Saturn's magnetic field is curiously warped, and one of its moons may be to blame

Saturn has a lopsided magnetic field, far different from the mostly even magnetosphere of Earth, new research suggests. The ringed gas giant's wonky magnetic shield may be the result of its rapid rotation (a day on Saturn lasts just 10.7 hours), and the effects of its moons, especially the icy ocean moon Enceladus.

The team behind this research reached these findings when they examined six years of data regarding Saturn collected by the Cassini spacecraft, which orbited the gas giant between 2004 and 2017. The aim of this research was to discover where the magnetic field lines of Saturn start to curve back into the planet’s poles, where they funnel charged particles down into the atmosphere, a point known as the "magnetic cusp."

The team found that Saturn's magnetic cusp is dragged to the right as seen from the sun. In terms of a clock face, Earth's magnetic cusp is at 12, while Saturn's is at 1 or 3. The discovery could be an important one for future spacecraft missions to the Saturnian system that aim to discover if its icy moon Enceladus is capable of supporting life in its global ocean.

"A better understanding of Saturn’s environment is especially urgent now as plans for our return to Saturn and its moon Enceladus start to be developed," team member Andrew Coates of University College of London's Mullard Space Science Laboratory said in a statement. "These results feed into the excitement that we are going back there. This time we will look for evidence of habitability and for potential signs of life."

Enceladus is such a drag

Saturn is the second-largest planet in the solar system after Jupiter, and the Saturnian magnetic field is ten times as wide as the gas giant itself. This new research determined that its uneven nature is the result of the planet's rapid rotation and the fact that, as it spins, Saturn drags a heavy soup of plasma around with it.

This matter is the result of gases emitted by the Saturnian moons, particularly Enceladus, which is known to spray out icy plumes that originate from its subsurface ocean.

"This study also provides critical evidence for a long-held theory – that the rapid spin of massive planets like Saturn with active moons replaces the solar wind as the dominant force shaping magnetospheres. It shows that Saturn’s magnetosphere, as well as the magnetospheres of other rapidly spinning gas giants, likely differ fundamentally from Earth's," Coates said. "Enceladus itself is a key driver of this environment, releasing huge amounts of water vapour that gets ionised, loading the magnetosphere with heavy plasma that is then pulled around as the planet spins.”

Team leader Zhonghua Yao of the University of Hong Kong explained that the differences between the structure of the Saturnian magnetic field and that of Earth's magnetosphere point to a unified fundamental process that governs how outflows of charged particles from the sun, known as the solar wind, interact with different planets.

This discovery could be vital in understanding how winds from other stars interact with planets beyond the solar system.

"Comprehensive terrestrial observations reveal the working mechanisms of Earth, while comparative studies between planets inform us of the fundamental laws that can be applied to understand other systems, such as exoplanets," Yao said.

Particularly useful for Yao and colleagues was data from two of Cassini's instruments, the Cassini Magnetometer (MAG) and the Cassini Plasma Spectrometer (CAPS), both of which detected incidents during which the spacecraft passed through Saturn's magnetic cusp.
This revealed 67 such occasions between 2004 and 2010.

Using this to simulate the shape of Saturn's magnetic field, the team found that the way the solar wind interacts with the ringed planet's magnetosphere is similar to interactions with the magnetic field of its fellow gas giant, Jupiter.

The team's research was published on Wednesday (April 1) in the journal Nature Communications.