Having already staked its claim in our solar system as the largest planet, Jupiter now stands out from its planetary kin for another reason: its shape-shifting magnetic field.
In a recent study, researchers compared observations of Jupiter’s magnetic field from NASA’s Juno spacecraft with those taken by Pioneer 10, Pioneer 11, Voyager 1, and Ulysses. They found that Jupiter’s field had changed in just a few short decades—a phenomenon known as secular variation.
One theory for the variation over time points to zonal winds—large storm systems in Jupiter’s atmosphere that extend to over 1,800 miles deep. At this depth, the hydrogen inside Jupiter is under so much pressure and at such high temperatures that it behaves like the molten iron in Earth’s core to produce a magnetic field.
“If the [zonal] winds are that deep, they would distort the magnetic field,” says Eli Galanti, a planetary scientist at the Weizmann Institute of Science, who was not involved in the study but researches these disturbances using gravity measurements. Galanti says Moore’s research shows that changes in flow could explain the historical movements of the magnetic field.
If these storms interfere with the flow of hydrogen, they could also explain why Jupiter’s magnetic field has such a peculiar “shape.”
Earth’s magnetic field is often represented with a simple bar magnet, but Jupiter’s field, according to data compiled from Juno in 2018, seems to be much more complex.
Magnetic field visualizations typically use a range of colors to represent the nature of a planet’s field: Shades from red to yellow illustrate where the field emerges, and shades from green to blue show where it reenters. Using this convention, Earth’s South Pole is painted red and the North Pole is blue.
Jupiter’s magnetic field is not as demarcated. It resembles a bruised peach with two distinct, deep blue blotches against swaths of red and yellow.
Rather than originating from Jupiter’s true north, the Jovian magnetic field springs from a broader band lower in the northern hemisphere. Arguably more intriguing is Jupiter’s southern hemisphere. Juno’s data suggest the planet’s magnetic field arcs out from magnetic north and returns through two different points, one near the south pole and another just below the equator. Researchers affectionately name this concentrated southerly zone the Great Blue Spot, an homage to Jupiter’s famous Great Red Spot.
Before Juno, astronomers had a limited view of Jupiter. Kimberly Moore, a graduate student at Harvard University who led both the recent study and last year’s data compilation, says that Earth’s magnetic field is like that produced by a tilted dipole, or an axis with opposite magnetic polarity on each end. “But (for Jupiter) we are learning you can’t use such a simple picture,” Moore says.
“We are looking at a completely different Jupiter than the one that was anticipated a few years ago,” says Mohamed Zaghoo, a research scientist at the Laboratory for Laser Energetics at the University of Rochester who was not involved in the study.
“The amazing thing about Jupiter is that because it’s a gaseous planet there is no crust to obscure our measurements of the planet’s dynamo (the engine that drives the magnetic field),” Zaghoo says. In Earth—about two-thirds of the way into the planet’s interior at the core-mantle boundary—molten iron constantly moves and creates electric currents that generate the magnetic field we observe at the surface.
In contrast, Jupiter doesn’t have a firm surface and is largely composed of hydrogen, which does not ordinarily conduct electricity as a gas. However, under the planet’s extreme pressure, liquid metallic hydrogen forms, named for its ability to conduct electric currents like metals do.
Metallic hydrogen circulates inside Jupiter due to the convection currents created by the difference between high internal temperatures and lower outer temperatures. This constant churning is believed to generate electric currents inside Jupiter, which in turn produce the magnetic field measured by Juno.
The irregular pattern of the magnetic field might be explained by the changing properties of metallic hydrogen as it descends deeper inside the planet. “You can think of it like you’re diving down an ocean, where different layers have different densities and conductivities,” says Zaghoo. In his work, Zaghoo recreates the conditions of hydrogen at different depths in Jupiter and compares how observations here on Earth line up with models made from the space mission.
But studying the flow of metallic hydrogen might not tell the whole story. Other, more stable layers of fluid deep inside Jupiter could also be responsible for the magnetic field, Moore and her colleagues suggest. If these layers contain heavier elements like helium, they might divert the flow of electric currents and consequently alter the magnetic fields that radiate from within.
“So far Juno has provided more questions than answers.” says Zaghoo. “The biggest lesson we’ve learned from Juno is humility.”
As the space probe continues orbiting Jupiter, it might offer more clues to its magnetic mystery. Juno will keep gathering information until July 2021, when it will plunge toward Jupiter and disintegrate in the planet’s atmosphere.
In the meantime, scientists like Moore hope to glean as much information as they can about Jupiter’s magnetic field.
“The magnetic field is much stronger and generated much closer to the surface on Jupiter than on Earth,” Moore says. “The insights we are getting into how these fields are generated on Jupiter might help us understand how they are generated here on Earth.”