Giant magnetic bubble ejected from Uranus

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NASA/JPL-Caltech

Scientists revisited old data captured by the NASA spacecraft Voyager 2 from 1986 and found “one more secret” about the mysterious seventh planet, icy-cold Uranus.

Three decades after the Voyager 2 reached within 50,600 miles of Uranus, reanalysis of data has uncovered that the spacecraft flew through a plasmoid - a giant magnetic bubble - that may have been pushing Uranus’s atmosphere out into space. 

The researchers say that this raises questions about the planet’s “one-of-a-kind” magnetic environment. 

Atmospheric leaks or “escapes” are governed by a planet’s magnetic field. 

Planetary atmospheres all over the universe are regularly leaking into space. Hydrogen leaks from Venus, while electrically-charged air is ejected from Saturn and Jupiter. 

A planet’s magnetic field acts as a gatekeeper, it can help to protect it from the solar wind released by the sun  - but it can also allow opportunities for atmospheres to escape if lines in the magnetic field tangle together. 

The impact of atmospheric leaks are tiny looking at them from the perspective of human timescales, but over millions or billions of years there can be huge consequences.

NASA says that even the Earth’s atmosphere leaks - but there’s no need to worry, it’s understood the atmosphere will remain for a least another billion years or so. 

To understand how and why a planet’s atmosphere changes, scientists pay close attention to magnetism. 

Uranus is mysteriously unique 

Unlike any other planet in the solar system, Uranus spins almost exactly on its side - t’s spin axis is tilted by a  huge 98 degrees. Its magnetic field axis points 60 degrees away from that spin axis, so as the planet spins, its magnetosphere, which is the space produced by its magnetic field, actually wobbles. Physicists still don't know how to model this motion. 

The unique nature of Uranus’s magnetic field and it’s distinct structure drew space physicists Gina DiBraccio and Dan Gershman to a project that involved building a plan for a new mission to the "ice giants" Uranus and Neptune.

The strange magnetic field surrounding Uranus that was last measured during the Voyager 2 flyby in 1986 - “seemed like a good place to start” according to the physicists. 

The team reexamined the Voyager 2 magnetometer readings, which monitored the strength and direction of the magnetic fields near Uranus as the spacecraft flew by. With no preconceptions of potential findings the scientists zoomed in closer than previous studies. They plotted a new data point every 1.92 second and believed to detect a plasmoid that escaped from the planet’s atmosphere.

With enough time, escaping plasmoids can drain the ions from a planet's atmosphere, fundamentally changing its composition.

The plasmoid observation filled only 60 seconds of Voyager 2's 45-hour-long flight by Uranus. “It appeared as a quick up-down blip in the magnetometer data. But if you plotted it in 3D, it would look like a cylinder," Gershman said.

By examining readings from inside the plasmoid, as Voyager 2 passed through it, this provided a hint to its origins.

The plasmoid, which resembled a cylinder, was 127,000 miles long and 250,000 miles across. 

Some plasmoids have a twisted internal magnetic field, but in the case of Uranus’s DiBraccio and Gershman observed smooth, closed magnetic loops. Loop-like plasmoids are often formed as a spinning planet flings bits of its atmosphere to space. "Centrifugal forces take over, and the plasmoid pinches off," Gershman said. 

According to their estimates, plasmoids like the one observed could be the cause of between 15% and 55% of atmospheric mass loss for Uranus. It could potentially be the main way that Uranus is losing its atmosphere to space.

It is difficult to determine how the plasmoid escape has impacted Uranus over time, seeing as there is only one set of observations from Voyager 2. Only future observations will provide more information about what's actually happening with the planet and its runaway atmosphere.

"Scientists are currently exploring future opportunities to visit Uranus and Neptune," DiBraccio said. 

"This includes mission concepts that would send spacecraft to orbit one of these planets and even send a probe into its atmosphere."

NASA estimates that a mission like this could launch around 2030.

Reference: 

DiBraccio, G.A., et al., ‘Voyager 2 constraints on plasmoid-based transport at Uranus’, Geophysical Research Letters (2019). DOI: 10.1029/2019GL083909

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