Rozenn Robidel

The icy moons of the outer solar system have been found in the forefront of planetary exploration, due to their diverse geomorphology coupled with numerous discoveries hinting to possible habitable conditions. Of paramount importance for constraining the internal structure and properties of these bodies are high-precision cartographic products, allowing for linking surface morphologies and composition to internal activity. These cartographic products are constructed by reconciling numerous different observations, which, due to the complexities of an orbital tour, are recorded under a great range of different illumination and observation conditions, with varying spatial sampling. The aim of this study is to provide a complete pipeline for producing Super-Resolution (SR) maps for icy moons, by combining all the overlapping observations for a specific target. The development and testing of our pipeline are carried out for Enceladus, using the hyperspectral cubes recorded by the Visual and Infrared Mapping Spectrometer (VIMS) instrument, on board the Cassini Saturn orbiter. We first navigate the data, calculating accurate projections that can be combined into global mosaics of Enceladus. We then implement a global photometric correction model that compensates for the different acquisition conditions. Finally, we combine all the overlapping observations to calculate the new SR maps of Enceladus in the infrared. The new SR global map of Enceladus provides new insight into the moon’s geomorphology as revealed from its surface spectral heterogeneity at finer spatial resolution, demonstrating the capability of SR to augment our current understanding of icy moons and help prepare for future observations.

Etude de l’exosphère de Mercure par le spectromètre PHEBUS de la mission BepiColombo: Mercure est la planète tellurique la moins explorée du système solaire, trois missions seulement l’ont survolée. Mariner 10 a effectué trois survols de la planète en 1974 et 1975, découvrant une atmosphère très mince, similaire à celle de la Lune, contenant de l’hélium (He) et de l’hydrogène (H). Le sodium (Na), le potassium (K) et le calcium (Ca) ont été détectés plus tard à partir d’observations depuis la Terre. La mission MESSENGER, lancée en 2004, a non seulement survolé Mercure à trois reprises mais a également orbité la planète durant quatre ans. Elle a détecté deux nouvelles espèces, le magnésium (Mg) et le manganèse (Mn). En parallèle, deux autres espèces ont été découvertes grâce à des observations depuis la Terre : le fer (Fe) et l’aluminium (Al). BepiColombo est la troisième mission à destination de Mercure. Lancée en 2018, la mission conjointe ESA JAXA doit effectuer six survols de Mercure avant de s’insérer en orbite fin 2025. Trois survols de Mercure ont déjà été réalisés lors desquels PHEBUS (Probing the Hermean Exosphere By Ultraviolet Spectroscopy) a pu observer l’exosphère de Mercure. L’instrument est composé de deux détecteurs UV (EUV et FUV) et de deux canaux visibles (c404 et c422). Le canal c404 est dédié à la ligne d’émission du K à 404,7 nm tandis que le canal c422 est dédié à la ligne d’émission du Ca à 422,8 nm. La géométrie d’observation des deux premiers survols est similaire, la sonde approchant la planète côté nuit, traversant son ombre avant de se diriger côté jour. La sonde s’est approchée à ∼200 km de la surface au point le plus proche côté nuit. PHEBUS pointait vers le nord, dans une direction opposée au soleil. Le taux de comptage enregistré par les deux canaux visibles illustre la géométrie d’observation, notamment le passage dans l’ombre de Mercure. Le maximum est atteint peu après la sortie d’éclipse. Le taux de comptage diminue ensuite à mesure que la sonde s’éloigne de Mercure. Lors des survols, les signaux des deux canaux sont pollués par des pics sporadiques dont l’origine reste incertaine. Malgré ces pics, le canal c422 a clairement détecté du Ca. Le signal c422 montre non seulement une concentration de Ca à l’aube mais également une couronne de Ca très étendue du côté matin qui n’a pas été rapportée par MESSENGER. La hauteur d’échelle déduite de nos profils (2500-2800 km) est en accord avec la valeur rapportée par MESSENGER à un angle d’anomalie vraie similaire. En utilisant le modèle de Chamberlain, j’ai déterminé une température à l’exobase très élevée (>50 000K), ce qui est en accord avec les résultats de MESSENGER. La grande hauteur d’échelle et la température à l’exobase élevée impliquent un processus de libération très énergétique. Les données semblent également indiquer une asymétrie jour/nuit qui pourrait signifier que la source de Ca est principalement du côté jour ou être la conséquence d’un décalage de la source de Ca par rapport à la région de l’aube. La prochaine étape consiste à utiliser un modèle exosphérique 3D prenant en compte la photoionisation et la distribution non uniforme du Ca. Enfin, le pointage de PHEBUS lors de ces deux premiers survols n’a pas permis d’observer la potentielle composante froide du Ca puisque les altitudes <200 km n’ont pas été explorées.Le canal c404 a détecté une ou plusieurs espèces à basse altitude côté matin lors des survols, potentiellement du K ou du Mn. Cette ligne d’émission du K n’a jamais été détectée par MESSENGER.Quant au Mn, MESSENGER ne l’a pas observé continuellement, il n’a été détecté qu’à certaines saisons et dans certaines régions de l’exosphère.Ces conditions de détection sont bien différentes de celles de BepiColombo.Il est donc difficile d’identifier avec certitude l’espèce détectée. D’autres observations sont nécessaires.On peut tout de même mentionner que l’échelle de hauteur déduite du profil c404 est ∼135 km et que le modèle de Chamberlain appliqué au profil c404 semble indiquer une température à l’exobase <3000 K.

