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

TL;DR

This study combines gravity and magnetic field data from Cassini and Juno missions to better understand the deep zonal flows of Saturn and Jupiter, revealing their extent and decay profiles within the planets.

Contribution

It introduces a combined gravity-magnetic analysis method to constrain the vertical structure and decay of planetary zonal flows, improving upon gravity-only approaches.

Findings

Zonal flows extend nearly barotropically to 7,000 km (Saturn) and 2,000 km (Jupiter).

Flow decay occurs rapidly in the semiconducting region, reducing to 1% within 1,000 km (Saturn) and 600 km (Jupiter).

Magnetic interactions likely influence the decay of flows in the semiconducting region.

Abstract

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 kilometers 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 7,000~km (2,000~km) and then decay rapidly in the semiconducting region, so that within the next 1,000 km (600~km) their value reduces to about 1\% 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.