Uranus and Neptune's Odd Magnetic Fields Explained by New Study

Not a collision

Uranus is tilted by 97 degrees, possibly after a collision with a long-lost Earth-sized planet. But Neptune's field is skewed too, with no such impact, so the cause is likely intrinsic to ice giants.

The U.S.-UK team ran computer simulations based on Voyager 2 data. Uranus and Neptune hold ammonia, hydrogen, helium, methane and water around a probable rocky core, and have 14.5 and 17 times Earth's mass. "Deep inside Uranus and Neptune, pressures and temperatures become extremely high," said lead researcher Chi Yan of the University of Texas at Austin. Water molecules can break into ions, so the material "may become increasingly electrically conductive with depth." With that assumption, the simulations reproduced both planets' odd fields.

The models also suggest the fields can change within decades. "Earth's magnetic tilt drifts by only a degree or so over 50 years," Yan said, while on the ice giants the tilt can swing by tens of degrees in some stretches.

Testing the model

Tom Stallard of Northumbria University, a co-author, leads James Webb Space Telescope observations of faint infrared auroras on both planets. Because auroras trace the magnetic field, combining them with Voyager 2 data and the simulations could show how the fields look today compared with the 1980s, a test of the model.

A definitive answer will need a new mission. NASA has prioritized a return to Uranus in the early 2040s, and China's Tianwen-4, launching to Jupiter in 2029, will send a small probe on to Uranus via a gravitational slingshot. The findings appeared on 3 November in Communications Physics.