Dark ribbon around Jupiter reveals magnetic mystery

Planetary scientists studying images of Jupiter taken from some of the Earth’s largest telescopes have discovered a mysterious dark “ribbon” winding around the giant planet’s magnetic equator.

The equator is the region equidistant between a planet’s north and south poles. Unlike the geographic equator, the magnetic equator defined by the north and south magnetic poles, which on Jupiter, as on Earth, are tilted about 10 to 11 degrees from true north.

Not that this ribbon is visible to human eyes. In a paper published this week in Nature Astronomy, a team led by Tom Stallard, a planetary astronomer at the University of Leicester in the UK, found it by studying infrared emissions from H3+ ions, which are created by interactions between the planet’s magnetic field and an upper layer of its atmosphere known as the ionosphere. 

“That molecule gets hot and starts glowing,” he says. “It’s a probe of the interaction between the atmosphere and the surrounding space environment.”

Most H3+ observations on Jupiter, he says, have focused on the magnetic poles, where the ions are associated with the planet’s aurorae. 

Away from the poles, these emissions are much less intense. “It’s about 100 times weaker,” Stallard says. “The only way [to see it] is by using really big telescopes for a long time.” 

That much observing time on such telescopes is hard to come by, so Stallard tried a different tack. Working with colleagues in the U.S. and Japan, he painstakingly stitched together 13,501 images taken in the late 1990s. These images were intended to support magnetic field studies by NASA’s Galileo spacecraft, which at the time was orbiting Jupiter. But their field of view was wide enough to encompass the rest of the planet. “I thought, ‘What if you take all that data and add it together?’” Stallard says.

It was a slow, painstaking process. But when it was done, he says, “We had the equivalent of several days of staring at every single part of the planet.” 

The dark band at the equator was a surprise—suggesting that the interactions between Jupiter’s magnetic field and its ionosphere are much more complex than anticipated. 

Of particular interest, Stallard says, is that the band is very narrow, with sharply defined edges. Most likely he says, the band marks places where Jupiter’s magnetic field is parallel to its surface, rather than plunging into it, as it does closer to the poles. But why the edges are so sharp is a mystery.

Such discoveries, Stallard says, are important not just to help us understand Jupiter. They also help us understand our own planet. “We think of Jupiter as alien,” he says, “but it’s more of a cousin.”

Studying its similarities and differences, he says, can help us learn more about our own world — something that’s particularly important when it comes to understanding the ionosphere. Radio communications regularly bounce signals off the Earth’s ionosphere. Satellites move within it and can be strongly affected by the way solar storms affect it. “I don’t think you can understand the Earth’s system until you have a broader understanding of other planets,” Stallard says.

Other scientists applaud the finding. “This is a very impressive scientific result,” says Randy Gladstone, a planetary scientist at the Southwest Research Institute, San Antonio, Texas. 

In part it dovetails nicely with new observations being taken by NASA’s Jupiter-orbiting Juno spacecraft, for which Gladstone is the lead investigator for one instrument. But he is even more impressed by the methodology, which involved compiling all those thousand of images one by one. “That is an enormous amount of work to go to, when no useful outcome was guaranteed,” he says.