Bilal Benmahi

Context . Uranus belongs to the category of ice giants that are common in our Galaxy. However, Uranus is one of the least explored and understood planets in our Solar System. Aims . This work aims to constrain the deep oxygen abundance of Uranus to better understand its formation. Another goal concerns the origin of exogeneous species, such as CO and HCN, found in the upper stratospheres of giant planets. Methods . We used spectral mapping observations of the CO (J=3–2) and HCN (J=4–3) rotational submillimeter lines obtained with ALMA in 2022 and 2024. We combined them with radiative transfer and thermochemical modeling to determine the tropospheric abundance of CO and the deep O/H ratio of Uranus. We used radiative transfer simulations with physical models of various sources of external CO and HCN to constrain the vertical and meridional distributions of these species and narrow down the nature of their external sources. We also applied a wind retrieval algorithm to search for zonal winds in the stratosphere of Uranus at the levels probed by the CO and HCN lines. Results . We unambiguously detect tropospheric CO for the first time with a mole fraction of 5.8±0.3 ppb and stratospheric HCN with a mole faction of (1.8±0.2)×10 ⁻¹¹ restricted to pressures lower than 0.2 mbar. Thermochemical calculations suggest that the deep interior of Uranus is enriched in oxygen with respect to the protoplanetary nebula by at least a factor of 52 ₋₂₀ ⁺³⁰ . We also find that the stratospheric CO is rather uniform over the observed latitudes and that the CO lines are best fit by an old comet impact model, in which a large comet hit the planet several centuries ago. CO therefore has a dual origin in Uranus. Finally, we do not detect stratospheric winds from these data, but the CO data indicate that zonal winds in the 10°S–10°N latitudinal range are likely retrograde at submillibar pressures.

Le Jupiter Icy Moon Explorer (JUICE) est la prochaine grande mission de l’Agence spatiale Européenne. JUICE embarquera 10 instruments et sera lancé en 2023 pour étudier le système jovien à partir de 2031 et pendant 4 ans. Parmi ses instruments, le Submillimeter Wave Instrument (SWI, PI P. Hartogh, MPS, Allemagne) nous intéresse plus particulièrement. Il s’agit d’un télescope de 29 cm de diamètre qui observera Jupiter et les satellites galiléens dans le domaine submillimétrique. SWI est destiné à étudier la composition ainsi que la dynamique des atmosphères de ces corps mais aussi les surfaces des satellites. Ce travail de thèse consiste à analyser et interpréter des observations de Jupiter et de Saturne, prises avec des télescopes au sol et spatiaux, pour préparer le programme scientifique de JUICE/SWI.Tout d’abord, nous avons étudié l’évolution temporelle de l’abondance d’H2O dans la stratosphère de Jupiter à partir de deux décennies de données recueillies par le télescope spatial Odin. Cette première étude nous permet notamment de prédire l’observabilité par SWI d’H2O dans la stratosphère de Jupiter grâce à nos modélisations photochimiques.Nous avons également analysé des observations ALMA (Atacama Large Millimeter/submillimeter Array) de CO et d’HCN, qui ont initialement été déposés dans la stratosphère de Jupiter lors de la désintégration de la comète Shoemaker-Levy-9 en 1994. Comme H2O, ces composés seront observables dans les bandes spectrales de SWI. Nous avons développé une méthode qui permet de mesurer directement l’effet Doppler induit par les vents atmosphériques sur les raies spectrales, grâce à la très haute résolution spectrale et spatiale d’ALMA. Cette méthode, qui permet d’étudier la dynamique atmosphérique des planètes géantes, est basée sur la technique de modélisation Markov Chain Monte Carlo.Nos résultats dans la haute stratosphère de Jupiter indiquent que des jets sont présents dans la zone équatoriale. Ces mesures de vent, jumelées avec des mesures concomitantes de la température stratosphériques, nous ont permis de caractériser la circulation stratosphérique équatoriale grâce à l’équation du vent thermique. Nous avons ainsi mis en évidence un empilement vertical de jets équatoriaux dirigés alternativement vers l’ouest et vers l’est.Nous avons appliqué cette même technique de mesure de vents à des observations ALMA de CO et d’HCN dans la stratosphère de Saturne. Nos résultats révèlent une circulation équatoriale inédite. Nous retrouvons, en effet, le grand jet équatorial de Saturne, auparavant observé par Hubble et Cassini au niveau des nuages entre 60 et 2000 mbar de pression, à 0.1 mbar, c’est-à-dire 500 km plus haut que les observations Hubble et Cassini.Par ailleurs, nous avons découvert que les zones polaires de Jupiter et de Saturne sont le siège d’une chimie et d’une dynamique particulières. En effet, dans la stratosphère de Jupiter, nos observations Odin et ALMA montrent qu’H2O et HCN sont détruits plus rapidement dans ces zones qu’aux latitudes basses et moyennes. Par ailleurs, nous avons détecté la présence de jets auroraux qui sont positionnés sous les ovales auroraux nord et sud de Jupiter. Il semble qu’un jet similaire soit également présent sous l’ovale auroral nord de Saturne. Nous pensons que ces jets pourraient former des vortex géants jusqu’à l’ionosphère où une dynamique similaire a été détectée sur Jupiter. Il se pourrait donc que l’on soit en présence de vortex gigantesques qui participeraient au confinement de la matière dans les zones aurorales où des électrons énergétiques précipitent et contribuent à la chimie. C’est pourquoi nous avons développé un outil de modélisation du transport des électrons magnétosphériques afin de le coupler à un modèle photochimique ion-neutre dans les zones aurorales de Jupiter et ainsi mieux comprendre la chimie et la dynamique de ces régions particulière. À terme, nous prévoyons d’appliquer ce modèle également à Saturne.

