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International audience; The Cassini mission has opened many science questions related to Saturn's atmospheric dynamics: jet-streams (including the hexagonal-shaped northern polar jet), giant convective storms, stratospheric equatorial oscillations, interhemispheric transport. This inspired our group to develop a Global Climate Model for Saturn that couples a massively-parallel icosahedral-grid hydrodynamical solver with detailed physical parameterizations (notably correlated-k radiative transfer) for Saturn's troposphere and stratosphere. Multiple test simulations led us to determine the best modeling settings to run simulations in which (amongst other requirements) the conservation of angular momentum is ensured with good accuracy. Multi-year simulations carried out with a spatial resolution of 1/2° in latitude/longitude allowed us to analyze the mechanisms accounting for jet acceleration (and possible latitudinal migration) in the troposphere and the stratosphere of Saturn, the formation of large-scale vortices and waves, and the occurrence of a zonostrophic regime. The perspectives for improvements of the reference Saturn simulations will be reviewed. We will also present simulations carried out for Jupiter (presently observed by the Juno mission) with the exact same modeling platform, to discuss comparative dynamics between both gas giants.

Remote sensing observations suffer significant limitations when used to study the bulk atmospheric compositionof the giant planets of our solar system. This impacts our knowledge of the formation of these planets andthe physics of their atmospheres. A remarkable example of the superiority of in situ probe measurements wasillustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases’abundances and the precise measurement of the helium mixing ratio were only made available through in situmeasurements by the Galileo probe. Here we describe the main scientific goals to be addressed by the futurein situ exploration of Saturn, Uranus, and Neptune, placing the Galileo probe exploration of Jupiter in a broadercontext. An atmospheric entry probe targeting the 10-bar level would yield insight into two broad themes: i) theformation history of the giant planets and that of the Solar System, and ii) the processes at play in planetaryatmospheres. The probe would descend under parachute to measure composition, structure, and dynamics, withdata returned to Earth using a Carrier Relay Spacecraft as a relay station. An atmospheric probe could representa significant ESA contribution to a future NASA New Frontiers or flagship mission to be launched toward Saturn,Uranus, and/or Neptune.

The SuperCam instrument [1,2] onboard Mars2020 disposes of a variety of active and passive techniques, including passive spectroscopy in the 0.40-0.85 (VIS) and 1.3 to 2.6 microns (IR, [3,4]) wavelength ranges. Since the landing on Mars of Perseverance in February 2021, Supercam has acquired numerous observations of its near and distant environment, exploring the geological and mineralogical context of Jezero crater. In addition, several measurements were devoted to probing the atmosphere surrounding the Perseverance rover. The technique of using sky spectra in passive mode, known as "passive sky", has already been demonstrated with ChemCam on the Mars Science Laboratory (MSL) rover [4]. SuperCam provides a superset of the ChemCam capabilities used in [4], and in particular adds a near-infrared component that includes absorption and scattering characteristics of key gases and aerosols/clouds. "Passive sky" measurements have typically been performed every other week to allow a consistent monitoring of the seasonal evolution of the main quantities (CO2, O2, H2O, CO, aerosols/clouds). Particular attention was given to joint measurements of O2 and CO, as they appear as key components of the Martian chemical cycle and have never been measured together at the same time on the surface of Mars. As the 2 m wavelength region is used for the first time at the surface of Mars, it enables the detection of CO (around 2.35 m). CO possesses a small absorption that has made it difficult to identify in SuperCam spectra so far. An overview of SuperCam's progress to date in its attempt to characterize the Martian atmosphere at Jezero will be presented. References : [1] Wiens, R.C., et al. , 2021. Space Sci Rev 217, 4, [2] Maurice, S., et al., 2021. Space Sci Rev 217, 47, [3] Royer, C., et al.., 2020. Review of Scientific Instruments 91, 063105, [4] Fouchet, T., et al., 2021, Icarus, submitted. [5] McConnochie T. H et al., 2018. Icarus 307, 294

