The rings of Uranus are even stranger than we thought

The rings that encircle Uranus may look similar in images, but their compositions are surprisingly different. Understanding the rings could help us unravel the many mysteries of this strange, dynamic system.

The outer rings of Uranus – the mu and nu rings – are so faint, and the whole system so distant, that it is difficult to observe them in any detail. Imke de Pater at the University of California, Berkeley and her colleagues combined nearly two decades of data from the Keck Telescope in Hawai’i, the Hubble space telescope, and the James Webb Space Telescope to get a better look.

They found that the mu ring, which is the outermost one, looked very blue, and the light shining off it indicated that it is made of tiny grains of ice. The nu ring, on the other hand, appears red, and it is rich in dust and relatively complex organic molecules called tholins.

The particles that make up the mu ring probably come from a small Uranian moon called Mab, so this indicates that Mab is icy, not rocky like the other moons that orbit near it. But it is unclear exactly how those tiny bits of ice got removed from Mab so that they could become a ring.

In some ways, this situation is similar to Saturn’s E ring, which is supplied by the frozen moon Enceladus – but Enceladus spews water from its buried ocean in huge plumes, and that’s probably not happening on Mab, which is only about 12 kilometres across.

“We don’t think that plumes would be possible on such a tiny moon like Mab, but still the parallels are exciting,” says Tracy Becker at the Southwest Research Institute in Texas, who was not involved with this work. Instead, it’s more likely that small rocks called micrometeoroids are hitting Mab’s surface and sending specks of ice flying.

The nu ring being dusty is less of a surprise, but the rocky objects that must be supplying all that dust have not been discovered, which indicates that they must be relatively small. The researchers also found that the nu ring changes in brightness over time – its shine halved between 2003 and 2006 – which may indicate that there was a major collision in the rings before 2003 that caused it to temporarily brighten.

The remaining question is not necessarily why the two rings are so different, but rather why the bodies that supply them with material are so different despite orbiting in similar areas around Uranus. “All the rocky bodies might have come from a moon that just fell apart or was slammed into pieces, but Mab is different,” says de Pater. “That really gets back to the origin of the whole system and what’s happened in the past.” Mab may be a shard of one of Uranus’s larger, more distant moons, which are icy, but then we don’t know what broke it off or why it migrated towards the planet.

“This gives us two or three more really important puzzle pieces to start putting the Uranus system into perspective, and it is suggesting that there are actually quite a few more puzzle pieces than expected,” says Becker. “Maybe the puzzle’s a little bit bigger and harder than we thought, and we need a lot more pieces before we can fully understand that system.”