the radiation acceleration as functions of velocity (Panel a) and true anomaly angle (TAA, Panel b) for Na, Ca, and Mg at Mercury.

We will explore seasonal variations in the extended Na tail out to 1500 R_M for proposed exospheric Na source processes from Mercury such as photon stimulated desorption, micrometeoroid impact vaporization, and ion sputtering. Because of the difficulty of observing the tail from ground-based telescopes, observations have only been made at a small range of true anomaly angles. Our models will result in predictions of the extent of the tail at other true anomalies and suggest methods by which it can be observed.

Figure 1: (a) Radiation acceleration, a_r,  for Na (red), Mg (magenta), and Ca (blue) as a function of an atom’s radial velocity relative to the Sun at a solar distance of 0.39 AU. The dotted lines show ±10 km s^-1, Mercury’s minimum and maximum radial velocity relative to the Sun. (b) Radiation acceleration of the same species as a function of Mercury’s TAA for an atom at rest relative to Mercury. The scale of the left shows the magnitude of a_r near Mercury. The scale on the right shows for both (a) and (b) the ratio of the radiation acceleration to the surface gravitational acceleration. From Burger et al., submitted (2025).

References

Baumgardner, J., Wilson, J., Mendillo, M. 2008. Imaging the sources and full extent of the sodium tail of the planet Mercury. Geophysical Research Letters 35. doi:10.1029/2007GL032337

Burger, M. H., et al., 2025. Effects of the Changing g-Value on Mercury’s Exospheric Structure, Planetary Science Journal, submitted.

Chamberlain, J. W. 1961. Physics of the aurora and airglow. International Geophysics Series, New York: Academic Press, 1961.

Schmidt, C. A., Wilson, J. K., Baumgardner, J., Mendillo, M. 2010. Orbital effects on Mercury’s escaping sodium exosphere. Icarus 207, 9–16. doi:10.1016/j.icarus.2009.10.017

How to cite: Burger, M., Killen, R., Vervack, R., and Schmidt, C.: Models of Seasonal Variations in Mercury’s Extended Sodium Tail Due to Variations in the g-Value, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-685, https://doi.org/10.5194/epsc-dps2025-685, 2025.

Observations of Mercury’s sodium exosphere reveal a distinctive annual variability, with two intensity peaks occurring near aphelion and perihelion. We identify a "hot pole" of micrometeoroid impact vaporization (MMIV) as a critical source of sodium, which is active for all Mercury orbit, with prevalence on the dawn side, but whose effect is evident to explain Mercury’s aphelion Sodium peak. On a yearly cycle, the MMIV is more enhanced in two regions, located at 90° and 270° longitude at the equator. Using new data from the Themis telescope, we confirmed this hypothesis by employing a Markov Chain Monte Carlo (MCMC) method to rigorously test the relevance of this asymmetric source. This model accurately captures both the magnitude and the seasonal patterns of Mercury’s Na exosphere. The results highlight the combined effect of Mercury’s rotation and orbital position in modulating sodium source and loss processes, explaining the observed dawn-side enhancements. Additionally, the model underscores the necessity of a persistent sodium supply on the nightside, potentially driven by plasma interactions or micrometeoroid precipitation, while also shedding light on the still-puzzling dusk-side sodium increases observed by MESSENGER during its inbound passes.

Figure: Yearly-averaged MMIV precipitation onto the surface of Mercury (from Mura et al., 2023 https://doi.org/10.1016/j.icarus.2023.115441). The two regions at 90 and 270 of longitudes are "hot poles" of MMIV precipitation.

How to cite: Mura, A., Milillo, A., Mangano, V., Leblanc, F., Moroni, M., Plainaki, C., Massetti, S., Orsini, S., Stumpo, M., and Dibartolomeo, P.: The effect of asymmetrical MMIV flux on the yearly variability of Mercury's Sodium exosphere, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1538, https://doi.org/10.5194/epsc-dps2025-1538, 2025.

The Strofio neutral mass spectrometer aboard the BepiColombo Mercury Planetary Orbiter (MPO) will deliver unprecedented in situ mass-resolved measurements of Mercury’s exosphere. Designed to detect neutral atoms and molecules with high mass resolution via time-of-flight analysis (m/Δm ≥ 60), Strofio enables identification of exospheric constituents as the spacecraft traverses low-altitude polar orbits. Despite lacking intrinsic energy resolution, Strofio offers valuable directional sensitivity through its collimated field of view, spacecraft-relative velocity, and TOF measurements. These allow partial reconstruction of the particles’ distribution function. However, interpreting the resulting signals to infer exospheric source distributions requires a dedicated modeling framework that accounts for the ballistic, collisionless nature of Mercury’s exosphere and the instrument’s measurement geometry.

