Uranus’ magnetic forces switch ‘on and off’
Voyager 2, NASA’s legendary interstellar probe, continues to deliver.
Thirty-one years after the spacecraft cruised past Uranus, researchers have capitalized on the spacecraft’s recordings to take a fresh look at the icy giant’s magnetosphere.
Typically, a planet’s magnetic field stays sturdy — a stalwart shield against the sun’s radiation. But Uranus’ magnetosphere swivels, switching its invisible armor on-and-off, according to new work from the Georgia Institute of Technology. While “quirky” barely begins to describe these magnetic forces, Uranus’ situation may signify the norm across the cosmos and be key to refining our search for habitable worlds.
“The scientific community wants to go back to Uranus, in light of all these exoplanet discoveries,” Carol Paty, a Georgia Tech planetary scientist who led the project, told NewsHour. “A large fraction of these exoplanets are Uranus, Neptune in size.”
That’s fascinating because of Uranus’ many contrasts with Earth. Our planet spins like a top, as it goes around the sun, much like the other planets in the solar system. Uranus rotates on its side like a paddle boat wheel, meaning one of its two poles face the sun for much of the year.
Stand on Uranus’ northern hemisphere on a summer solstice, and the sun wouldn’t set. It’d circle overhead like an orb on a crib mobile. Day by day during the summer — which lasts 21 years — the sun would drop until you experienced what seemed like an Earth day, with an equal amount of daytime and nighttime, on the autumn equinox. Push forward in time, and suddenly the tilt is against you. Everyday, you see fainter and fainter hints of sun rays on the horizon, until the winter solstice, when your day and night are completely black.
In 1986, Voyager 2 took a snapshot of Uranus’ magnetosphere. It, too, defied expectations.
On Earth, the magnetosphere emanates, like from any magnet with two poles, invisible and shaped like butterfly wings. This alignment keeps the solar wind out of our atmosphere, except at the poles, where pass through the field’s cusp and mingle with the atmosphere to create auroras.
But, Voyager 2 found Uranus’ magnetic fields were lopsided — its “wings” twisted off-center by 60 degrees.
Paty wondered what this off-kilter arrangement might mean for the planet’s ability to buffer solar wind, as the planet orbits the sun and rotates through its 17-hour day. So, she and her graduate student Xin Cao built a computer simulation, based primarily on Voyager 2’s snapshot of Uranus’ magnetosphere.
“They used a supercomputing approach, rather than a pen-and-paper approach,” said Krista Soderlund, a planetary fluid dynamicist at the University of Texas Institute for Geophysics who wasn’t involved in the project. Soderlund said by doing so, the model painted a high-resolution picture of the chemical ions in Uranus atmosphere, which were detected when Voyager 2 came within 50,600 miles the planet’s clouds.
Paty and Cao’s model predicted that Uranus’ magnetosphere tumbles “very fast, like a child cartwheeling down a hill head over heels.” As a consequence, the magnetosphere is sometimes in position to protect the planet from solar wind. At other times, it can’t. Its buffering ability switches on and off.
This bizarre setup excites geophysicists like Soderlund because of what it might reveal about Uranus’ insides. Earth’s magnetosphere is thought to be generated by liquid metal coursing around the planet’s solid inner core. This fluid movement, called convection, is partially dictated by the Earth’s rotation and causes the metal to conduct electricity, like a dynamo. The result is a giant electromagnet and the magnetosphere.
Uranus’ weird magnetosphere suggests its liquid dynamo is thin and relatively close to the planet’s surface, Soderlund said. In 2013, her team argued the chemical composition of the liquid layer may also play a part, especially given Uranus’ dynamo may be composed of charged water rather than liquid metal.
“We put out this idea that Uranus and Neptune would have a much more vigorous convection going on, that’ll be less influenced by the planet’s rotation,” Soderlund said. “You’ll have more messy convection happening in the interior, which leads to a messier magnetic field structure.”
Soderlund and others plan to use Paty’s model to refine their assessments about Uranus.
“It’s good timing to have this scientific study published about Uranus, at the same time NASA is looking into possibly proposing a mission to go back,” said University of Iowa space physicist George Hospodarsky, who wasn’t involved in Paty’s project. Two weeks ago, NASA and the European Space Agency completed a study about future missions to Uranus and Neptune, with an eye toward possibly sending an orbiter to the ice giants.
Theses mission could prove whether or not Paty’s model is accurate, Hospodarsky said, but also answer a bigger question about the cosmos. He said astrophysicists have been slowly building a unified model of magnetospheres, based on data from missions to Jupiter, Saturn, Mercury and Earth.
“That’s actually one of the reasons they do these models for some of the outer planets,” Hospodarsky said. “If you have a model, and the model is good, it should really work for Earth, Jupiter, Saturn — whatever planet you apply it to.”
Such a model could explain why Earth’s magnetosphere is so ideal for supporting life — without it, we’d be charred to death by solar wind — or why Mars lacks one.
That can come in handy for predicting if an exoplanet has a suitable magnetosphere for habitation, in case we ever develop the technology to visit one.