Some of our understanding of Uranus may be false, say physicists at NASA’s Jet Propulsion Laboratory who have revisited Voyager 2 data before and after its 1986 flyby of this ice-giant planet. The new analyses could shed more light on some of the mysterious and hitherto unexplainable measurements made by the spacecraft. For example, why did it register a strongly asymmetric, plasma-free magnetosphere – something that is unheard of for planets in our solar system – and belts of highly energetic electrons?
Voyager 2 reached Uranus, the seventh planet in our solar system, 38 years ago. The spacecraft gathered its data in just five days and the discoveries from this one and, so far, only flyby provide most of our understanding of this ice giant. Two major findings that delighted astronomers were its 10 new moons and two rings. Other observations perplexed researchers, however.
One of these, explains Jamie Jasinski, who led this new study, was the observation of the second most intense electron radiation belt after Jupiter’s. How such a belt could be maintained or even exist at Uranus lacked an explanation until now. “The other mystery was that the magnetosphere did not have any plasma,” he says. “Indeed, we have been calling the Uranian magnetosphere a ‘vacuum magnetosphere’ because of how empty it is.”
Unrepresentative conditions
These observations, however, may not be representative of the conditions that usually prevail at Uranus, Jasinski explains, because they were simply made during an anomalous period. Indeed, just before the flyby, unusual solar activity squashed the planet’s magnetosphere down to about 20% of its original volume. Such a situation exists only very rarely and was likely responsible for creating a plasma-free magnetosphere with the observed highly excited electron radiation belts.
Jasinski and colleagues came to their conclusions by analysing Voyager 2 data of the solar wind (a stream of charged particles emanating from the Sun) upstream of Uranus for the few days before the flyby started. They saw that the dynamic pressure of the solar wind increased by a factor of 20, meaning that it dramatically compressed the magnetosphere of Uranus. They then looked at eight months of solar wind data obtained by the spacecraft at Uranus’ orbit and found that the solar wind conditions present during the flyby only occur 4% of the time.
“The flyby therefore occurred during the maximum peak solar wind intensity in that entire eight-month period,” explains Jasinski.
The scientific picture we have of Uranus since the Voyager 2 flyby is that it has an extreme magnetospheric environment, he says. But maybe the flyby just happened to occur during some strange activity rather than it being like that generally.
The timing was just wrong
Jasinski previously worked on NASA’s MESSENGER mission to Mercury. Out of the thousands of orbits made by this spacecraft around the planet over a four-year period, there were occasional times where activity from the Sun completely eroded the entire magnetic field. “That really highlighted for me that if we had made an observation during one of those events, we would have a very different idea of Mercury.”
Following this line of thought, Jasinski asked himself whether we had simply observed Uranus during a similar anomalous time. “The Voyager 2 flyby lasted just five days, so we may have observed Uranus at just the ‘wrong time’,” he says.
One of the most important take-home messages from this study is that we can’t take the results from just one flyby as a being a good representation of the Uranus system, he tells Physics World. Future missions must therefore be designed so that a spacecraft remains in orbit for a few years, enabling variations to be observed over long time periods.
Why we need to go back to Uranus
One of the reasons that we need to go back to Uranus, Jasinski says, is to find out whether any of its moons have subsurface liquid oceans. To observe such oceans with a spacecraft, the moons need to be inside the magnetosphere. This is because the magnetosphere, as it rotates, provides a predictable, steadily varying magnetic field at the moon. This field can then induce a magnetic field response from the ocean that can be measured by the spacecraft. The conductivity of the ocean – and therefore the magnetic signal from the moon – will vary with the depth, thickness and salinity of the ocean.
If the moon is outside the magnetosphere, this steady and predictable external field does not exist and it can no longer drive the induction response. We cannot therefore, detect a magnetic field from the ocean if the moon is outside the magnetosphere.
Before these latest results, researchers thought that the outermost moons, Titania and Oberon, would spend a significant part of their orbit around the planet outside of the magnetosphere, Jasinski explains. This is because we thought that Uranus’s magnetosphere was generally small. However, in light of the new findings, this is probably not true and both moons will orbit inside the magnetosphere since it is much larger than previously thought.
Titania and Oberon are the most likely candidates for harbouring oceans, he adds, because they are slightly larger than the other moons. This means that they can retain heat better and therefore be warmer and less likely to be completely frozen.
Things we don’t know about Uranus (and Neptune)
“A future mission to Uranus is critical in collecting the scientific measurements to answer some of the most intriguing science questions in our solar system,” says Jasinski. “Only by going back to Uranus and orbiting the planet can we really gain an understanding of this curious planet.”
Happily, in 2022, the US National Academies outlined that a Uranus Orbiter and Probe mission should be a future NASA flagship mission that NASA should prioritize in the coming decade. Such a mission would help us unravel the nature of Uranus’s magnetosphere and its interaction with the planet’s atmosphere, moons and rings, and with the solar wind. “Of course, modern instrumentation would also revolutionize the type of discoveries we would make compared to previous missions,” says Jasinski.
The present study is detailed in Nature Astronomy.
