Helium Rain on Saturn May Explain Its Unusual Magnetic Field

Credit:  NASA/JPL-Caltech.

Artist’s impression of the Cassini spacecraft as it dove toward Saturn’s atmosphere during its “Grand Finale” series of close approaches. Credit:  NASA/JPL-Caltech.

In 2017, the Cassini spacecraft ended its thirteen-year tenure at Saturn by plunging into its atmosphere. The close observations of Saturn’s magnetic field made during the Cassini Grand Finale orbits revealed several unusual properties. Unlike other planets in the solar system, Saturn’s magnetic field is extremely axisymmetric, meaning that it is symmetric around its axis of rotation. A study by Chi Yan and Sabine Stanley at Johns Hopkins University has suggested a possible reason for this phenomenon:  a thick layer of helium rain deep in Saturn’s atmosphere.

The magnetic fields of giant planets like Jupiter and Saturn are driven by a dynamo formed by the convective movement of their deep atmospheres, where the extreme pressures force hydrogen and helium into metallic physical states. However, there is a pressure and temperature range where hydrogen can be metallic while helium is still liquid, and they do not mix. Helium would “rain” out from the hydrogen at these depths, forming a layer that may inhibit the dynamo and shape the magnetic field. Yan and Stanley modeled a magnetic dynamo in the deep, convective interior, where metallic hydrogen and helium mix, surrounded by a thick helium rain-out layer extending from about 40% to 70% of Saturn’s radius. They found that this helium rain-out layer could stabilize the atmosphere against convection and lead to the axisymmetry of the magnetic field observed by Cassini. Yan and Stanley also found that the symmetry of the field is affected by the heat flux at the top of the rain-out layer with the best matches to Saturn’s observed magnetic field, requiring a stronger heat flux from the poles than the equatorial region. These simulations show one possible method for indirectly examining the exotic conditions within the interiors of the giant planets. READ MORE