Quirk may explain odd magnetism of Neptune, Uranus

The abnormal magnetic fields of Uranus and Neptune – whose magnetic poles lie near their equators – may be a side effect of stable planetary cores that hinder convection. Harvard University scientists report in the March 11 issue of the journal Nature that they’ve used a computer model, similar to those used in weather forecasting, to establish a possible link between the two planets’ strange magnetic fields and their internal composition.

“The discovery that Uranus’ and Neptune’s magnetic poles are so different than the geological poles around which the planets rotate destroyed the paradigm set by Earth, Jupiter, and Saturn,” says Jeremy Bloxham, professor of geophysics and Harvard College Professor. “Although various explanations for Uranus’ and Neptune’s unusual magnetic fields have been proposed, their cause has remained unexplained.”

Scientists have been intrigued by Uranus’ and Neptune’s quirky magnetic fields ever since the two planets were first visited by NASA’s Voyager II spacecraft in the 1980s. Some have suggested that the unorthodox magnetism could result from sluggish core convection caused by partially frozen cores, or that perhaps the faraway bodies were in the process of reversing their magnetic poles, as has happened occasionally on Earth and other planets.

“Since planetary magnetic fields are generated by complex fluid motions in electrically conducting regions of the planets, a process known as dynamo action, they are intricately linked to the structure and evolution of planetary interiors,” write Bloxham and co-author Sabine Stanley, a graduate student in Harvard’s Department of Earth and Planetary Science. “Determining why Uranus and Neptune have different field morphologies is not only critical for studying these planets’ interiors, but is essential for understanding the dynamics of magnetic field generation in all planets.”

Using the Kuang and Bloxham numerical dynamo method, Bloxham and Stanley found that a core structure characterized by limited convection was consistent with the magnetic fields observed on Uranus and Neptune. Compared with the iron core of the Earth, which is solid in its innermost third, these two planetary outliers may be characterized by cores that are stably fluid across two-thirds of their radius, with just a thin layer of convective fluid surrounding.

This larger stable mass would hinder convection in Uranus’ and Neptune’s cores, meaning that fluids in the core cannot circulate as freely. Scientists have known for some time that magnetic fields are influenced by the churning of fluids in planetary cores, but the fine details of this process are poorly understood.