Context. Ultraviolet (UV) auroral emissions on Jupiter offer a unique window into the coupling between the planet’s magnetosphere and upper atmosphere. Color ratios derived from molecular hydrogen emissions provide valuable diagnostics for the energy of precipitating electrons and the structure of the auroral atmosphere. Aims. We aim to characterize the horizontal and vertical variability of hydrocarbon absorption in Jupiter’s auroral atmosphere using UV data from the Juno-UVS spectrograph and to investigate potential departures from the expected structure. Methods. We constructed color ratio maps sensitive to CH ₄ and C ₂ H ₂ absorptions for two of Juno’s close approaches to Jupiter, perijoves (PJs) 6 and 10, by integrating auroral H ₂ emission over hydrocarbon-sensitive spectral intervals. For CH ₄ , we redefined the absorbed spectral band, replacing the traditionally used 125-130 nm interval with 135-140 nm to mitigate higher order calibration issues. In regions of intense auroral brightness, we developed a correction method to account for spectral distortion due to detector nonlinearities at high fluxes. Results. The CH ₄ and C ₂ H ₂ absorptions generally follow the expected vertical distribution, with the CH ₄ density extending to higher altitudes than C ₂ H ₂ . However, several localized regions show unexpected spatial distribution of the absorption. In PJ6, such anomalies are attributed to instrumental nonlinearities. Following the correction, the CR distributions become consistent with standard hydrocarbon vertical distributions. In PJ10, however, some anomalous patterns persist despite the correction. Spectral modeling studies indicate that these can be explained by modifying the relative abundances of CH ₄ and C ₂ H ₂ , suggesting horizontal compositional variability and possible deviations in the homopause altitude between species. Conclusions. Our results confirm that hydrocarbon absorption in Jupiter’s aurora is vertically stratified, but that the altitude of this stratification itself varies horizontally with latitude and longitude. These findings underscore the importance of accounting for local atmospheric composition when interpreting UV auroral spectra and retrieving electron energy distributions. Juno-UVS thus provides a powerful diagnostic for probing both the auroral precipitation and upper atmospheric structure. The framework developed here enables more accurate retrievals of electron energies and provides new constraints on the spatial variability of Jupiter’s upper atmosphere. A systematic application to the full Juno dataset will offer deeper insights into the temporal and spatial dynamics of the Jovian aurora.

