Eight and a half years after her big tour of the solar system, NASA’s Voyager 2 spaceship was ready for another meeting. It was January 24, 1986 and he would soon meet the mysterious seventh planet, freezing Uranus.
In the next few hours, Voyager 2 flew within 50,600 miles (81,433 kilometers) from the tops of the clouds of Uranus, collecting data that revealed two new rings, 11 new moons and temperatures below minus 353 degrees Fahrenheit (minus 214 degrees Celsius). The dataset is still the only close measure we’ve ever done on the planet.
Three decades later, scientists reproposed that the data found another secret.
Unbeknownst to the entire space physics community, 34 years ago Voyager 2 flew through a plasmoid, a giant magnetic bubble that could have brought Uranus’ atmosphere into space. The discovery, reported in Geophysical research letters, raises new questions about the unique magnetic environment.
A strange wobbly magnetic sphere
Planetary atmospheres throughout the solar system lose in space. Hydrogen flows from Venus to join the solar wind, the continuous flow of particles coming out of the Sun. Jupiter and Saturn expel the globes of their electrically charged air. Even the losses of the Earth’s atmosphere. (Don’t worry, it will remain for about another billion years.)
The effects are minuscule on human time scales, but if left long enough, atmospheric escape can substantially alter the fate of a planet. For an emblematic case, look at Mars.
“Mars was a wet planet with a dense atmosphere,” said Gina DiBraccio, a space physicist at NASA’s Goddard Space Flight Center and project scientist for Mars Atmosphere and Volatile Evolution, or MAVEN mission. “It has evolved over time” – 4 billion years of loss in space – “to become the arid planet we see today”.
Atmospheric escape is driven by a planet’s magnetic field, which can both help and hinder the process. Scientists believe that magnetic fields can protect a planet by repelling explosions that hit the atmosphere of the solar wind. But they can also create escape opportunities, such as the giant globs freed from Saturn and Jupiter when the magnetic field lines get tangled. However, to understand how atmospheres change, scientists pay close attention to magnetism.
This is another reason why Uranus is such a mystery. The 1986 Voyager 2 flyby revealed how magnetically strange the planet is.
“The structure, the way it moves …”, said DiBraccio, “Uranus is really alone.”
Unlike any other planet in our solar system, Uranus turns almost perfectly on its side – like a pig on a spit roast – completing a barrel roll once every 17 hours. Its magnetic field axis points 60 degrees away from the rotation axis, so as the planet turns, its magnetosphere – the space dug by its magnetic field – swings like a badly thrown soccer ball. Scientists still don’t know how to model it.
This oddity attracted DiBraccio and his coauthor Dan Gershman, a fellow space physicist from Goddard, to the project. Both of them were part of a team that worked out plans for a new mission for the “ice giants” Uranus and Neptune, and were looking for mysteries to solve. Uranus’ strange magnetic field, last measured more than 30 years ago, seemed like a good place to start.
They then downloaded the Voyager 2 magnetometer readings, which monitored the strength and direction of magnetic fields near Uranus as the spacecraft flew. Without having any idea what they would find, they moved closer to previous studies, drawing a new data point every 1.92 seconds. The soft lines have given way to jagged peaks and dips. And that’s when they saw him: a small zigzag with a great story.
“Do you think it could be … a plasmoid?” Gershman asked DiBraccio, seeing the squiggle.
Little known at the time of Voyager 2’s overflight, plasmoids are now recognized as an important way in which planets lose mass. These gigantic bubbles of plasma, or electrified gas, detach from the end of the magnetotail of a planet, the part of its magnetic field rejected by the Sun like a windsock. With enough time, fleeing plasmoids can drain ions from a planet’s atmosphere, radically changing its composition. They had been observed on Earth and other planets, but no one had yet detected plasmoids on Uranus – yet.
DiBraccio managed the data through its processing pipeline and the results came back clean. “I think it definitely is,” he said.
The bubble escapes
The plasmoid DiBraccio and Gershman found occupied only 60 seconds of Uranus’ 45-hour flight of Voyager 2. It seemed like a quick reversal in the magnetometer data. “But if I tracked it in 3-D, it would look like a cylinder,” said Gershman.
Comparing their results with the plasmoids observed at Jupiter, Saturn and Mercury, they estimated a cylindrical shape at least 127,000 miles long (204,000 kilometers) and up to about 250,000 miles (400,000 kilometers) wide. Like all planetary plasmoids, it was full of charged particles, primarily ionized hydrogen, the authors believe.
The readings from inside the plasmoid – as Voyager 2 crossed it – hinted at its origins. While some plasmoids have a twisted internal magnetic field, DiBraccio and Gershman have observed smooth and closed magnetic rings. Such loop-shaped plasmoids typically form when a spinning planet throws fragments of its atmosphere into space. “Centrifugal forces take over and plasmoid pinches,” said Gershman. According to their estimates, plasmoids like that could account for between 15 and 55% of the atmospheric mass loss in Uranus, a higher percentage than in Jupiter or Saturn. It could be the dominant way in which Uranus spreads its atmosphere in space.
How has the plasmoid escape changed Uranus over time? With a single series of observations, it is difficult to say.
“Imagine if a spacecraft had just flown through this room and tried to characterize the whole Earth,” said DiBraccio. “Obviously it won’t show you anything about what the Sahara or Antarctica looks like.”
But the results help to focus new questions on the planet. The remaining mystery is part of the draw. “That’s why I love planetary science,” said DiBraccio. “You always go to a place you don’t really know.”
How does Uranus sound?
Gina A. DiBraccio et al. Voyager 2 constraints on plasmoid-based transport in Uranus, Geophysical research letters (2019). DOI: 10.1029 / 2019GL083909
Revisiting decades-old Voyager 2 data, scientists find another secret about Uranus (2020, March 26)
recovered on March 26, 2020
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