Interpreting the Equatorially Antisymmetric Gravitational Field of Saturn Measured by the Cassini Grand Finale

Abstract

Abstract The equatorially antisymmetric gravitational field of Saturn, represented by the four gravitational coefficients J 3, J 5, J 7, and J 9, has been measured by the Cassini Grand Finale. Since the rotational distortion and the internal structure do not contribute to the asymmetric gravity field, the spectrum of the odd coefficients offers a direct window into the structure and amplitude of fluid motion in Saturn. We show that the observed cloud-top asymmetric winds extending into Saturn’s interior cannot produce the measured gravity field, suggesting that the cloud-top winds are confined within a thin weather layer and hence do not contribute to the observed gravity. We interpret the asymmetric gravitational field by searching for an a priori unknown zonal circulation taking place in the molecular envelope. We find that a zonal circulation confined in the 20,000 km thick molecular layer produces the coefficients J 3, J 5, J 7, J 9 in agreement with measurements. The depth of the circulation is consistent with that inferred from the equatorially symmetric gravitational field of Saturn.