A new look at Voyager 2 data explains one of Uranus’s long-standing mysteries
The spacecraft saw Uranus’s magnetic field at a weird time, so our picture of the planet and its moons actually represents an edge case rather than the norm.
Michigan Engineering News
Experts
Xianzhe Jia
Scientists have considered Uranus an oddball in our solar system, but a new analysis suggests that view may be biased by unusual circumstances.
When Voyager 2 flew by the distant planet in 1986, scientists noticed that it seemed to trap energized particles of plasma, the electrically conductive gas covering our solar system, unlike any other planet orbiting our sun. The planet’s magnetic field had an intense layer of radiation that was second only to Jupiter’s notoriously brutal radiation belts.
Unlike other planets that generate their own magnetic fields, the radiation came almost entirely from very energetic electrons. The rest of Uranus’s magnetic field was mostly devoid of other kinds of charged particles, which planets ordinarily collect from the solar wind and moons orbiting inside their magnetic fields.
“This is a mystery that has baffled the scientific community ever since the Voyager 2 flyby,” said Xianzhe Jia, a professor of climate and space sciences and engineering at U-M and a co-author of the study published in Nature Astronomy.
The new analysis of the Voyager 2 data revealed that the odd magnetic field was caused by a rare event. Just before the spacecraft reached Uranus, the planet’s magnetic field had likely been squashed by a jet of unusually dense solar wind, as evidenced by a steady increase in solar wind pressure leading up to Voyager 2’s encounter with Uranus. The pounding likely compressed the electrons inside Uranus’s magnetic field into the radiation belts while other charged particles were squeezed out, like juice from a lemon.
“If Voyager 2 had arrived just a few days earlier, it would have observed a completely different magnetosphere at Uranus,” said Jamie Jasinski of NASA’s Jet Propulsion Laboratory in southern California and the corresponding author of the study. “The spacecraft saw Uranus in conditions that only occur about 4% of the time.”
An accurate picture of Uranus’s magnetic environment is important for scientists to understand whether its moons could be ocean worlds. Scientists discovered the ocean on Jupiter’s moon Europa by studying the moon’s magnetic field, which was induced by variations in Jupiter’s magnetic field surrounding Europa. Although two of Uranus’s moons have layers of ice similar to Europa, scientists couldn’t find any evidence of an ocean in Uranus’s weak magnetic field. If Voyager 2 had arrived before Uranus’s magnetic field was squashed, the story might have been different.
“Although we thought that the two outer moons of Uranus, Titania and Oberon, regularly move in and out of Uranus’s magnetic field, this was just because the magnetic field was squashed. Our new analysis shows that the moons actually spend the majority of their time within the planet’s magnetic field,” said Jia.
“The moons’ magnetic environment is likely influenced by variations in Uranus’s magnetic field, similar to the conditions that lead to the discovery of Europa’s ocean. Our new analysis suggests that we could look for subsurface oceans on Uranus’s moons too,” Jia added.
Read NASA JPL’s press release for more information.