Scientists announced on May 12 that one of Uranus’s faint outer rings is made mostly of water ice, changing what researchers know about the planet’s ring system. Matthew Hedman, a professor of physics at University of Idaho, contributed to the study by analyzing telescope data to help determine the rings’ composition.
An international team used observations from the W. M. Keck Observatory, the Hubble Space Telescope, and the James Webb Space Telescope to create a complete spectrum of two outer rings called μ (mu) and ν (nu). The findings show that these two 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 mainly of dark, rocky material similar to other nearby moons and rings. Scientists believe it likely formed from debris created by collisions between larger objects. In contrast, the μ ring stands out because it is made up of extremely small particles of water ice and appears blue in visible light—a rare trait among planetary rings.
Researchers think that Mab, a small moon about 10 kilometers across that also seems to be mostly ice, feeds material into the μ ring. Icy grains knocked loose from Mab’s surface can escape its weak gravity but remain trapped around Uranus as part of this unique ring.
“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.”
Previous studies suspected there was ice in this ring but could not confirm it until now because Earth-based observations are affected by atmospheric water vapor. The James Webb Space Telescope detected clear signs of water ice in space.
The results raise new questions since Mab and its associated μ ring appear much icier than nearby moons or other rings which are darker and more rock-rich. Researchers do not yet know why this difference exists.
“There’s something unusual happening in this part of the Uranus system that we’re still trying to understand,” Hedman said.
Some evidence suggests that brightness changes in the μ ring over years could mean Mab releases fresh material periodically into orbit around Uranus.
“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 James Webb Space Telescope for annual observations over five years so scientists can track changes in these features going forward.
