The magnetic fields of Uranus and Neptune are really, seriously messed up. And we don’t know why.
The magnetic fields of most planets (if they even have one) are pretty straightforward. The planet spins in a certain direction, and the field roughly lines up with that direction of spin. Sure, the fields may wander a little bit here and there, but generally speaking everything makes sense.
And then there are the ice giants, Uranus and Neptune. In the case of Uranus, the planet itself spins almost perpendicular to the rest of the solar system, but its magnetic field is in almost the usual up-down direction. With Neptune, the magnetic field is a full 47 degrees away from the spin direction. In addition, the magnetic fields are offset away from the centers of both planets.
What’s going on?
Scientists have long guessed that something funky is happening within the planets. Both planets are thought to host large convective layers, somewhere between the core and the atmosphere, where super-pressurized water and methane exist in a “superionic state”, with properties of both liquids and solids. The superionic water and methane circulate in up-down patterns, and since they are charged, the planets might generate their magnetic fields there, rather than in the cores.
To test this idea, Tomoaki Kimura and Motohiko Murakami from the Department of Earth Sciences at ETH Zurich studied the properties of ammonia in a super-pressurized state using a diamond cell anvil. By squeezing the sample and heating it to over 2,000 degrees Celsius, they were able to recreate the interiors of the ice giants.
They found that the superionic ammonia might indeed be stable at those pressures and temperatures, suggesting that it might exist inside those planets. But crucially, the ammonia was not viscous enough to form a stable layer deeper within the planet. In other words, for the convective layer idea to work, it needs to sit on top of a stable layer, and it seems difficult to get ammonia to play both roles. And without a convective layer, we can’t explain the magnetic field structure.
For now, the mystery of the ice giant magnetic fields goes unsolved.