Uranus Ejected a Giant Plasma Bubble During Voyager 2’s Visit

Uranus is unquestionably weird. Swirling with mostly water, methane and ammonia, the solar system’s seventh planet is tipped over at 98 degrees, so its magnetic poles take turns directly facing the sun. And its magnetic field is strangely misaligned with the planet’s rotation, causing it to wildly lurch about.

Back in 1986, the ice giant world got what remains its only visitor from Earth — Voyager 2, which is now more than 11 billion miles from Earth, but at that time flew a mere 50,600 miles above Uranus’s cloudy skies. As it passed, Voyager 2 heard an odd magnetic whisper, a signal so ephemeral that it went unnoticed.

More than three decades later, scientists were taking a deep dive into the venerable spacecraft’s data pool, hoping to find scientific mysteries that could help support a return mission to Uranus and its ice giant sibling, Neptune. They unearthed that magnetic hiccup, and realized it represented the detection of a mass of electrically excited gas with a width 10 times Earth’s circumference.

This ginormous bubble was a jettisoned part of Uranus’s atmosphere. Although only one was spotted, other gassy missiles may also be launched every 17 hours, the time it takes Uranus to complete one rotation.

This process is draining the planet’s atmosphere, but scientists aren’t anticipating a vanishing act.

“Even with moderate gassiness, it’s likely that Uranus will be able to hold on to most of its atmosphere for the remainder of the solar system’s life,” said Paul Byrne, a planetary geologist at North Carolina State University who was not involved in the research. “Uranus just has that much gas.”

Gina DiBraccio and Dan Gershman, space physicists at NASA’s Goddard Space Flight Center, were enthralled by Uranus’s pandemonious magnetic field, so they plumbed Voyager 2’s data to see if they could find any hidden curiosities. While reviewing data from the robotic probe’s 45-hour-long close encounter with Uranus in January 1986, they spied a 60-second jolt in its magnetic recording.

As highlighted in a recent NASA blog post, based on a paper published last year, this anomaly was the signal emitted by a 250,000-mile thick cylindrical mass of electrified hydrogen gas from Uranus. The ice giant was losing its atmosphere, a fate it shares with many worlds.

Mars’s ancient, substantial atmosphere kept its abundant surface water liquid. But roughly four billion years ago, its outer core stopped swirling, its global magnetic field collapsed and the solar wind stripped away much of its atmosphere, transforming it into an arid realm. Earth’s magnetic field largely shields its atmosphere from this destructive behavior, but 100 tons of our planet’s gases still disappear into space every single day.

Uranus’s global magnetic field may help it avoid a Mars-like fate. But as Voyager 2 discovered, that protection doesn’t stop it from discharging gassy cannonballs into space.

That colossal, vaporous blob was something called a plasmoid. Plasmoids have been found around several planets, and like snowflakes, no two are alike. This one’s neat, closed magnetic loops suggest it was flung off Uranus by the planet’s speedy rotation.

Plasmoids are known to roll off the elongated magnetic tail that stretches behind a planet’s dark side. If Voyager 2 had spent more time in Uranus’s magnetic tail, said Dr. Gershman, many more plasmoids could have been seen. But we’ll have to go back to spot more.

“This discovery provides yet another reason for blasting off a dedicated mission to Uranus,” Dr. Byrne said.

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