The rings around Uranus have mystified astronomers for almost half a century. Now, thanks to a combination of ground- and space-based telescopic observations, scientists think they’ve worked out where two of these rings came from.
A blue-tinged ring, named Mu, seems to be made of icy shards knocked off a nearby moon by micrometeorite impacts, researchers report in the April Journal of Geophysical Research: Planets. Conversely, Nu, a reddish ring, is composed of rocky particles probably sourced in a similar manner from an unseen rocky moon or moons.
Ring systems — like those around Jupiter, Saturn and Neptune — can form in myriad ways, including through moon-on-moon collisions or from a moon being ripped apart by the planet’s gravity. But for Uranus, it seems that “impacts have played a huge role, and still play a role,” says Imke de Pater, an astronomer at the University of California, Berkeley.
At 19 times the distance from the sun than Earth, Uranus is tremendously difficult to spy on. To date, just one spacecraft has flown past it: Voyager 2, in 1986. Not much is known about the planet with any certainty, but astronomers do know it is frigid, gassy, orbited by at least 28 moons and surrounded by 13 faint rings, some of which were first spotted in 1977.
The outermost rings, Mu and Nu — transliterations of the Greek letters μ and ν — are an oddity. Despite being adjacent, they look quite different. “It’s typical of Uranus to spice things up,” says James O’Donoghue, an astronomer at the University of Reading in England, who wasn’t involved with the study.
Prior observations with the W.M. Keck Observatory in Hawaii and the Hubble Space Telescope suggested the Mu ring was blue, signifying it was made of very small particles, while Nu was red-hued, suggesting it was dusty — both inferences based on how sunlight scatters off them.
For the new study, de Pater and her colleagues used a combination of Keck, Hubble and James Webb Space Telescope observations of Mu and Nu to try and unravel the reason for their disparate appearances. JWST, which observed Uranus periodically from 2023 to 2025, could perceive the rings with a remarkably high resolution. And by looking at the rings in infrared, the team was able to ascertain the sizes, distributions and compositions of the ring particles.
The blue Mu ring has the spectral signature of water ice. The most likely source of Mu, the researchers suggest, is a moon named Mab, a gelid orb no more than 12 kilometers wide embedded in the ring itself.
Saturn also has a blue ring: the E ring, constantly replenished by geysers shooting out of the icy shell of the moon Enceladus. Mab, though, is far too small to have a source of internal heat driving similar geysers. Instead, “micrometeorite impacts on Mab might produce this ring,” de Pater says. Ice chips flying off the moon could end up in orbit around Uranus.
The diminutive Mab could itself be a creation of an impact event. “Maybe it’s a chunk of Miranda,” de Pater says, referring to a large icy moon orbiting farther from Uranus than Mab.
The red-hued Nu ring is made of rocky confetti and organic matter. Dust escapes quickly into space, so Nu requires constant replenishment. The researchers posit that such an endless stream of dust could come from regular meteorite impacts on one or several as-yet-undiscovered rocky moons nearby. These moons might be incredibly difficult to spot from Earth, but at least astronomers know where to search for them.
“It would be very cool to discover a moon from the dust ring,” O’Donoghue says, as no moon has been found that way before.
The planet’s outermost circlets aren’t entirely scrutable just yet. Mu, for example, seems to brighten and dim over time, and it’s not at all clear why. Even with JWST’s sharp eyes, Uranus will probably keep running rings around astronomers for the foreseeable future.
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