BepiColombo, the joint ESA/JAXA mission to Mercury, was launched in October 2018 and is scheduled to arrive at Mercury in November 2026 after an 8-year cruise. Like other planetary missions, its scientific objectives focus mostly on the nominal, orbiting phase of the mission. However, due to the long duration of the cruise phase covering distances between 1.2 and 0.3 AU, the BepiColombo mission has been able to outstandingly contribute to characterise the solar wind and transient events encountered by the spacecraft, as well as planetary environments during the flybys of Earth, Venus, and Mercury, and contribute to the characterisation of the space radiation environment in the inner Solar System and its evolution with solar activity. In this paper, we provide an overview of the cruise observations of BepiColombo, highlighting the most relevant science cases, with the aim of demonstrating the importance of planetary missions to perform cruise observations, to contribute to a broader understanding of Space Weather in the Solar System, and in turn, increase the scientific return of the mission. Graphical Abstract

Enceladus is one of the most intriguing icy moons of the solar system, exhibiting active cryovolcanism and continuous plume activity at its south pole [1]. The surface in this region is geologically young and frequently resurfaced by plume fallout and tectonic activity [2]. While numerous studies have mapped compositional variations using Cassini-VIMS data [3,4], few have quantified the microphysical properties of water ice—including grain size, crystallinity and surface roughness—over the full VIMS spectral range. Understanding these parameters is critical for interpreting the thermal and geological evolution of the surface and for tracing exchanges between the subsurface ocean and the surface.Here, we present a comprehensive study in which we fit VIMS reflectance spectra across the 1.0–5 µm range to estimate key microphysical properties of Enceladus's surface. We apply a Hapke-based radiative transfer model [5,6] in a Bayesian Markov Chain Monte Carlo (MCMC) framework [7] to retrieve the posterior distributions of ice crystallinity, grain size and surface roughness for selected observations. We use the optical constants of Mastrapa et al. [8], which provide laboratory measurements of crystalline and amorphous water ice at temperatures from 20 to 150 K. The full-spectrum inversion enables robust fitting of broad and narrow features, including the diagnostic absorption band at 1.65 µm, which appears only in crystalline ice [9]. This band is not used as an isolated fitting constraint but is reproduced by the model if crystalline ice is present at the appropriate temperature. We find that its shape and central wavelength are well matched in many spectra, especially in the South terrains, where the inversion yields low temperatures (

