Uranus’s two outer rings show starkly different origins

Astronomers using the W. M. Keck Observatory on Maunakea, Hawaiʻi Island are revealing new insight into the composition and origins of Uranus’s two outer rings. Using data from the Keck Observatory Archive (KOA), combined with observations taken by the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST), researchers constructed the first complete reflectance spectrum (sunlight reflected off the rings) of the μ and ν rings, confirming their colors and uncovering their detailed composition.

These rings are peculiar because they are extremely faint and orbit within the planet’s crowded system of 14 inner moons.

“By decoding the light from these rings, we can trace both their particle size distribution and composition, which sheds light on their origins, offering new insight into how the Uranian system and planets like it formed and evolved,” said Imke de Pater, professor at the University of California, Berkeley, and lead author of the study.

The findings point to two very different origin stories. The study, led by the University of California, Berkeley, is published in the Journal of Geophysical Research: Planets.

Two very different formation stories

Though they orbit the same planet, Uranus’s μ and ν rings are fundamentally different. Prior observations with the combined Keck Observatory and HST showed that the μ ring appeared blue, a signature of extremely small particles, while the ν ring’s reddish hue points to a more typical dusty ring. Why the rings were so different remained a mystery, though.

When JWST came on-line and observed Uranus, the research team used all its data, taken at different infrared wavelengths, in combination with Keck Observatory and HST observations to construct a complete spectrum from the visible through to infrared. By analyzing how sunlight reflects off the rings, the team identified a strong absorption feature near a wavelength of 3 microns (3 millionths of a meter) visible in the infrared for both rings.

Beyond that shared feature, the differences become clear when simulating the detailed spectra: the μ ring closely matches the spectral signature of water ice, while the ν ring is clearly composed of rocky material, mixed with approximately 10%–15% carbon-rich organic compounds commonly found in the outer solar system.

The μ ring seems to be made up of tiny icy grains knocked off the planet’s small (12-km sized) moon, Mab, by micrometeorite impacts. Interestingly, the icy composition of the μ ring also confirms that the moon Mab is composed mostly of water-ice.

“In contrast, the ν ring material is sourced from micrometeorite impacts on and collisions between unseen rocky bodies rich in organic materials, which must orbit between some of the known moons,” said de Pater. “One interesting question is why the parent bodies sourcing these rings are so different in composition.”

A ring system revealed over time

Uranus’s rings were first discovered in 1977, when astronomers observed a star dim multiple times as the planet passed in front of it, indicating a surrounding ring system. At the time, only Saturn was known to have rings, making Uranus the second known ringed planet in our solar system.

Unlike Saturn’s bright, easily visible rings, Uranus’s rings are faint and narrow, making them far more difficult to study. Over the decades, additional rings were identified through NASA’s Voyager 2 spacecraft and HST observations, gradually revealing a more complex system.

Keck Observatory, together with HST, played a key role in our understanding of the Uranian system. HST observations in 2003–2005 led to the discovery of the µ and ν rings and Keck Observatory helped characterize them, including providing the first evidence that the μ ring is blue, whereas the ν ring appears red. These color differences hinted at fundamental variations in particle size and composition, but the available data were limited.

The only other ring in our solar system that is blue is Saturn’s E ring. This ring is produced through geyser activity on the moon Enceladus, which orbits Saturn in the E ring, and geysers spew out tiny icy grains. Mab, however, seems too small to be volcanically active.

A major breakthrough came in 2007, when Earth passed through Uranus’s ring plane and faint dusty rings became much brighter, allowing de Pater and her team to view the rings edge-on. Using the NIRC2 instrument on the Keck II telescope, the team captured rare near-infrared observations of the μ ring, providing some of the first detailed views of this faint structure at these wavelengths.

To build on the Keck Observatory and HST observations, the team combined them with measurements from JWST. The combination of data allowed scientists to construct a complete spectrum of the μ and ν rings for the first time, revealing their particle sizes and composition.

What comes next

The new results about the µ and ν rings raise an interesting question: Why is Mab, the source of the µ ring, so different from Uranus’s other, rockier inner moons?

“I suspect we will need closeup images from a future spacecraft mission to Uranus in order to answer that question,” said Mark Showalter, co-author and senior research scientist at the SETI Institute.

Meanwhile, continued monitoring of the system from Keck Observatory, HST, and JWST, can play a critical role.

“We see hints that the µ ring’s brightness changes over time, and what could be causing those changes is still a mystery,” added Matt Hedman, co-author and professor at the University of Idaho.