The posided magnetic fields of Uranus and Neptune may stem from extreme conditions lying within their deep interior. Unravelling the mystery behind the magnetic fields of these worlds is giving us new insight into the little-known interiors of the ice giants.
Only one spacecraft has ever visited Uranus and Neptune, the Voyager 2 spacecraft which spend past Uranus in January 1986 and Neptune in August 1989. Voyager 2 found that their planetary magnetic fields were, to put it bluntly, wonky. Uranus’ magnetic field was offset from its rotational axis by 59 degrees and isn’t even centered on the planet’s core, but instead is only two-thirds of the way down. Neptune is similar, with a disordered magnetic field offset by 47 degrees and centered halfway between the core and its visible cloud tops. If we compare these planets to Earth, which has a stable magnetic field tilted by just 11 degrees to our planet’s rotation, we can see how extreme the magnetic fields of the ice giants are.
Various explanations for the strange magnetic fields of the ice giants have been put forward over the years. Uranus as a whole is tilted by 97 degrees and effectively rolls around the sun, and speculation is that billions of years ago it was knocked on its side by a collision with a long-lost Earth-sized planet. Could this collision have also skewed its magnetic field?
This might be a plausible explanation were Neptune’s magnetic field also not similarly skewed without having experienced a collision. Clearly, whatever the cause, it must be intrinsic to the nature of the two ice giant planets. Now, a new study by scientists in the U.S. and the UK, based on computer simulations and Voyager 2’s measurements of the two planets’ magnetic fields, may have found a solution.
The solution to the weird magnetic fields of the solar system's ice giants may come down to how planets generate their magnetic field through the dynamo effect. This arises from the swirling, convective motion of an electrically conducting liquid within a rotating body, which is able to generate and maintain a powerful magnetic field.
Inside Earth, the electrically conducting fluid is the molten iron in the outer part of our planet’s core, and the magnetic field that it generates has clear north and south poles – what we call a dipole, a phenomenon that we can also see in the two magnetic poles of a simple bar magnet.
However, the conditions inside Uranus and Neptune are somewhat different. As ice giants, they contain a lot of volatiles – ammonia, hydrogen, helium, methane, and water surrounding a probable rocky core. Furthermore, Uranus and Neptune are substantially larger than Earth, with 14.5 and 17 times the mass of our planet, respectively. This increased mass creates crushing conditions within the interior of the ice giants.
"Deep inside Uranus and Neptune, pressures and temperatures become extremely high," research team leader Chi Yan of the University of Texas at Austin, who led the new findings, said in a statement. "Under those conditions, which can cause water molecules to break apart into ions, the material may become increasingly electrically conductive with depth."
Given the increasing electrical conductivity and the compositions of the two planets, the computer simulations were able to replicate the bizarre magnetic fields of Uranus and Neptune. This showed how the magnetic-field structure co-evolves alongside changes in the fluid motion driven by convection and the temperature inside the planets.
It also seems that this co-evolution might not yet be over. The simulations indicate that the magnetic fields of both planets can change quite significantly in the space of just a few decades.
"Earth’s magnetic tilt drifts by only a degree or so over 50 years," said Yan. "Our models show the ice giants can behave very differently. In some stretches, the tilt swings by tens of degrees within decades. In others, it shifts only slightly over the same span."
NASA have prioritized a new mission to return to Uranus in the early 2040s, while the China National Space Administration’s Tianwen-4 mission launching to Jupiter in 2029 will deploy a small probe to carry onwards to Uranus via a gravitational slingshot. Until then, scientists are coming up with creative ways to not have to rely only on the 40-year-old Voyager 2 measurements of the magnetic fields.
For example, Tom Stallard of Northumbria University in the UK, who is a co-author with Yan on the new research paper, has been leading James Webb Space Telescope (JWST) observations of the faint infrared auroral lights on both Uranus and Neptune. Aurorae are generated when charged particles on the solar wind interact with a planet’s magnetic field, driving some of those charged particles into the atmosphere above the magnetic poles. (Neptune actually has a quadrupolar component, muddling its defined north and south magnetic poles.) As such, the location of aurorae is a good indicator of what’s going on with a planet’s magnetic field.
"We are busy working on what’s contained within these observations, but the shape and structure of the aurora, as well as other weird patterns in the ionosphere [upper atmosphere], promise to provide us with a way of understanding how the magnetic fields have changed, [which is] an important stepping stone," said Stallard.
The hope is that by combining JWST’s observations of the aurora with Voyager 2’s magnetic-field measurements and the results of the computer simulations, it will be possible to estimate the configuration of the magnetic fields on Uranus and Neptune today and see how they compare to the 1980s.
This would provide an experimental test to help show whether the new model describing the magnetic fields is on the right track, although we won’t know for sure until a new mission arrives in the ice giants’ backyard.
The findings were described on 3 November in the journal Communications Physics.