One of the goals of this thesis is to interpret the observations of the main hydrocarbons(C2H2 and C2H6) from Cassini (NASA/ESA) on Jupiter and Saturn. The one-dimensional photochemical models are insufficient to explain these spatially resolved observations. I have developed the first two-dimensional (altitude-latitude) seasonal photochemical model for the giant planets, which predicts their chemical composition.Without meridional transport, Saturn’s chemical composition follows the insolation variations. The C2H2 and C2H6 abundances measured by Cassini (Guerlet et al., 2009)are reproduced from the equator up to mid-latitudes, at pressures higher than 0.1mbar.At higher latitudes, the disagreements suggest either a stratospheric circulation cell orthe signature of ion-neutral chemistry. For the first time, I have coupled our seasonal photochemical model with the seasonal radiative model of Greathouse et al. (2008). I predict that the seasonal temperature peak is shifted half a season earlier, with respect to previous models, at high latitudes in the higher stratosphere.Jupiter shows weak seasonal variations of chemical composition, only controlled by its orbital eccentricity. The observed meridional distributions of C2H2 and C2H6 show opposition trends (Nixon et al., 2010). C2H6 observed distribution is reproduced when Isuppose a combination of meridional diffusion and stratospheric circulation, while causingat the same time a stronger agreement with the C2H2 observations. Accounting for theion-neutral chemistry might preferentially affect C2H2 and potentially play a key role on hydrocarbon abundances in Jupiter’s stratosphere., L’un des objectifs de cette thèse est d’interpréter les observations des principaux hydrocarbures(C2H2 et C2H6) effectuées par Cassini (NASA/ESA) sur Jupiter et Saturne. Les modèles photochimiques à une dimension sont insuffisants pour interpréter ces observations spatialement résolues. J’ai développé le premier modèle photochimique saisonnier à deux dimensions (altitude-latitude) des planètes géantes qui calcule leur composition chimique.En l’absence de transport méridional, la composition chimique de Saturne suit les variations d’ensoleillement. Les abondances de C2H2 et C2H6 mesurées par Cassini (Guerletet al., 2009) sont reproduites jusqu’aux latitudes moyennes, à des pressions supérieures à0,1mbar. Les écarts notés dans l’hémisphère sud suggèrent la présence de dynamique ou d’une chimie entre les ions et les espèces neutres. J’ai couplé, pour la première fois, mon modèle photochimique avec le modèle radiatif de Greathouse et al. (2008). Nous prédisons un décalage du pic saisonnier de température, par rapport aux précédents modèles, d’une demi-saison à haute altitude et aux hautes latitudes.Jupiter présente de faibles variations saisonnières de composition chimique, uniquement contrôlées par son excentricité. Les distributions méridionales observées de C2H2 etC2H6 présentent des tendances opposées (Nixon et al., 2010). Mon modèle est en accord avec les observations de C2H6 lorsque j’invoque une combinaison de diffusion méridionale et de circulation stratosphérique, tout en provoquant un plus grand désaccord avec les observations de C2H2. La chimie ionique pourrait principalement affecter C2H2 et jouer un rôle important dans l’atmosphère de Jupiter.

The goal of the thesis is to measure the temperature and composition fields in Saturn's stratosphere. In order to do so, we analyze spectra of the thermal emission of the planet acquired in limb geometry by CIRS, an infrared spectrometer aboard the Cassini spacecraft. These spectral signatures are analyzed using a radiative transfer model coupled to a retrieval algorithm in order to measure vertical profiles of the temperature and of the abundance of atmospheric gases. We have thus obtained the first maps, from 70N to 80 S, and on several scale heights, of the temperature and abundance of five hydrocarbons. The temperature map we obtain is used to quantify the thermal response of the atmosphere to seasonal variations in isolation. We compare the gradients we measure to the radiative seasonal model of Greathouse et al. [2008]. We also report the discovery of an equatorial oscillation in temperature and zonal wind proles in the stratosphere of Saturn, similar to the Earth's quasi-biennial oscillation, and we present its temporal evolution between 2005 and 2010. This analysis of the hydrocarbon's distributions and their comparaison to a seasonal photochemical model [Moses and Greathouse, 2005] shows signs of meridional transport in the lower stratosphere, from the equator to the poles, and vertical transport in the upper stratosphere. Finally, we present a detailed analysis of the radiative exhanges in the stratosphere using the Net Exhange Rates formulation. This work will form the basis for a future parameterization of radiative transfer computation, a necessary step in developing a general circulation model of Saturn's stratosphere.; L'objectif de cette thèse est de mesurer les champs de température et de composition de la stratosphère de Saturne. Pour cela, j'ai analysé des spectres de l'émission thermique de la planète acquis au limbe par CIRS, un spectromètre infrarouge à bord de la sonde Cassini. L'analyse de ces signatures spectrales à l'aide d'un code de transfert de rayonnement et d'une méthode d'inversion permet de mesurer les profils verticaux de la température et de l'abondance des constituants. J'ai ainsi réalisé les premières cartographies, de 70°N à 80°S et sur plusieurs échelles de hauteur, de la température et de l'abondance de 5 hydrocarbures. La carte de température obtenue permet de quantifier la réponse de l'atmosphère aux variations saisonnières de l'ensoleillement. Les gradients mesurés sont comparés au modèle radiatif saisonnier de Greathouse et la. [2008]. Je montre la découverte d'une oscillation équatoriale de température et de vent zonal dans la stratosphère de Saturne, analogue à l'oscillation quasi-biennale terrestre, et je présente son évolutioin entre 2005 et 2010. L'analyse de la distribution des hydrocarbures et la comparaison à un modèle de photochimie saisonnier [Moses and greathouse, 2005] met en évidence des indices de transport méridien dans la basse stratosphère. Enfin, je présente un travail d'analyse des échanges radiatifs dans la stratosphère, de l'équateur vers les pôles, et de transport vertical dans la haute stratosphère à l'aide d'une formulation en puissances nettes échangées. Ce travail servira de base à une future paramétrisation du transfert de rayonnement, étape nécessaire au développement d'un modèle de circulation générale de la stratosphère de Saturne.