Mysteries about Uranus that have baffled scientists for decades may have been the result of an unusually powerful solar storm that happened to occur as a spacecraft visited the planet, a new study involving UCL researchers has found.
NASA's Voyager 2, which flew by Uranus in 1986, provided scientists' first, and so far only, close glimpse of the planet, shaping their understanding of it in the decades since.
The mission found oddities, though. The planet's radiation belts - areas of charged particles trapped on magnetic field lines - were incredibly intense, second only to Jupiter's. Yet the rest of Uranus's magnetosphere (magnetic bubble) was nearly empty of plasma (ionised gas), meaning no apparent source of charged particles to feed those belts.
The new study, published in Nature Astronomy, found that a "hurricane" of extreme solar weather at the time of the flyby likely squashed the planet's magnetic bubble, pushing plasma out of it, and intensified radiation belts by feeding electrons into them.
Because of the nearly empty magnetosphere, Uranus's five moons were assumed to be inert dead worlds, with no ongoing activity.
The new findings suggest they could be geologically active after all and, combined with other recent discoveries, mean that they may have oceans. The moons could have been spewing ions into the surrounding bubble all along, with these ions temporarily blasted away by solar storms during Voyager 2's flyby.
Co-author Dr William Dunn, of UCL's Department of Physics and Astronomy, said: "Almost everything we know about Uranus is based on Voyager 2's two-day flyby. This new study shows that a lot of the planet's bizarre behaviour can be explained by the scale of the space weather event that occurred during that visit.
"We now know even less than we thought about what a typical day in the Uranian system might look like and are even more in need of a second spacecraft to visit to truly understand this mysterious, icy world.
"A big piece of evidence against there being oceans on Uranus's moons was the lack of detection of any water-related particles around the planet - Voyager 2 didn't find water ions. But now we can explain that: the solar storm basically would have blown all that material away."
Lead author Dr Jamie Jasinski of NASA's Jet Propulsion Laboratory (JPL), who was previously based at the Mullard Space Science Laboratory at UCL, 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."
A possible NASA space mission to Uranus is currently being developed following the US National Academies' 2023 Planetary Science and Astrobiology Decadal Survey prioritised the planetary system as a target for a future mission.
Dr Dunn said: "The design of the upcoming NASA flagship mission to Uranus should be carefully considered in the context of these findings. For instance, we might want instruments that could detect nudges to the magnetic field from a moon's salty ocean and instruments that could measure all the particles in the system to test whether we find water or other important material from the moons.
"Based on our discoveries of planets outside our solar system, Uranus-type planets are the most common in the Universe, giving us even more cause to try to better understand the Uranian system."
Dr Linda Spilker, based at JPL, was among the Voyager 2 mission scientists. She 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 about 13 billion miles (21 billion kilometers) from Earth.