The icy moons of the outer solar system (Ganymede, Enceladus, Titan, etc.) have been found in the forefront of planetary exploration, due to their diverse geomorphology coupled with numerous discoveries hinting to possible habitable conditions: geothermal activity, oceans of liquid water under the ice crusts, abundance of volatiles, and in some cases complex organics, to name a few. Of paramount importance for constraining the internal structure and properties of these bodies are high-precision cartographic products, allowing for linking surface morphologies and composition to internal activity. These cartographic products are constructed by reconciling numerous different observations, which, due to the complexities of an orbital tour, are recorded under a great range of different illumination and observation conditions, with varying spatial sampling. The aim of this study is to provide to the community a complete pipeline for producing super-resolution (SR) maps for icy moons, by combining all the overlapping observations for a specific target. The development and testing of our pipeline are carried out using the hyperspectral cubes recorded by the Visual and Infrared Mapping Spectrometer (VIMS) instrument, on board the Cassini Saturn orbiter, during its 13-year long investigation of the Saturn system that ended in 2017. We focus our efforts on Enceladus, a prime astrobiological target, which contrary to Titan does not possess a thick atmosphere, the influence of which needs to be removed from the data before implementing the super-resolution methodology for surface cartography. We first navigate the data, calculating accurate projections that can be combined into global mosaics of Enceladus. We then implement a global photometric correction model that compensates for the different acquisition conditions, allowing for the construction of global mosaics with diminished visible seams, improved homogeneity at the global scale and sharper definition of surface features at the local scale. In the third step of our methodology, we combine all the overlapping observations of the same area to calculate the new SR maps of Enceladus in the infrared. Utilizing the new, SR global map of Enceladus, we provide new insight into the moon’s geomorphology as revealed from its surface spectral heterogeneity, demonstrating the capability of the SR approach to augment our understanding of existing data sets of icy moons and help prepare for future observations.

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.

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 daysideFigure 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...

Radar-bright features near Mercury’s poles were first identified through ground-based radar observations, which suggested the presence of water ice at or near the surface in these regions [1,2]. Subsequent data from MESSENGER revealed a strong correlation between these radar-bright deposits and deep impact craters that remain in persistent shadow – areas where sunlight never reaches the crater floors and where surface temperatures are low enough for water ice to be thermally stable [3,4]. This stability is especially likely if the ice is insulated by a layer of organic-rich volatile compounds, as suggested by topographic, thermal, and spectral measurements from MESSENGER [5-8].In these permanently shaded regions (Figure 1), the main source of illumination is solar Lyman-α radiation at 121.6 nm, produced by the scattering of solar photons by interplanetary hydrogen. This process generates a diffuse ultraviolet glow that uniformly bathes Mercury’s surface. If water ice is present, it is expected to induce detectable variations in surface albedo at this wavelength, as the Lyman-α reflectance from icy areas should be lower than that of the surrounding regolith.Figure 1: North Polar Stereographic view of Mercury. The pink circles represent the Permanent Shadowed Regions (PSRs). The EUV (Extreme Ultraviolet) detector of the PHEBUS (Probing the Hermean Exosphere By Ultraviolet Spectroscopy) instrument on board BepiColombo [9], operating in the 55–155 nm wavelength range, is well-suited to investigate this phenomenon. PHEBUS/EUV will enable the detection of variations of surface reflectance variations at Lyman-α, offering a unique opportunity to confirm the presence of surface or near-surface water ice in Mercury’s polar regions. In this study, we identify potential observation opportunities for these PSRs with PHEBUS/EUV during the nominal science phase of the BepiColombo mission, starting in April 2027. Observations of Mercury’s polar regions on the night side, near the terminator, with PHEBUS are scheduled during the so-called “terminator seasons” (Figure 2). However, these observations come with a risk that needs to be carefully considered: because the observations involve pointing close to the illuminated surface of Mercury, even a small misalignment could allow direct sunlight to enter the baffle and irreversibly damage the detector. Scheduling the observations toward the end of the mission would ensure that most exospheric data will have already been acquired, minimizing the risk of detector degradation.Figure 2: The diagram shows the orbit of Mercury as seen from the Sun's north pole and the polar orbits of BepiColombo (green). It also indicates the boundaries of the four seasonal phases during Mercury's 88-day orbital period: in blue, the perihelion season, in orange the aphelion season and in magenta, the terminator seasons. References:[1] Harmon, J. K., & Slade, M. A. (1992). Radar mapping of Mercury: Full-disk images and polar anomalies. Science, 258(5082), 640-643.[2] Slade, M. A., Butler, B. J., & Muhleman, D. O. (1992). Mercury radar imaging: Evidence for polar ice. Science, 258(5082), 635-640.[3] Chabot, N. L., Ernst, C. M., Denevi, B. W., et al. (2012). Areas of permanent shadow in Mercury's south polar region ascertained by MESSENGER orbital imaging. Geophysical Research Letters, 39(9).[4] Chabot, N. L., Ernst, C. M., Harmon, J. K., et al. (2013). Craters hosting radar‐bright deposits in Mercury's north polar region: Areas of persistent shadow determined from MESSENGER images. Journal of Geophysical Research: Planets, 118(1), 26-36.[5] Neumann, G. A., Cavanaugh, J. F., Sun, X., et al. (2013). Bright and dark polar deposits on Mercury: Evidence for surface volatiles. Science, 339(6117), 296-300.[6] Paige, D. A., Siegler, M. A., Harmon, J. K., et al. (2013). Thermal stability of volatiles in the north polar region of Mercury. Science, 339(6117), 300-303.[7] Chabot, N. L., Ernst, C. M., Denevi, B. W., et al. (2014). Images of surface volatiles in Mercury’s polar craters acquired by the MESSENGER spacecraft. Geology, 42(12), 1051-1054.[8] Chabot, N. L., Ernst, C. M., Paige, D. A., et al. (2016). Imaging Mercury's polar deposits during MESSENGER's low‐altitude campaign. Geophysical Research Letters, 43(18), 9461-9468.[9] Quémerais, E., Chaufray, J. Y., Koutroumpa, D., et al. (2020). PHEBUS on Bepi-Colombo: post-launch update and instrument performance. Space Science Reviews, 216, 1-23.

