Saturn's Deep Atmospheric Flows Revealed by the Cassini Grand Finale Gravity Measurements

Abstract

How deep do Saturn's zonal winds penetrate below the cloud level has been a decades‐long question, with important implications not only for the atmospheric dynamics but also for the interior density structure, composition, magnetic field, and core mass. The Cassini Grand Finale gravity experiment enables answering this question for the first time, with the premise that the planet's gravity harmonics are affected not only by the rigid body density structure but also by its flow field. Using a wide range of rigid body interior models and an adjoint based optimization for the flow field using thermal wind balance, we calculate the flow structure below the cloud level and its depth. We find that with a wind profile, largely consistent with the observed winds, when extended to a depth of around 8,800 km, all the gravity harmonics measured by Cassini are explained. This solution is in agreement with considerations of angular momentum conservation and is consistent with magnetohydrodynamics constraints.

Linking zonal winds and gravity: the relative importance of dynamic self-gravity

“…For Saturn, J 3 , J 5 and J 10 seem precise enough to warrant including DSG effects. The estimates of Kong et al (2017) and Galanti et al (2019) about the relative impact of the DSG is compatible with our results. Including the DSG term generally increases the amplitude of the gravity coefficients.…”

Global climate modeling of Saturn's atmosphere. Part III: Global statistical picture of zonostrophic turbulence in high-resolution 3D-turbulent simulations

“…at the 3-bar level), the bounding condition is stress-free and a bottom drag u b = −αu is imposed with a frictional rate α = 1/τ R similar to Schneider and Liu (2009) and Liu and Schneider (2010). This large-scale frictional dissipation emulates to zeroth order, at the bottom of our model, the magneto-hydrodynamic drag (MHD) acting on jets extending at the bottom of the atmospheric layer (Liu et al, 2008) several thousand kilometers below the visible cloud layer (Galanti et al, 2019). Improving on this admittedly simplistic bottom boundary condition is an entire research goal on its own and is left for future investigations.…”

Saturn atmospheric dynamics one year after Cassini: Long-lived features and time variations in the drift of the Hexagon

“…The survival of the hexagon during the polar night, the stability of the polar jet to the varying solar irradiation, the indirect determination of winds extending down at least to the ∼ 10 bar level, and the recent determination of deep winds in Saturn's mid to low latitudes (Galanti et al, 2019) convincingly agree that the polar jet extends deep in the atmosphere, well below levels affected by seasonal variations. Still, the hexagon's drift rate changes in time and the presence of the NPS and coincident drift rate in 1980-1981 and 1990-1991 periods, and its absence since the first images of the north polar area in 2007 seem to indicate the importance of this feature in modifying the hexagon's drift rate.…”