Study reveals icy origins of mysterious Uranus ring

MOSCOW, Idaho — Scientists have confirmed that one of Uranus’s faint outer rings is made primarily of water ice — a surprising discovery that is reshaping what researchers know about how some of the planet’s rings formed.

Matthew Hedman, a professor of physics at University of Idaho, contributed to the study by analyzing telescope data to help determine the rings’ composition.

Using observations from the W. M. Keck Observatory, the Hubble Space Telescope and the James Webb Space Telescope, an international team constructed the first complete spectrum of two of Uranus’s outer rings, known as the μ (mu) and ν (nu) rings. The results show the rings have very different compositions and likely formed in different ways.

“We’re finally able to see what these rings are made of,” Hedman said. “That gives us a clearer picture of how they formed and how the Uranus system has evolved over time.”

The ν ring appears to be composed of dark, rocky material similar to other rings and small moons near Uranus. Researchers believe it likely formed from debris created by collisions between larger objects.

The μ ring, however, stands out. It is made up of extremely small particles of water ice and appears blue in visible light — a rare trait among planetary rings.

Researchers believe the μ ring is fed by material from a small moon called Mab, which is only about 10 kilometers across and also appears to be composed primarily of ice. Icy grains knocked loose from Mab’s surface can escape the moon’s weak gravity and spread into orbit around Uranus.

“That material doesn’t fall back to the moon, but it can’t escape Uranus either,” Hedman said. “So, it ends up forming a ring.”

Scientists had long suspected the μ ring might contain ice, but earlier observations could not confirm it. The James Webb Space Telescope made the difference by detecting a clear signature of water ice — something that is difficult to measure from Earth because of interference from water in the atmosphere.

The findings also raise new questions. Mab and the μ ring appear to be much icier than nearby moons and rings, which are darker and more rock-rich. Researchers are still working to understand why.

“There’s something unusual happening in this part of the Uranus system that we’re still trying to understand,” Hedman said.

Some observations also suggest the ring’s brightness varies over a period of years, which could mean Mab periodically releases fresh material.

“To really understand what’s happening, we need to keep watching it,” Hedman said. “There are hints the ring may be changing over time. If that’s the case, it means we’re watching an active system, not something frozen in place.”

A research team, including Hedman, has been awarded time on the James Webb Space Telescope to observe Uranus annually over the next five years. The repeated observations will allow scientists to track changes in the ring and better understand how it forms and evolves.

The study was led by researchers at the University of California, Berkeley, and is published in the Journal of Geophysical Research: Planets.

Media contact

Matthew Hedman
Professor of Physics
208-885-6389
mhedman@uidaho.edu