Two-Decade Study Reveals Different Origins and Compositions of Uranus’s Outermost Mu and Nu Rings

The mu and nu rings, the outermost two rings of Uranus, display markedly different compositions and behaviors, according to a study that combined nearly two decades of observations from the Keck Telescope in Hawai’i, the Hubble Space Telescope and the James Webb Space Telescope.

Imke de Pater at the University of California, Berkeley led the team whose analysis showed the mu ring, the outermost ring of Uranus, appears very blue. The light reflected from it indicated the mu ring is made of tiny grains of ice whose particles probably come from a small Uranian moon called Mab.

Mab is about 12 kilometres across and is icy, unlike the other moons that orbit near it. Exactly how the tiny bits of ice are removed from Mab to form the ring remains unclear. The situation bears some resemblance to Saturn’s E ring, which is supplied by the frozen moon Enceladus, yet plumes are considered unlikely on such a small body.

“We don’t think that plumes would be possible on such a tiny moon like Mab, but still the parallels are exciting,” Tracy Becker at the Southwest Research Institute in Texas, who was not involved with the work, said. In contrast, the nu ring appears red. It is rich in dust and relatively complex organic molecules called tholins.

The rocky objects supplying that dust have not been discovered, suggesting they are relatively small. The nu ring also changes in brightness over time. Its shine halved between 2003 and 2006, which may indicate that a major collision in the nu ring occurred before 2003, causing temporary brightening.

The remaining question is why the bodies supplying the two rings 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. That really gets back to the origin of the whole system and what’s happened in the past,” Imke de Pater said.

Mab may be a shard of one of Uranus’s larger, more distant moons, which are icy, but it is not known what broke it off or why it migrated inward. The study adds to the puzzle of the Uranian system. “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.

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,” Tracy Becker stated. 1029/2025JE009404.