Context . Saturn’s Great Storm of 2010–2011 has produced two stratospheric hot spots, the “beacons,” that eventually merged to produce a gigantic one in April and May 2011. This beacon perturbed stratospheric temperatures, hydrocarbon, and water abundances for several years. Aims . We aim to assess whether the beacon induced any perturbation in another oxygen species, namely CO. A second goal is to measure how the vortex perturbed the stratospheric wind regime. Methods . We conducted interferometric observations of Saturn in the submillimeter range with SMA and ALMA to spatially resolve the CO (J=3–2) and (J=2–1) emissions, respectively. We used a previously determined CO vertical profile as a template, to search for (i) the meridional distribution of CO and (ii) variations of the CO abundance associated with the storm. The high spatial and spectral resolutions of the ALMA observations enabled us to retrieve the winds from the Doppler shifts induced by the winds on the lines. Results . Despite limitations resulting from the removal of baseline ripples, we find a relatively constant meridional distribution of CO. The average CO mole fraction implied by the adopted and rescaled 220-year-old-comet-impact vertical profile is (1.7±0.7)×10 ⁻⁷ at 0.3 mbar, i.e., where the contribution functions peak. We also find that the CO abundance has not been noticeably altered in the beacon. The winds measured at 1 mbar show striking differences with those measured in 2018, after the demise of the beacon. We find the signature of the vortex as an anticyclonic feature. The equatorial prograde jet is 100–200 m s ⁻¹ slower, and broader in latitude, than in quiescent conditions. We also detect several prograde jets in the southern hemisphere. Finally, we detect a retrograde jet at 74°N which could be a polar jet caused by the interaction of the Saturn magnetosphere with its atmosphere. Conclusions . With Saturn’s equinox season approaching, new wind measurements would enable the findings presented in this paper to be confirmed by probing the two hemispheres equally and searching for a southern retrograde polar jet.

Jupiter’s polar stratosphere exhibits unique chemical and dynamical processes that shape its atmospheric composition and structure. Hydrocarbon observations from multiple instruments (e.g., Voyager/IRIS, Cassini/CIRS, IRTF/TEXES, Juno/UVS, and JWST/MIRI) reveal abundance enhancements and strong latitudinal variations of C2 hydrocarbon species within the auroral regions. These enhancements are attributed to the influence of auroral energy deposition and possibly enhanced vertical mixing. IRTF/TEXES and JWST/MIRI observations also provide new constraints on the vertical structure of the polar atmosphere, suggesting that the methane homopause is located at higher altitudes in auroral regions than at lower latitudes. In addition, some observations indicate that Jupiter’s previously known aerosol layer is situated at higher altitudes in the polar regions (above about 20 mbar) compared to lower latitudes (around about 50 mbar). Magnetosphere–ionosphere–thermosphere coupling in Jupiter’s polar regions generates ionospheric winds with velocities of several km/s, which may propagate downward to the 0.1 mbar level, where neutral winds appear to be co-located with those measured at higher altitudes. Jupiter’s polar atmosphere thus constitutes a highly complex system in which magnetospheric forcing is strongly coupled with chemistry and dynamics from the ionosphere down to the tropopause. Understanding the distribution of hydrocarbons at high latitudes, and the extent to which they control the atmospheric radiative balance, is crucial for constraining upper-atmospheric dynamics. In this contribution, we provide a general overview of the physical and chemical processes governing Jupiter’s polar regions and present preliminary results from the JAFAR project, designed to investigate Jupiter’s polar atmosphere and the fate of aerosols, in preparation for the JUICE arrival in 2031.

