THE rings around Uranus have mystified astronomers for nearly half a century. Now, through a combination of telescopic observations on the ground and in space, scientists believe they have determined the origin of two of these rings.
A blue-tinted ring, named Mu, seems be made up of shards of ice torn from a nearby moon by micrometeorite impactsthe researchers report in the April report Geophysical Research Journal: Planets. Conversely, Nu, a reddish ring, is composed of rock particles likely similarly derived from one or more unseen rock moons.
Ring systems – like those around Jupiter, Saturn and Neptune – can form in multiple ways, including from moon-moon collisions or from a moon being torn apart by the planet’s gravity. But for Uranus, it appears that “impacts played a huge role, and still play a role,” says Imke de Pater, an astronomer at the University of California, Berkeley.
Located 19 times the distance of the Sun from Earth, Uranus is extremely difficult to spy on. To date, only one spaceship flew over it: Voyager 2, in 1986. Not much is known for certain about the planet, but astronomers do know that it is frigid, gaseous, orbited by at least 28 moons, and surrounded by 13 faintly visible 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. Although they are adjacent, they look very different. “It’s typical of Uranus to spice things up,” says James O’Donoghue, an astronomer at the University of Reading in England, who was not involved in the study.
Faint glow from the ring
Uranus’ outermost rings, Nu and Mu, sparkle in this infrared image from the James Webb Space Telescope. The dark ring is a digital mask used to dim the glare of Uranus (bright disk, middle) and its main ring system bright by a factor of 100 so that the much fainter outer rings can be seen. The thin, bright circle just inside the ring is the epsilon ring, the outermost of Uranus’ main rings.
Previous observations with the WM Keck Observatory in Hawaii and the Hubble Space Telescope suggested that the Mu ring was blue, meaning it was made up of very small particles, while Nu was red in color, suggesting it was dusty – both inferences being based on the way sunlight scatters.
For the new study, de Pater and colleagues used a combination of observations of Mu and Nu by the Keck, Hubble and James Webb space telescopes to try to understand the reason for their disparate appearances. JWST, which observed Uranus periodically from 2023 to 2025, was able to perceive the rings with remarkably high resolution. And by observing the rings in infrared, the team was able to determine the size, distribution and composition of the ring particles.
The blue Mu ring has the spectral signature of water ice. Mu’s most likely source, 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 erupting from the icy shell of the moon Enceladus. Mab, however, is far too small to have an internal heat source powering similar geysers. Instead, “micrometeorite impacts on Mab could produce this ring,” says de Pater. Shards of ice flying off the Moon could end up in orbit around Uranus.
The diminutive Mab could itself be the creation of an impact event. “Maybe it’s a Miranda piece,” says de Pater, referring to a large icy moon orbiting farther from Uranus than Mab.
The red-hued Nu ring is made of rock confetti and organic matter. Dust escapes quickly into space, so Nu requires constant replenishment. The researchers postulate that such an incessant flow of dust could come from regular meteorite impacts on one or more as-yet-unknown rocky moons nearby. These moons can be incredibly difficult to spot from Earth, but at least astronomers know where to look for them.
“It would be really cool to discover a moon from the dust ring,” O’Donoghue says, because no moon has been found this way before.
The outermost circles of the planet are not yet fully observable. Mu, for example, seems to lighten and fade over time, and it’s not at all clear why. Even with JWST’s keen eyes, Uranus will likely continue to spin its rings around astronomers for the foreseeable future.