This work presents a signal inversion framework developed to translate Strofio’s detection count rates into surface production properties, including source location, emission mechanisms, and relative contribution of physical processes. The method is based on a Liouville Algorithm (LA), which propagates virtual particles backward in time from the point of detection through Mercury’s gravitational field to the planetary surface. Liouville’s theorem ensures that phase space density is conserved along these trajectories, allowing the surface source distribution to be mapped to the detector location. By integrating over the instrument’s known field of view, the algorithm calculates the expected count rate for a given surface emission model.

To support global exospheric context, the Exospheric Global Model or EGM (Leblanc et al., 2017) – a 3D Forward Monte Carlo model – is also included in the framework. EGM simulates large ensembles of particles launched from the surface, tracking their trajectories under gravity and/or optional forces. It is particularly useful for modeling multi-hop transport and determining steady-state surface coverage for volatile species. The LA model and EGM are used in a hybrid configuration where the FMC estimates the steady-state distribution of re-emitted particles, and the LA predicts the resulting Strofio signal. This combined approach ensures physical self-consistency and computational efficiency.

Importantly, the framework incorporates Strofio’s exact detection geometry and TOF characteristics. The instrument’s collimation angle, sensitivity function, and mass resolution are included in the integration to faithfully simulate the expected signal. Loss processes such as photoionization or detector dead time can also be included in the signal mapping, ensuring realistic comparison with observed data.

In summary, this framework enables Strofio to fulfill its science goals by linking count rates in time-of-flight spectra to the physical and spatial properties of Mercury’s surface sources. By combining Liouville-based inversion with targeted forward simulations and instrument-specific integration, the model transforms directional data into actionable scientific insight.

How to cite: Steichen, V., Livi, S., Leblanc, F., and Schroeder, J.:  Decoding Neutral Emission Mechanisms in Mercury’s Exosphere from STROFIO Mass Spectra, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-759, https://doi.org/10.5194/epsc-dps2025-759, 2025.

The ESA-JAXA joint mission BepiColombo is still on the track to Mercury. The two spacecraft for BepiColombo, Mio (Mercury Magnetospheric Orbiter: MMO) and Mercury Planetary Orbiter (MPO), are combined with MMO Sun Shield (MOSIF) and Mercury Transfer Module (MTM) during the cruise phase. BepiColombo will arrive at Mercury in November 2026, and it has 8-years cruise with the heliocentric distance range of 0.3-1.2 AU. The long cruise phase also includes 9 planetary flybys: once at the Earth, twice at Venus, and 6 times at Mercury. On 8 January 2025 we completed the last (6th) Mercury flyby successfully. Even before arrival, we already obtained fruitful science data from Mercury during the Mercury flybys. We performed science observations with almost all the instruments onboard Mio and successfully obtained comprehensive data of Mercury’s magnetosphere such as magnetic fields, plasma particles, and waves. Here we present the overview and initial results of the science observations during the Mercury flybys.

How to cite: Murakami, G. and Jones, G.: Overview and initial results of BepiColombo Mercury flybys, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1292, https://doi.org/10.5194/epsc-dps2025-1292, 2025.

Mercury hosts a dynamic and highly variable magnetosphere shaped by its weak intrinsic magnetic field and the intense pressure of the solar wind. Previous observations from spacecraft sent to the planet have provided key insights into Mercury’s magnetospheric structure and energetic particle populations, revealing transient and highly variable energetic electron enhancements within the planet’s magnetosphere. We present BepiColombo/SIXS observations of energetic electron populations in Mercury’s magnetosphere during the spacecraft’s first three flybys of the planet. Although no such populations were observed during the first flyby, strong energetic electron signatures were observed during the second and third flybys. We also present energetic electron and proton observations during the fourth and sixth flybys showing strong planetary shielding and magnetic mirroring effects during SEP events. Additionally, we present the highest time resolution energy spectra (> 70 keV) produced at Mercury. These observations are further discussed in the context of previous observations (e.g. MESSENGER).

How to cite: Edwards, L., Kilpua, E., Vainio, R., Grande, M., Lawrence, D., Aizawa, S., Lehtolainen, A., and Esko, E.: Energetic Particle Observations During BepiColombo’s Mercury Flybys, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1795, https://doi.org/10.5194/epsc-dps2025-1795, 2025.

On 8 January 2025, BepiColombo performed its sixth and final Mercury gravity assist, following a unique trajectory during cruise from the night-side southern hemisphere to the day-side northern hemisphere, crossing the cusp region at altitudes as low as ~295 km. This high-latitude, low-altitude path provided an unprecedented opportunity to investigate Mercury’s inner magnetosphere dynamics. Despite limitations due to the spacecraft’s cruise-mode configuration, the magnetometer (MPO-MAG; Heyner et al., 2021) and the Mercury Plasma Particle Experiment (MPPE; Saito et al., 2021), particularly the Mass Spectrum Analyzer (MSA; Delcourt et al., 2016) and Mercury Ion Analyzer (MIA), acquired valuable ion measurements.