The Io Plasma Torus (IPT) plays a key role in the workings of the Jupiter System at large. It is a complex system driven by Io’s volcanism and surface-atmosphere that interacts with numerous Jupiter System objects. Despite significant and varied modelling efforts, the description of its spatial structure and temporal variability remains challenging, especially because of insufficient data coverage.In this EGU Poster, we analyze the IPT's spectral emissions and spatio-temporal dynamics to establish the optical specifications for a future ground-based observation system. Our main conclusions are as follows:1- Temporal variability: the large diversity of objects in the Jupiter System with which the IPT interacts, and the complex, highly non-linear nature of these interactions, contribute to the strong observed temporal variability of the IPT, which displays a broad range of time scales, from hourly to multi-decadal. Capturing all timescales requires hourly, intercalibrated observations, necessitating dedicated space platforms and/or a longitudinal ground network.2- Spatial scales: the Io system includes interconnected objects of very different spatial extensions, from tens of km with Io’s volcanoes and plumes, to more than 1000 Rj with the nebula(e). Hence, an observation system covering the Io system in a comprehensive way will need to combine observations with very diverse spatial coverages, from sharp AO observations (e.g. 0.02’’ achieved with the LBT) to 5.5°.3- Spectral extension: electromagnetic emissions generated by the Io system cover a very large part of the electromagnetic spectrum, and the different components of the system emit to a large part in different wavelength ranges and in the different spectral lines corresponding to different neutral and ionized species. Hence, a combined set of telescopes covering this system in a comprehensive way will have to optimally combine observations of different spectral lines in different spectral ranges. 4- Complementary observations: beyond the body of UV / EUV observations from space, ground-based observations over the last 50 years have also borne very valuable fruit. Indeed, instrumentation necessary to image the IPT does not necessarily need to be expensive: simple designs using smart "amateur-class" equipment already allow for the observation of the brightest lines. As spaceborne and ground-based observations offer complementary advantages and limitations, a future comprehensive observation system for the IPT will likely have to combine both.Given stringent time resolution constraints, and aiming at an affordable budget envelope, the development of a longitudinal network of telescopes appears as particularly cost-effective and promising. It could build on the successful IPT telescopes that already exist in different longitude sectors and complement them with one or several ones at key locations, including the European and African sectors. In France, such an effort will be coordinated at the national level, allowing one to take full advantage of synergies between radio observations at Nançay and new optical observations. In this poster, we outline design guidelines for a smart, multi-site, multi-spectral system capturing the IPT's spatiotemporal dynamics and coupling processes that will address the choice of telescope as well as of the spatial (coronagraph) and spectral (color filters) filtering systems.

Ganymede, the only satellite in the Solar System possessing an intrinsic magnetic field, is continuously immersed in Jupiter’s magnetosphere. The relative motion of this conducting body through the jovian magnetic field, together with the presence of closed magnetic field regions around Ganymede, leads to the precipitation of energetic electrons along these closed field lines, producing ultraviolet auroral emissions.In sunlit auroral regions, these emissions are superimposed on the solar radiation reflected by the surface, making an accurate knowledge of the surface spectral reflectance essential for a proper interpretation of the observed UV spectra.We used Juno/UVS observations obtained during the 34th perijove to derive the spectral reflectance of Ganymede’s surface in the 140-205 nm range. The analysis was performed with a non-local thermodynamic equilibrium radiative transfer model initially developed for auroral emission studies, which self-consistently includes the reflection of the incident solar flux by the surface. By fitting the UVS spectra in illuminated auroral regions, we retrieved spatially resolved reflectance values.The inferred reflectance exhibits strong spatial and spectral variability, ranging from about 0.1% to 8% over the [140-205] nm interval, revealing a highly heterogeneous surface. This variability is likely the signature of long-term irradiation by energetic particles, which modifies the physical structure, crystallinity, and chemical composition of surface ice. The resulting UV reflectance maps show no clear correlation with visible-wavelength surface morphology, indicating that irradiation-driven processes dominate over geological features in controlling the UV albedo.These new reflectance constraints constitute a key input for future modeling of Ganymede’s ultraviolet aurora and will be particularly valuable for the interpretation of upcoming observations by the JUICE/UVS instrument.

