Latest insights into Saturn's weird magnetic field only make things weirder

Leicester space scientists, who have been involved in the Cassini mission for decades, discuss the ‘gold mine’ of information the mission has revealed about the ringed planet and their personal connections to the project.

NASA’s Cassini mission, on a quest to reveal the mysteries of Saturn, took a series of daring dives between the planet and its inmost ring in September 2017 before burning up in the planet’s atmosphere.

New research has provided a first analysis of the data from the magnetometer instrument, built and run by Imperial College London physicists and supported by University of Leicester scientists. The data shows that the planet's magnetic field has a tilt of less than 0.01⁰. 

The results are published today in a special issue of Science reporting the first end-of-mission results.

It was thought that magnetic fields around planets can only form when there is a discernible tilt between the rotation axis of the planet and the magnetic field axis. This is the situation on Earth, where the magnetic poles are offset from the geographic poles.

This tilt sustains currents in a liquid metal layer deep inside a planet – on Earth this is a liquid iron-nickel layer around the solid iron core, and on Saturn it’s believed to be a metallic hydrogen layer surrounding a small rocky core.

“Each time we more accurately measure the tilt of Saturn’s magnetic field, it gets smaller, until now we are in a position where it is smaller than a hundredth of a degree. This is in sharp contrast to the Earth’s magnetic field tilt of 11 degrees. It may still be that Saturn’s turbulent atmosphere of thick gases is obscuring some of the magnetic data, but it looks increasingly likely we will have to rethink the ways different kinds of planets can form magnetic fields,” says Principal investigator for the magnetometer Professor Michele Dougherty, from the Department of Physics at Imperial. 

Planetary scientists from our University have played key roles in the mission since the 1990s, and Cassini’s breath-taking discoveries have helped shape Leicester’s research into planetary magnetospheres, ionospheres and atmospheres since the robotic explorer launched in 1997.

The mission has been instrumental in launching not only the pioneering probe itself, but also the careers of space scientists who still conduct research at the University today and have long-standing and personal connections to the mission. 

Professors Emma Bunce and Stan Cowley from our Department of Physics and Astronomy are Co-Investigators on the magnetometer instrument, who have, with Dr Gabrielle Provan, worked with processing Cassini’s data for decades.

“It has been a privilege to be a part of the Cassini mission’s exploration of the planet Saturn, through our involvement with the Imperial College magnetometer team,” says Professor Emma Bunce. “The Grand Finale orbits, passing between the rings and the planet itself, have provided exquisite detail of the internally generated magnetic field of Saturn and have also provided some unexpected features. We will be studying this data for some time to come, despite the mission having ended last year. 

“For me personally, I have been working on this mission from the end of my PhD all the way through to becoming a Professor – so I owe a great deal to this wonderful project!”

“It has been a truly amazing opportunity to be a planetary scientist at the University of Leicester during the Cassini era,” says Dr Provan, who has been interpreting data from Cassini for a decade. “The University of Leicester’s research on Saturn is world-leading and we will continue to uncover the mysteries of the ringed planet for years to come.” 

Professor Cowley, who has been involved in the Cassini mission for almost three decades, said: “The data on Saturn’s magnetic field acquired during the final close fly-by orbits of Cassini in 2017 is providing a gold mine of novel results of which those in this Science paper are only the first.  

“I have been involved in the Cassini mission for almost 30 years, and although sorry to see it end as a fireball in Saturn’s atmosphere last September, it has really gone out with a scientific bang and not with a whimper, which is a great way to go.”

While the almost negligible tilt is a surprising mystery to scientists, the team did spot some other interesting structures in the magnetic field close to the planet, which may hold further clues.

Closer to Saturn, they saw signals that hint at a secondary source of magnetism for the planet. Above the deep liquid hydrogen layer creating the main planet-wide magnetic field, they suggest there is a shallower layer producing many much smaller, stable magnetic fields.

There also seems to be an electric current flowing between the inner ring, the D ring, and the planet. The rings cut across the main magnetic field lines as they lie around the planet’s equator, and may play a role in shaping magnetic fields generated externally from the planet.

The team are looking closer at these phenomena and modelling possible structures for the interior of Saturn. They will also be combining their data with that of other instruments on Cassini. For example, combining the magnetometer data with gravity data could allow them to work out the mass, size and density of the planet’s core.

• ‘Saturn’s magnetic field revealed by the Cassini Grand Finale. By Michele K. Dougherty, Hao Cao, Krishan K. Khurana, Gregory J. Hunt, Gabrielle Provan, Stephen Kellock, Marcia E. Burton, Thomas A. Burk, Emma J. Bunce, Stanley W. H. Cowley, Margaret G. Kivelson, Christopher T. Russell, and David J. Southwood is published in Science.