Voyager 2 took this photo of Uranus when the spacecraft passed the ice giant in 1986. Credit: NASA/JPL-Caltech
In 1986, Voyager 2 flew past the ice giant Uranus. It was humanity’s first close-up view of the outer planet, and remains the closest any spacecraft has ever seen. During this trip, the spacecraft found 10 previously unknown moons, two rings, and a truly bizarre magnetic field that has baffled scientists ever since.
Most other planets have magnetic fields that roughly align with their poles, which point up and down, out of the plane of the solar system. But Uranus is unique in that it spins on its side, like a ball rolling around the Sun. Despite this oddity, its magnetic field does not appear to originate in the center, but somewhere far to the south, perhaps up to a third of the distance of the planet. And it’s tilted 59 degrees.
Additionally, Voyager 2 noted a strangely intense magnetosphere during its passage, with strong radiation belts and a bizarre lack of plasma. Researchers have wondered about these findings for the past 40 years.
Now, a new study published on November 11 in Natural astronomy revisited data from Voyager 2 and found that Uranus was indeed undergoing a burst of solar time, putting it in a state that researchers estimate represents Uranus only four percent of the time and reframing what little we know about our distant near frozen.
Putting it into perspective
Jamie Jasinski, of NASA’s Jet Propulsion Laboratory in California, is the lead author of the new study. In the paper, he and co-authors point out that previous studies used Voyager 2’s measurements of the solar wind as it passed through Uranus’ bow shock, the outer edge of its magnetosphere, to frame the rest of the observations. But looking at the spacecraft’s measurements, it’s clear that the planet – and the spacecraft – were engulfed by a burst of solar wind at that very moment. Looking at the big picture, about eight months of data, it was clear to them that this is not the norm for Uranus and explains why its magnetosphere looked so strange.
The planet is and remains tilted, which causes some oddities in the magnetosphere. But the inexplicable part for the researchers was the lack of observed plasma. The moons of Uranus (we now know of 28, including five large ones) were expected to behave like the other outer moons of the solar system: icy and probably capable of emitting ions (charged particles) and their dissociated electrons in the vicinity of the planet. But Voyager 2 saw none of this material.
The spacecraft saw intense belts of electronic radiation, second only to that of Jupiter. But how these belts formed and persist, without a plasma source, has also baffled astronomers.
A burst of solar wind solves both of these puzzles. Just as a gust of wind on Earth causes a flag to swell and break, solar wind can compress and crush a planet’s magnetosphere. “In the case of Uranus, we calculated that the increase in pressure before the Voyager 2 flyby could have caused a fivefold reduction in the volume of the daytime magnetosphere,” Jasinski said in a statement.
The squeezing effect can in turn push electrons into the radiation belts and push plasma out of the system. Astronomers have observed exactly this effect on both Earth and Saturn, lending credibility to their theory.
It needs more data
The authors caution, however, that while their new shot of Uranus in an unusual state during Voyager 2’s flyby makes sense, it remains that the spacecraft provided the only close-up data we’ve ever gotten from the ice giant.
A dedicated mission to Uranus with an orbiter and probe is in the planning stage and will investigate the planet and its moons for 4.5 years. That mission was deemed the highest priority by the U.S. National Academies’ most recent Decadal Planetary Survey, which guides funding and strategy for NASA and the National Science Foundation, among others. But given the timing of the planet’s orbit and seasons, a launch is needed within the next decade, and time and budget are already closing in on such a plan.
Astronomers are discovering thousands of worlds beyond our solar system. But there are still mysteries nearby. Uranus, studied only briefly and from afar, could still have some surprises in store for us.
