Scientists may have solved a long-standing mystery surrounding the ice giant Uranus and its faint radiation belts. It is possible that the weakness of the belts is related to the planet’s strangely tilted and lopsided magnetic field; the field can cause “jams” for particles to crash into the world.
The mystery dates back to Voyager 2’s visit to Uranus in January 1986, well before the probe left the Solar System in 2018. The spacecraft found that Uranus’ magnetic field is asymmetric, tilted roughly 60° from its spin axis. Voyager 2 also found that Uranus’ radiation belts, consisting of particles trapped by this magnetic field, are about 100 times weaker than predicted.
The new research, based on simulations made using data from Voyager 2, suggests that these two strange aspects of the ice giant are connected.
“It has a magnetic field unlike any other in the Solar System. Most planets that have strong internal magnetic fields, such as Earth, Jupiter and Saturn. They have a very ‘traditional’ magnetic field shape, which is known as a dipole,” lead author Matthew Acevski told Space.com. “This is the same magnetic field shape you would expect from your everyday bar magnet. With Uranus this is not the case; Uranus’ field is highly asymmetric – and is getting closer to the planets’ surface.’
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Acewski explained that this research highlights how Uranus’ magnetic asymmetry deforms the structure of the planet’s proton radiation belts, particularly near the region traversed by Voyager 2.
“My hypothesis was that the magnetic asymmetry distorts the proton radiation belts, forming regions around the planet where the radiation belts are more compressed,” Acewski said, “and therefore of stronger intensity; other regions where they are more common, resulting in lower intensity.
“If Voyager 2 flew through a region where radiation belts are more prevalent, this could explain its observations of weaker-than-expected proton radiation belts.”
Solar System Anomaly
The coldest planet in the solar system and the seventh planet from the sun, Uranus is odd among the other worlds of our planetary system. The ice giant rolls like a space ball tilted in one direction at an angle of 97 degrees from the plane of its orbit. This means that when it rotates, it does so somewhat “sideways”. It is the only planet in the solar system that does this.
The tilt, believed to be the result of a collision with an Earth-sized object in the distant past, causes Uranus to have the most extreme seasons in the Solar System, with a winter that lasts 21 years. Completing an orbit once every 84 Earth years, Uranus is also only one of two planets in the Solar System (the other being Venus) that orbits the sun in the opposite direction to all the other planets.
About four times wider than Earth and located about 19 times farther from the sun than our planet is, Uranus is surrounded by 13 faint rings and at least 28 moons. Uranus also has auroras, similar to the northern and southern lights of Earth, but because of the planet’s tilted magnetic field, they do not appear over its poles, as they do over our planet, Jupiter and even Saturn.
As with all planets that have magnetic fields, charged particles are trapped around Uranus, creating radiation belts – but why these radiation belts appear so faint has remained a puzzle for five decades.
The team’s simulation abandoned the idea that Uranus’ magnetic field acts as a dipole and used a more complex quadrupole magnetic field to reproduce its uneven nature.
This revealed that particles speed up and slow down as they pass through regions of different field strength. Changes in the speed of the particles cause them to clump together in some regions and become more dispersed in others. This effect only occurs when a complex quadrupole magnetic field is included in the simulation, which is why it has never been seen before.
“We found that the magnetic asymmetry of Uranus can lead to regions around the planet where protons move more slowly and are more compressed, and other regions where they move faster and are more spread out,” Acewski said. “It’s analogous to how traffic jams form on a ring road. When cars are moving slower, it causes denser traffic; if cars are moving faster, the traffic is more spread out.”
Acewski and his colleagues theorize that when Voyager 2 visited Uranus, it passed through a weak zone in the ice giant’s radiation belt.
“We projected Voyager 2’s trajectory onto this profile and found that the spacecraft actually flew through a region of ‘fast drift,’ which would mean it should have observed a lower-than-normal intensity of the proton radiation belt,” Acewski said. “It is important to note that our particle simulations show that this result accounts for a maximum variation of approximately 20% of the proton intensity around the planet.”
That means the team’s model can’t fully account for the 100 times lower intensity observed by Voyager 2.
“It is possible that whatever primary effect caused these much fainter proton radiation belts was compounded by this effect we found,” Acewski continued. “We were extremely surprised by the results. It’s amazing to see how much influence magnetic asymmetry can have on the structure of the radiation belt. This is something that was not known before.”
Acewski indicated that the results he and the team obtained could help inform future spacecraft missions to Uranus. So far, Voyager 2 is the only spacecraft to have visited the ice giant. This means that direct data about the world is extremely limited.
Plans are being developed at NASA to launch a mission to Uranus as soon as 2030. Such a mission could help experimentally verify the conclusion of this simulation.
“What we need to verify these simulations is a flagship spacecraft mission to Uranus to get new in-situ measurements of the planet over several years, not just a few hours as Voyager 2 did.” , Acevski said. “A new mission may also allow us to reveal new physics that we can’t even predict with simulations.”
“Because this is a planet with a magnetic field we’ve never seen before, it’s entirely possible that completely new phenomena will be discovered that would expand our understanding of planetary science.”
Acevski is certainly not done with this strange world of the solar system yet. The ice giant is a particular fascination for the explorer.
“Uranium presents a unique challenge for science, which I find great pleasure in tackling.” It’s truly amazing how much you can uncover with so little data, and we’re literally just scratching the surface,” Acevski concluded. “To date, not many people have studied the ice giant planets, Uranus and Neptune, despite the fact that they exhibit such strange characteristics, especially in their magnetic fields, and so drawing attention to the strange phenomena that can happen there, is a very exciting prospect for me.”
The team’s research was published in June in the journal Geophysical Research Letters.
