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Gastine T.,Max Planck Institute For Sonnensytemforschung | Wicht J.,Max Planck Institute For Sonnensytemforschung | Aurnou J.M.,University of California at Los Angeles
Icarus | Year: 2013

The surface zonal winds observed in the giant planets form a complex jet pattern with alternating prograde and retrograde direction. While the main equatorial band is prograde on the gas giants, both ice giants have a pronounced retrograde equatorial jet.We use three-dimensional numerical models of compressible convection in rotating spherical shells to explore the properties of zonal flows in different regimes where either rotation or buoyancy dominates the force balance. We conduct a systematic parameter study to quantify the dependence of zonal flows on the background density stratification and the driving of convection.In our numerical models, we find that the direction of the equatorial zonal wind is controlled by the ratio of the global-scale buoyancy force and the Coriolis force. The prograde equatorial band maintained by Reynolds stresses is found in the rotation-dominated regime. In cases where buoyancy dominates Coriolis force, the angular momentum per unit mass is homogenized and the equatorial band is retrograde, reminiscent to those observed in the ice giants. In this regime, the amplitude of the zonal jets depends on the background density contrast with strongly stratified models producing stronger jets than comparable weakly stratified cases. Furthermore, our results can help to explain the transition between solar-like (i.e. prograde at the equator) and the "anti-solar" differential rotations (i.e. retrograde at the equator) found in anelastic models of stellar convection zones.In the strongly stratified cases, we find that the leading order force balance can significantly vary with depth. While the flow in the deep interior is dominated by rotation, buoyancy can indeed become larger than Coriolis force in a thin region close to the surface. This so-called "transitional regime" has a visible signature in the main equatorial jet which shows a pronounced dimple where flow amplitudes notably decay towards the equator. A similar dimple is observed on Jupiter, which suggests that convection in the planet interior could possibly operate in this regime. © 2013 Elsevier Inc. Source


Gastine T.,Max Planck Institute For Sonnensytemforschung | Heimpel M.,University of Alberta | Wicht J.,Max Planck Institute For Sonnensytemforschung
Physics of the Earth and Planetary Interiors | Year: 2014

The surface winds of Jupiter and Saturn are primarily zonal. Each planet exhibits strong prograde equatorial flow flanked by multiple alternating zonal winds at higher latitudes. The depth to which these flows penetrate has long been debated and is still an unsolved problem. Previous rotating convection models that obtained multiple high latitude zonal jets comparable to those on the giant planets assumed an incompressible (Boussinesq) fluid, which is unrealistic for gas giant planets. Later models of compressible rotating convection obtained only few high latitude jets which were not amenable to scaling analysis. Here we present 3-D numerical simulations of compressible convection in rapidly-rotating spherical shells. To explore the formation and scaling of high-latitude zonal jets, we consider models with a strong radial density variation and a range of Ekman numbers, while maintaining a zonal flow Rossby number characteristic of Saturn.All of our simulations show a strong prograde equatorial jet outside the tangent cylinder. At low Ekman numbers several alternating jets form in each hemisphere inside the tangent cylinder. To analyze jet scaling of our numerical models and of Jupiter and Saturn, we extend Rhines scaling based on a topographic β-parameter, which was previously applied to an incompressible fluid in a spherical shell, to compressible fluids. The jet-widths predicted by this modified Rhines length are found to be in relatively good agreement with our numerical model results and with cloud tracking observations of Jupiter and Saturn. © 2014 Elsevier B.V. Source


Hickson K.M.,CNRS Institute of Molecular Sciences | Loison J.C.,CNRS Institute of Molecular Sciences | Cavalie T.,Max Planck Institute For Sonnensytemforschung | Hebrard E.,University of Lorraine | And 2 more authors.
Astronomy and Astrophysics | Year: 2014

Aims. We studied the hypothesis that micrometeorites and Enceladus' plume activity could carry sulfur-bearing species into the upper atmosphere of Titan, in a manner similar to oxygen-bearing species.Methods. We have developed a detailed photochemical model of sulfur compounds in the atmosphere of Titan that couples hydrocarbon, nitrogen, oxygen, and sulfur chemistries.Results. Photochemical processes produce mainly CS and H2CS in the upper atmosphere of Titan and C3S, H2S and CH3SH in the lower atmosphere. Mole fractions of these compounds depend significantly on the source of sulfur species.Conclusions. A possible future detection of CS (or the determination of a low upper limit) could be used to distinguish the two scenarios for the origin of sulfur species, which then could help to differentiate the various scenarios for the origin of H2O, CO, and CO2 in the stratosphere of Titan. © ESO, 2014. Source


Gastine T.,Max Planck Institute For Sonnensytemforschung | Wicht J.,Max Planck Institute For Sonnensytemforschung
Icarus | Year: 2012

The banded structures observed on the surfaces of the gas giants are associated with strong zonal winds alternating in direction with latitude. We use three-dimensional numerical simulations of compressible convection in the anelastic approximation to explore the properties of zonal winds in rapidly rotating spherical shells. Since the model is restricted to the electrically insulating outer envelope, we therefore neglect magnetic effects.A systematic parametric study for various density scaleheights and Rayleigh numbers allows to explore the dependence of convection and zonal jets on these parameters and to derive scaling laws.While the density stratification affects the local flow amplitude and the convective scales, global quantities and zonal jets properties remain fairly independent of the density stratification. The zonal jets are maintained by Reynolds stresses, which rely on the correlation between zonal and cylindrically radial flow components. The gradual loss of this correlation with increasing supercriticality hampers all our simulations and explains why the additional compressional source of vorticity hardly affects zonal flows.All these common features may explain why previous Boussinesq models were already successful in reproducing the morphology of zonal jets in gas giants. © 2012 Elsevier Inc. Source

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