Uranus is a “sideways” planet. It’s tilted at a little more than 90°, so it rolls along in its orbit like a giant blue-green bowling ball. And planetary scientists may have had sideways ideas about Uranus for almost 4 decades.
A new study published in Nature Astronomy suggests that the planet’s magnetosphere probably isn’t as odd as reported by the Voyager 2 spacecraft during its 1986 flyby. It turns out that at the time, the magnetosphere was being squeezed by especially strong waves of plasma from the Sun, creating unusual conditions.
“Voyager 2 found that the magnetosphere was devoid of plasma, which was unexpected and a big mystery,” said lead author Jamie Jasinski, a space plasma physicist at the Jet Propulsion Laboratory. “The other big mystery was that Uranus had very strong radiation belts—second only to Jupiter’s. So it was all a big conundrum.”
Voyager 2 first encountered Uranus’s magnetosphere on 24 January 1986, just 11 hours before its closest approach. In addition to a lack of plasma (known as a vacuum magnetosphere) and strong radiation belts, the craft discovered a magnetic field offset 59° from the planet’s rotation axis and that the source of the field was offset from the center of Uranus by about one third of the planet’s radius.
The discoveries tagged Uranus as a magnetic outlier, much different from the other planets.
Jasinski said he wondered whether Voyager might have encountered Uranus “at the wrong time”—during a period of intense solar activity that could have drastically compressed the magnetosphere. So Jasinski and colleagues analyzed the magnetic field observations Voyager made during the 2 weeks before the craft reached the planet.
They found that the intensity of the solar wind increased dramatically just a few days before the encounter. “Two weeks before Voyager 2 got there, the solar wind pressure was very low,” Jasinski said. “But we saw that a few days before the flyby it went up by a factor of 20. That obviously affected the size of the magnetosphere. From then on, the analysis wrote itself.”
“I don’t know that this says, ‘Aha, this explains the mysteries of the vacuum magnetosphere and the enhanced radiation belts.’ But it’s an important piece of the puzzle.”
The Sun-facing side of the magnetopause (the boundary between solar wind and the planet’s magnetosphere) was squeezed inward from about 28 or 29 times the radius of Uranus shortly before the encounter to just 17 radii when Voyager arrived, Jasinski said. The squeeze could have emptied the plasma from that region of the magnetosphere while “energizing” the radiation belts.
“I don’t know that this says, ‘Aha, this explains the mysteries of the vacuum magnetosphere and the enhanced radiation belts,’” said Carol Paty, a planetary scientist at the University of Oregon who was not involved in the study. “But it’s an important piece of the puzzle.”
Planning for a New Voyage
The study was motivated in part by the possibility of a combination orbiter-probe mission to Uranus in the next decade, which was ranked as the highest priority for a future “flagship” project in the most recent planetary science decadal survey.
“The community is gearing up for the next mission, so we thought we’d go back and look at some of the old data,” said study team member Xianzhe Jia, a scientist at the University of Michigan who specializes in planetary magnetospheres.
“At the time of the [Voyager 2] encounter, everyone was focused on the time the spacecraft was close to Uranus,” Jia said. “We said, let’s zoom out and see what happens. We wanted to understand what the typical conditions might be like at Uranus.”
That understanding could yield new insights into the planet’s interior structure and the mechanism that generates its oddly offset magnetic field. It also could help planners select the proper instruments for a new mission, particularly those designed to probe the magnetic environment.
That’s especially important for studying some of the planet’s large moons, which, like some of the moons of Jupiter and Saturn, may hide oceans of liquid water below their icy crusts. As the planet’s magnetic field sweeps across a moon, it can induce an electric current in the ocean, generating a magnetic response that can alter that region of the planetary magnetic field. Repeated observations of the moon can use those changes to reveal the ocean’s presence along with a general idea of its depth, distance below the surface, and other details. “It’s been a really successful tool for studying icy worlds,” Paty said.
The technique requires a moon to be inside its planet’s magnetosphere. Voyager’s observations, however, showed that two of the possible ocean moons—Titania, the planet’s largest moon, and Oberon, the second largest—were either outside or near the boundary of the magnetosphere. But the new analysis shows that Titania should be outside the magnetosphere less than 4% of the time and Oberon less than 13%, increasing the odds of detecting subsurface oceans.
“This paper is really well timed to open our minds to thinking about new perspectives for a new mission,” Paty said. “Perspective is everything. We had the entire Cassini mission to learn about Saturn’s magnetosphere, and some of the things we saw, we didn’t even know to look for at Uranus in 1986.”
“This shows that we shouldn’t take Voyager 2 as the be-all and end-all of what we know about Uranus,” Jasinski said. “It was really just a snapshot—a moment in time that was heavily affected by the solar wind. It changed the discoveries we would have made. We really need to reset the way we think.”
—Damond Benningfield, Science Writer
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