Combined magnetic and gravity measurements probe the deep zonal flows of the gas giants

During the past few years, both the Cassini mission at Saturn and the Juno mission at Jupiter provided measurements with unprecedented accuracy of the gravity and magnetic fields of the two gas giants. Using the gravity measurements, it was found that the strong zonal flows observed at the cloud level of the gas giants are likely to extend thousands of kilometres deep into the planetary interior. However, the gravity measurements alone, which are by definition an integrative measure of mass, cannot constrain with high certainty the exact vertical structure of the flow. Taking into account the recent Cassini magnetic field measurements of Saturn, and past secular variations of Jupiter's magnetic field, we obtain an additional physical constraint on the vertical decay profile of the observed zonal flows on these planets. Our combined gravity-magnetic analysis reveals that the cloud-level winds on Saturn (Jupiter) extend with very little decay, i.e. barotropically, down to a depth of around 7000 km (2000 km) and then decay rapidly in the semiconducting region, so that within the next 1000 km (600 km) their value reduces to about 1 per cent of that at the cloud level. These results indicate that there is no significant mechanism acting to decay the flow in the outer neutral region, and that the interaction with the magnetic field in the semiconducting region might play a central role in the decay of the flows.