Depth of the dynamo region and zonal circulation of the molecular layer in Saturn inferred from its equatorially symmetric gravitational field

Abstract

ABSTRACT The high-precision equatorially symmetric gravitational field of Saturn (the even gravitational coefficients J2, J4, …, J12) measured by the Cassini Grand Finale reflects its internal structure, its non-spherical shape caused by rotation and its strong zonal circulation whose location is controlled by the depth of its dynamo. We construct a four-layer, non-spheroidal (i.e. its shape is irregular) model of Saturn comprised of an inner core, a metallic dynamo region, an outer molecular envelope and a thin transition layer between the metallic and molecular regions. The model produces the even zonal gravitational coefficients that are in agreement with those measured by the Cassini Grand Finale within the error bars. Our Saturnian model reveals that (i) the observed cloud-top winds extending to any depth on cylinders cannot explain the measured coefficients J2, J4, …, J12; (ii) a deep zonal flow confined in the 20 000 km thick molecular layer is required to interpret them; (iii) the profile of the zonal flow – whose direction is sufficiently alternating with several retrograde peaks – significantly differs from that of the surface winds, implying that the observed winds are confined to a shallow layer, do not extend deeply into Saturn and do not contribute to the observed gravity; and (iv) the Saturnian dynamo can substantially affect the structure of its equatorially symmetric gravitational field by stopping the zonal-flow penetration and by changing, because of the boundary condition at the metallic and molecular interface, the distribution of the dynamic density anomalies.