When NASA's Voyager 2 spacecraft flew past Uranus in January of 1986, it gave us our first up-close look at the distant ice giant. Now, over three decades later, the data collected by the probe's sensors has revealed that it flew straight through an immense magnetic bubble, known as a plasmoid, that was ejected from the planet's atmosphere.
Uranus is certainly the oddball of all the worlds orbiting our Sun.
Tilted over on its side by some cataclysmic event in the early years of the solar system, this ice giant has a bizarre 'wobbly' magnetic field that scientists are still trying to make sense of, even now.
"The structure, the way that it moves... Uranus is really on its own," Gina DiBraccio, a space physicist at NASA's Goddard Space Flight Center and project scientist for the MAVEN mission, said in a press release.
Roughly 34 years ago, Voyager 2 discovered rings around Uranus, found never-before-seen moons circling it, and it took readings of the planet's extremely frigid atmosphere.
Voyager 2 flies past Uranus on January 24, 1986. Credit: NASA/JPL-Caltech
According to a new study, data from Voyager 2's flyby also picked up one other thing, which was missed at the time.
DiBraccio and fellow NASA Goddard scientist Dan Gershman were in the midst of planning possible missions for NASA to revisit the Ice Giant planets, Uranus and Neptune, but to stay this time. One aspect of this process is to look for compelling mysteries that a planetary spacecraft could investigate and potentially solve. Poring through the data collected by Voyager 2's Magnetometer instrument, over three decades ago, they found something interesting.
As the spacecraft swung by, at around 80,000 km above Uranus' cloudtops, it picked up a weird magnetic blip that lasted for just one minute of the total 45-hour flyby.
Magnetometer data from Voyager 2's Uranus flyby, showing the data averaged over 8-minute periods (red), and the same data plotted every 1.92 seconds (black), revealing the zigzag signature of a 'plasmoid'. Credits: NASA/Dan Gershman
After examining this blip closely, DiBraccio and Gershman came to the conclusion that it was a 'plasmoid' - an immense magnetic bubble being expelled from Uranus' magnetic field.
A plasmoid often forms as a planet's magnetic field is shaped by the solar wind, and influenced by solar storms. While the magnetic field lines near to the planet form a fairly spherical 'shield', the flow of the solar wind causes the outer field lines to stretch back into a 'magnetotail' on the night side the planet. If a solar storm or a fast stream of the solar wind pushes the field lines in the magnetotail closer together, they can 'reconnect', closing off a magnetic bubble that then gets caught up in the solar wind's flow.
Magnetic reconnection in Earth's magnetotail forms a plasmoid bubble. Credit: NASA's Scientific Visualization Studio/Scott Sutherland
Based on what Voyager 2's Magnetometer picked up, the spacecraft apparently passed straight through one of these plasmoids as it was passing Uranus. At roughly 400,000 kilometres wide, this magnetic bubble was found to be filled with mostly ionized hydrogen. There was a bonus, too. The magnetic field inside the bubble was found to form smooth, closed loops. This told DiBraccio and Gershman that the ionized hydrogen was very likely a blob of Uranus' atmosphere, being carried away by the plasmoid.
While plasmoids are known to form in Earth's magnetotail, and they've been observed at Mercury, Jupiter and Saturn, this is the very first evidence that this process happens at Uranus too.
Currently, there is great interest in the space exploration community to send new missions out to the Ice Giants, as we have with the Gas Giants, Jupiter and Saturn. After Voyager 1 & 2's flybys of the outer planets, NASA sent Galileo, and then Juno, to orbit Jupiter. Cassini explored Saturn, and its rings and moons, for over 14 years.
Sending spacecraft to Uranus and Neptune would give us new up-close looks at these distant worlds. It would provide new data that could help scientists solve existing mysteries that were discovered, either by telescopes or the Voyager probes. It would undoubtedly give us new mysteries to study, about the Ice Giants themselves and about our solar system in general.