The shape of Jupiter and Saturn based on winds, occultations and gravity measurements

The shape of gaseous planets, defined as an equipotential surface for a specific pressure, can be calculated given measurements of the zonal gravity harmonics, and the wind profile and polar radius at a given level. For both Jupiter and Saturn, the gravity harmonics have been measured to high accuracy by the Juno and Cassini missions, respectively. Measurements for the zonal winds are also available via cloud tracking, buth a significant uncertainty is associated with them, stemming from the clouds altitudes, as well as time variations in the strength and location of the winds. With that, the wind profiles can be also constrained by the gravity measurements. Further constraint on the calculated shape of the two gas giants can be obtained from occultation measurements, which give radial dependent profiles of pressure for specific spatial location.

Here we propose a new method for calculating the shape of the gas giants, based on an optimization of the wind latitudinal profile, decay structure, and the polar radius, given both gravity and occultation measurements. We use thermal wind balance to relate the wind to the gravity measurements, and a shape model to relate the wind and polar radius to the occultation measurements. We perform the analysis for both the 0.1 and 1 bar pressure levels. We examine the ability to explain both types of measurements in each planet, and discuss the implication for the possible wind profiles and how they might change with pressure. We also discuss the solutions for the polar radius with respect to the currently used mean values.

Only a few occultation measurements are currently available in each planet, but in the coming years the Juno mission is expected to perform dozens of occultations for Jupiter, covering a wide spatial range, and there are several Cassini occultations performed for Saturn that are still waiting for analysis. Using the method proposed here, we expect the new measurements to help resolve the shape of the gas giants to better accuracy, and to allow better understanding of the wind structures and their depth dependence.