Updated Equipotential Shapes of Jupiter and Saturn Using Juno and Cassini Grand Finale Gravity Science Measurements

A commonly used shape model for the giant plants of Jupiter and Saturn is an oblate ellipsoid, a simplified model of the equipotential shape. The ellipsoidal shape models were originally derived from radio occultation data and gravity data after the Voyager flybys in 1979. Through precise Doppler tracking of NASA's Juno and Cassini spacecraft telecommunications links, zonal coefficients in a spherical harmonic expansion of the gravity field of Jupiter and Saturn have been resolved to degree 10, including the detection of nonzero odd zonal harmonics, which have been interpreted as differential rotation in the atmosphere for both planets. In this work, we construct the equipotential surfaces of Jupiter and Saturn using the recently measured gravity fields determined by Juno through perijove 8 and Cassini through end of mission. For both planets, even zonal harmonics dominate the equipotential shape, differing from the reference ellipsoid up to ~32 km at Jupiter and ~125 km at Saturn in the midlatitude regions. Saturn's internal rotation period estimated with ring seismology produces a shape that is fully consistent with the Pioneer and Voyager radio occultation measurements, and Jupiter's shape is fully consistent with the Pioneer and Voyager radio occultation measurements. With current equipotential theory, the recent analysis of the depth of the deep zonal flow on Jupiter and Saturn cannot fully be explained by the shape from current radio occultation measurements; additional occultation measurements and reanalysis of the Pioneer and Voyager radio occultations and will be useful for further constraining the shape of the planets.