When Voyager 2 made its historic flyby of Uranus in 1986, the spacecraft captured the best data humanity had gathered on the giant planet up to that point. But as scientists become better at analyzing cosmic signals, they’re also uncovering past mistakes by revisiting Voyager 2’s data, as yet another study points out.
This time, astronomers with the Southwest Research Institute offer an answer to a 39-year-old mystery surrounding Uranus’s radiation levels, reporting their research in Geophysical Research Letters.
Specifically, the unusual, “off the charts” energy levels of Uranus’s electron radiation belts—donut-shaped regions of energetic electrons surrounding a planet—were more likely the product of a solar wind storm. They were probably not caused by the planet’s natural radiation levels. It just so happened that Voyager 2 visited Uranus on a particularly unusual day, and that skewed our perception of the ice giant.
Indeed, scientists had mistakenly assumed such conditions were typical for the planet—even as the scientists themselves struggled to explain how that could be possible. But as the new analysis suggests, Uranus appeared to be having a bad weather day.
“Uranus provides a highly unique and dynamic magnetosphere, [but] our understanding is largely limited to a single flyby of the planet by Voyager 2,” study lead author Robert Allen told Gizmodo in an email.
Uranus, of the solar system
Earth is currently experiencing a steady wave of powerful solar storms due to the Sun being in its active stage, and our planet’s magnetic field has been feeling the impact. Intense plasma bursts and buffeting solar winds can severely warp the stability of a planet’s radiation levels and magnetic fields.
We’re naturally more familiar with how solar activity affects our own planet. But obviously, Earth isn’t the only thing orbiting the Sun—and all planets, asteroids, interstellar objects, and the like are subject to solar whimsy.
Granted, the only direct data we have on Uranus’s radiation environment comes from Voyager 2, as the researchers admit. Still, if there are outstanding questions about old datasets, revisiting them should be worthwhile, they said.
“Science has come a long way since the Voyager 2 flyby,” Allen noted in a statement.
Uranus’s bad day?
The new analysis partly takes inspiration from another recent study, which argued that Voyager 2 passed by Uranus when the planet’s magnetosphere was warped by solar energetic particles. Based on that reasoning, the researchers wondered whether accounting for extreme solar weather events could make the Voyager 2 data more consistent with the radiation structures found around other planets in the solar system.
The study thus takes a comparative approach between Voyager 2 data and Earth observations “using our modern understanding,” Allen told Gizmodo, enabling scientists to “further understand Uranus, as well as very fundamental aspects governing the physics of plasmas.”
For the analysis, Allen and colleagues gathered data from a 2019 event on Earth, during which our planet’s radiation belts experienced an energy spike from large solar wind storms.
A comparison showing space weather impacts of a fast solar wind structure driving intense solar storms at Earth in 2019 (second panel) and Uranus when Voyager 2 passed by the planet in 1986 (third panel). Credit: SwRI
The similarities were uncanny, the paper reported.
“If a similar mechanism interacted with the Uranian system, it would explain why Voyager 2 saw all this unexpected additional energy,” study co-author Sarah Vines explained in the release.
That said, the analysis perhaps raises more questions than it answers, the researchers said. If, as similar “debunks” demonstrate, there truly were so many misinterpretations of Voyager 2 data, how misguided are we in our understanding of Uranus? What do we misunderstand, and what have we yet to find?
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“Uranus is an incredible planetary environment unlike anything else in our solar system,” Allen said. “While these recent re-analyses have built upon our previous understanding of the Uranian system, our and other recent studies do highlight the need for new, direct observations in the future to dive deeper.”