BepiColombo, the ESA/JAXA joint mission, has already performed three out of the six flybys of Mercury scheduled during its journey to the innermost planet of our solar system. The first two flybys were conducted at a similar True Anomaly Angle (TAA~265°), while the third one occurred closer to the perihelion (TAA=311°).During these three flybys, several instruments observed the planet and its environment, including PHEBUS (Probing of Hermean Exosphere By Ultraviolet Spectroscopy), the UV spectrograph on board BepiColombo/MPO. The two visible channels of PHEBUS, centered at 404nm and 422nm, observed Mercury’s exosphere during each of the three flybys. The third flyby provided the first observations of the southern hemisphere of Mercury. Indeed, PHEBUS was pointing towards the south ecliptic direction during the third flyby while the instrument was pointing towards the north ecliptic direction during the first two flybys.We report the detection of the calcium (Ca) emission line at 422.8nm during each of the three flybys. Our results reveal that Mercury Ca exosphere is very extended on the morning side (up to ~10,000 km of altitude) and is enhanced near the dawn region. We then discuss year-to-year variability and potential source processes.We also report the detection of additional species with the visible channel centered at 404nm during the three flybys, potentially manganese (Mn) or potassium (K). The detection is confined to the predawn region. Mn was detected by MESSENGER at similar local times (2-5 A.M.) but at different TAA (0-70°). However, the K doublet near 404nm has never been detected by MESSENGER.Finally, we briefly discuss the geometric configuration of the next flybys of Mercury that will take place in September 2024, December 2024 and January 2025.

Mercury’s exospheric atoms are mainly ejected from the surface through several processes such as thermal input, UV irradiation, solar wind particle sputtering, and micro-meteoroid impact. Observations by the MESSENGER spacecraft have shown that Mercury magnesium (Mg) exosphere is related to its surface abundance. Additionally, Mg is an interesting species as its surface abundance reflects the non-uniformity of magma compositions. However, spatial distribution (especially in the latitude direction) and seasonal variability of Mg exosphere is not well understood due to its dark brightness and the geometry of observations.BepiColombo, the Mercury orbiting mission led by ESA and JAXA, is on its way to the planet. The 2nd and 3rd Mercury swing-bys were conducted on 22/06/2022 and 19/06/2023 (UTC), respectively, and many instruments observed the Mercury environments then. In this study, we analyzed Mg exosphere data from PHEBUS, the UV spectrometer onboard BepiColombo, to deduce temperature and production rate of Mg exosphere during each swing-by.As a result, similar signals were obtained through both swing-bys. Season, local time, and longitude of Mercury during both observations were similar, but boresights of PHEBUS were different (2nd: northward, 3rd: southward). These results show that Mg production rates have little year-to-year variability, which is consistent with the fact that Mg is mainly ejected by micro-meteoroid impact. Besides, these results mean that dust impact flux has little north-south asymmetry.In this presentation, we introduce the results obtained by observations of the spectrometer onboard BepiColombo, PHEBUS, during the 2nd and 3rd swing-bys.