We present new observations of cold planetary ions (Na⁺/Mg⁺ and K⁺/Ca⁺, down to ~10 eV/e) and a potential detection of additional species such as Si⁺ in the central plasma sheet. Energetic H⁺ ions (~10 keV/e) were also detected and are consistent with a ring current configuration, as supported by backward-traced test particle simulations.

Most notably, we report the first evidence of sporadic ion injections into Mercury’s polar regions. Dispersed H⁺ signatures (>1 keV/e) suggest injections of solar wind ions from the night-side magnetotail, while heavy ion dispersions indicate localized planetary ion injections and bounce motion near the planet. These results highlight dynamic processes, possibly substorm-like activity, shaping Mercury’s plasma environment.

How to cite: Hadid, L. and the MSA-MIA-MEA/MPPE-LEP, MPO-MAG and MGF: Planetary ion species and sporadic plasma sheet ion injections observed during BepiColombo’s sixth Mercury flyby, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-334, https://doi.org/10.5194/epsc-dps2025-334, 2025.

On 8 January 2025, the ESA/JAXA BepiColombo mission flew by Mercury for the sixth time at an altitude of 295 km. The spacecraft took on a unique route through Mercury’s magnetic and particle environment, crossing the equator opposite the Sun on Mercury’s night side before flying over the planet’s north pole. During eclipse, in the cold shadow of the planet, as well as above the northern pole the spacecraft passed through regions where charged particles precipitate from the planet’s magnetic tail and from the solar wind towards its surface. We will detail the original electron observations obtained by the Mercury Electron Analyzer during Mercury’s sixth flyby, and compare and contrast them with electron observations obtained during previous BepiColombo flybys. All together, these new observations will provide new insights into the diversity of structures observed in these regions and the underlying mechanisms responsible for their formation and dynamics.

How to cite: André, N. and the MEA-MPPE Team: Observations from the Mercury Electron Analyzer onboard BepiColombo during its sixth Mercury flyby  , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1442, https://doi.org/10.5194/epsc-dps2025-1442, 2025.

Mercury's environment is populated with a tenuous exosphere consisting mostly of sodium. After ionization by solar radiation, these sodium ions take peculiar paths through the magnetosphere; being trapped along dipolar magnetic field lines, re-impacting on the surface, or being lost downstream.
Using the global 3D hybrid model AIKEF, we will investigate the effect of different interplanetary magnetic field (IMF) directions onto the ion's pathways through the magnetosphere. These model cases are then compared to BepiColombo plasma observations made along the swingbys 12346, aiding the magnetometer observations in the determination of the upstream IMF direction cases.

How to cite: Exner, W., Teubenbacher, D., Robidel, R., and Krupp, N.: Sodium Ion Distribution in Mercury's Magnetosphere as seen from MSB12346, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-692, https://doi.org/10.5194/epsc-dps2025-692, 2025.

MESSEGER observations at Mercury have uncovered a highly dynamic magnetosphere, primarily driven by magnetic reconnection between the IMF and the planet’s intrinsic field. Frequent reconnection at Mercury’s dayside magnetopause efficiently couples the solar wind to the magnetosphere, resulting in the acceleration and heating of solar wind particles as they enter the magnetosphere. In this study, we present findings from global coupled fluid-kinetic simulations of Mercury’s magnetosphere (Li et al., 2024, JGR), focusing on the dynamics in the cusp region. These simulations utilize the MHD-AEPIC (MHD with Adaptively Embedded PIC) model, conducted with varying solar wind and IMF conditions. Our results reveal frequent occurrences of filamentary structures in the cusp region, marked by significant reductions in magnetic field strength and notable increases in plasma density and pressure. These features closely resemble those of the so-called “cusp filaments” observed by MESSENGER (e.g., Poh et al., 2016, JGR). By tracking their evolution in our simulations, we demonstrate that cusp filaments essentially represent the high-latitude extensions of flux transfer events (FTEs). Additionally, we examine the spatial distribution and variability of solar wind proton precipitation onto the planetary surface through the cusps, which is crucial for understanding surface weathering and exosphere generation at Mercury. Our simulations indicate global precipitation rates ranging from 0.7 to 2.5×10²⁵ particles per second, consistent with MESSENGER observations, and these rates increase with decreasing solar wind Mach number and IMF clock angle. Moreover, we observe a notable dawn-dusk asymmetry in proton precipitation patterns, with enhanced dawnside precipitation, aligning with asymmetries predicted in magnetopause reconnection occurrences by previous MHD-AEPIC simulations.

How to cite: Jia, X. and Li, C.: Dynamics in Mercury’s magnetospheric cusps: New insights from global coupled fluid-kinetic simulations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1088, https://doi.org/10.5194/epsc-dps2025-1088, 2025.