Saturn's magnetic shield is asymmetrical compared to Earth's, suggests a new study involving University College London (UCL) researchers, and this is likely a result of its fast rotation coupled with the heavy material it pulls around it.
Planetary magnetic fields (magnetospheres) shield planets from the highly charged particles of the solar wind. Saturn's field is vast, more than 10 times wider than the planet itself.
The new study, published in Nature Communications, looked at six years of data from the Cassini space mission to determine the precise location of Saturn's cusp - where magnetic field lines start to curve back into the planet's poles and funnel charged particles down into the atmosphere.
The team found that the cusp was dragged to the right as viewed from the Sun, and was located most often between 13:00 and 15:00 (as it might appear on a clockface), compared to 12:00 as it would be on Earth.
The researchers said this was likely because of Saturn's extremely fast rotation (a Saturn day is 10.7 hours) and the heavy "soup" of plasma (ionised gas) it pulls around it, a product of gases emitted by Saturn's moons, especially Enceladus. Together, these are thought to drag the magnetic field lines to the right. But more simulations are needed to confirm this interpretation.
The environment around Saturn is of particular interest given that its moon Enceladus, which has icy plumes emanating from a subsurface ocean, may even host life and is the planned destination of a major European Space Agency mission proposed for launch in the 2040s.
Co-author Professor Andrew Coates (Mullard Space Science Laboratory at UCL) said: "The cusp is the place where the solar wind can slip directly into the magnetosphere. Knowing the location of Saturn's cusp can help us better understand and map the whole magnetic bubble.
"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. 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.
"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."
"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."
The international study team was led by researchers at the Chinese Academy of Sciences, the Southern University of Science and Technology, and the University of Hong Kong.
Corresponding author Professor Zhonghua Yao (The University of Hong Kong) said: "The differences between Saturn's magnetic structure and that of Earth point to a unified fundamental process governing solar wind interaction across different planets. 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."
Lead author Dr Yan Xu (Southern University of Science and Technology in China) said: "By combining Cassini observations with simulations, we found that Saturn's rapid rotation and the plasma from its moon Enceladus together shape the asymmetric global distribution of the cusps. We hope this gives some useful reference for future exploration of Jupiter's and Saturn's space environments."
The researchers looked at data from two of Cassini's instruments (the Cassini Magnetometer, or MAG, and Cassini Plasma Spectrometer, CAPS) to detect moments where the spacecraft flew through Saturn's cusp. They found 67 such events in total across the years 2004 to 2010, indicated through such clues as the energy levels of electrons the sensors detected.
Drawing on this data, the researchers carried out simulations of the magnetic field, finding that interactions between the magnetic field and solar wind at the edge of the magnetosphere closely resembled that of Jupiter's.
A key source of data for the study came from the CAPS's electron sensor, which was developed by a team led by Professor Coates at the Mullard Space Science Laboratory at UCL.
The researchers received funding from the UK's Science & Technology Facilities Council and the National Natural Science Foundation of China, among other institutions.
