NASA published this original story on November 11, 2024. Edits by EarthSky.
When NASA's Voyager 2 spacecraft flew by Uranus in 1986, it provided scientists' first -- and, so far, only -- close glimpse of this strange, sideways-rotating outer planet. Alongside the discovery of new moons and rings, baffling new mysteries confronted scientists. The energized particles around the planet defied their understanding of how magnetic fields work to trap particle radiation. And so Uranus earned a reputation as an outlier in our solar system.
Now, new research analyzing the data collected during that flyby 38 years ago has found the source of that particular mystery is a cosmic coincidence: It turns out that in the days just before Voyager 2's flyby, the planet had been affected by an unusual kind of space weather. This space weather event squashed the planet's magnetic field, dramatically compressing Uranus' magnetosphere.
Jamie Jasinski of NASA's Jet Propulsion Laboratory in Southern California is the lead author of the new work published in Nature Astronomy on November 11, 2024. Jasinski said:
If Voyager 2 had arrived just a few days earlier, it would have observed a completely different magnetosphere at Uranus. The spacecraft saw Uranus in conditions that only occur about 4% of the time.
Magnetospheres serve as protective bubbles around planets (including Earth) with magnetic cores and magnetic fields. They shield worlds from jets of ionized gas -- or plasma -- that stream out from the sun in the solar wind. Learning more about how magnetospheres work is important for understanding our own planet. And it's also important to understand worlds in seldom-visited corners of our solar system and beyond.
That's why scientists were eager to study Uranus' magnetosphere. And what they saw in the Voyager 2 data in 1986 flummoxed them. Inside the planet's magnetosphere were electron radiation belts with an intensity second only to Jupiter's notoriously brutal radiation belts. But there was apparently no source of energized particles to feed those active belts. In fact, the rest of Uranus' magnetosphere was almost devoid of plasma.
The missing plasma also puzzled scientists because they knew that the five major Uranian moons in the magnetic bubble should have produced water ions, as icy moons around other outer planets do. They concluded that the moons must be inert with no ongoing activity.
So why didn't we observe plasma? And what was happening to beef up the radiation belts? The new data analysis points to the solar wind. When plasma from the sun pounded and compressed the magnetosphere, it likely drove plasma out of the system. The solar wind event also would have briefly intensified the dynamics of the magnetosphere, which would have fed the belts by injecting electrons into them.
The findings could be good news for those five major moons of Uranus: Some of them might be geologically active after all. With an explanation for the temporarily missing plasma, researchers say it's plausible that the moons actually may have been spewing ions into the surrounding bubble all along.
Planetary scientists are focusing on bolstering their knowledge about the mysterious Uranus system, which the National Academies' 2023 Planetary Science and Astrobiology Decadal Survey prioritized as a target for a future NASA mission.
JPL's Linda Spilker was among the Voyager 2 mission scientists glued to the images and other data that flowed in during the Uranus flyby in 1986. She remembers the anticipation and excitement of the event, which changed how scientists thought about the Uranian system.
Spilker, who has returned to the iconic mission to lead its science team as project scientist, said:
The flyby was packed with surprises, and we were searching for an explanation of its unusual behavior. The magnetosphere Voyager 2 measured was only a snapshot in time. This new work explains some of the apparent contradictions, and it will change our view of Uranus once again.
Voyager 2, now in interstellar space, is almost 13 billion miles (21 billion km) from Earth.
Bottom line: A new analysis of data shows that when the Voyager 2 spacecraft flew past Uranus in 1986, it saw a skewed view of the planet's magnetosphere because a large solar wind event had just buffeted Uranus.
Source: The anomalous state of Uranus's magnetosphere during the Voyager 2 flyby
Via NASA/JPL-Caltech