Aims. We analyzed ultraviolet observations of Ganymede recorded on June 7, 2021, by the Juno spacecraft, which observed the aurora at high spatial resolution in order to detect the presence of auroral sub-structures. The emission mainly comes from two oxygen lines at 130.4 and 135.6 nm, which are excited under electron precipitation. Methods. We produced longitude–latitude projections of the oxygen emission lines from Ganymede’s atmosphere to investigate the presence of small-scale auroral features. We adopted a magnetic field model of Ganymede’s magnetosphere to map between the moon surface and the surrounding space, in order to determine the regions magnetically connected to the aurora. Results. We find auroral patches on the leading, downstream side of Ganymede. Their typical size is ∼50 km, and they have a brightness of up to ∼200 Rayleigh. These features map approximately to the downstream reconnection region and seemingly resemble the terrestrial and Jovian mesoscale auroras associated with sub-storms and dawn storms, respectively. This could indicate that magnetospheres of celestial bodies host similar physical process(es) despite their different conditions and dynamics.

Context . The interaction between Io and the Jovian magnetosphere produces the auroral “Io footprint”, which can be observed in the ultraviolet thanks to the deexcitation of atmospheric H and H ₂ under electron precipitation. Since 2016, Jupiter has been explored with Juno’s Ultraviolet Spectrograph, which monitors the emission from 68 to 210 nm. Aims . We used Juno observations of the Io footprint near the planetary limb to determine its vertical ultraviolet emission profile in the northern hemisphere. We simulated emission as a function of the altitude and the mean energy of the precipitating electrons, and we used the results to determine the energy spectrum associated with the footprint profile. Methods . We estimated the source location of the Io footprint UV emission, and we extracted its vertical profile. We analyzed the variability of the emission altitude, and we derived the corresponding energy spectrum of precipitating electrons using TransPlanet. The results were compared with the in situ measurements from Juno’s particle detectors. Results . The main spot emission peaks around ~500 km, with a variability of ~200 km correlated with longitude. The emission of the footprint tail within 20° from the main spot peaks around ~500±300 km, and it is moderately correlated with the magnitude of the magnetic field near Jupiter. The trans-hemispheric electron beam spot is located at ~300 km, and the associated precipitation appears depleted at low energy. The retrieved energy spectrum of the precipitation shows remarkable agreement with the particle measurements. Conclusions . The longitudinal modulation of the Io footprint altitude suggests that the morphology of the torus affects the transmission of the Alfvén waves responsible for the acceleration of auroral electrons. The dependency of the footprint tail altitude on the magnetic field strength indicates that the magnetic field also plays a role in the acceleration mechanism.

Context . Ganymede is the only moon in the Solar System with an intrinsic magnetic field that actively interacts with the Jupiter magnetosphere. This precipitates energetic electrons that generate ultraviolet (UV) auroral emission. Aims . In sunlit auroral regions, the observed emission partly overlaps with the solar continuum reflected by the surface. An accurate modeling of the observed UV spectra therefore requires precise knowledge of the surface spectral reflectance. Methods . We analyzed Juno/UVS data acquired during the 34th perijove (PJ) flyby to constrain the Ganymede surface reflectance in the 140−205 nm range. We used the non-local thermal equilibrium radiative transfer model originally developed to simulate the auroral emission of Ganymede, which also accounts for the reflection of solar flux by the satellite surface, to fit the observed spectra in sunlit auroral regions. Results . Our results revealed that the reflectance varies strongly spatially and spectrally from 0.1% to 8% in the [140 nm; 205 nm] wavelength range. This indicates a significant surface heterogeneity. This variability likely reflects long-term interactions between the icy surface of Ganymede and precipitating energetic particles, which alter the ice structure and crystallinity and its chemical composition. In addition, the derived reflectance maps show no clear correlation with the visible surface features of Ganymede, suggesting that the UV reflectance is primarily shaped by irradiation-driven processes and not by the geological morphology. Conclusions . The resulting reflectance maps provide a critical input for future UV auroral emission modeling, particularly in preparation for observations by the Juice/UVS mission.