The solar system’s oddball planet has some pretty odd moons, too. The first infrared spectra of Uranus’s small inner moons, which will be presented on 18 December at the 2025 AGU Annual Meeting in New Orleans, have shown that their surfaces are much redder, much darker, and more water-poor than the larger moons orbiting far from the planet.
“We were trying to see how these properties varied across the rings and moons,” said Matt Hedman, a planetary scientist at the University of Idaho in Moscow and a coauthor on the research. “We didn’t have a lot of information about their spectra before because they’re hard to observe.”
The new observations also revealed that some moons were not quite where they should have been, highlighting how much more astronomers have to learn about the dynamics of the Uranian system.
Small, Dark, and Red
In 1986, Voyager 2 flew past Uranus in humanity’s only visit to the system. At that time, astronomers knew only of the planet’s five major moons and a handful of rings. Voyager 2 discovered 11 more moons and was able to roughly measure their sizes. Since then, scientists have used ground- and space-based telescopes to discover more than a dozen additional satellites, bringing Uranus’s moon total to 29.
Many of the more recently discovered moons are pretty tiny, from Sycorax at 150 kilometers across to Mab and Cupid at just 10 kilometers. Most of them also orbit within or just outside Uranus’s ring system, close to the much brighter planet.
All of these properties have made it tricky for astronomers to learn more about the smallest Uranian moons. That’s where the infrared powerhouse James Webb Space Telescope (JWST) comes in.
This diagram shows the orbital distances of Uranus’s inner moons and rings, to scale. Uranus is placed at the top of the diagram. Click image for larger version. Credit: Ruslik0/Wikimedia Commons, Public Domain
“Part of what makes JWST particularly good for this compared to, say, Hubble and other optical telescopes, is that in the infrared, Uranus is much fainter, so you can see the things orbiting it way more easily,” Hedman explained. What’s more, all of the spectral features the team was interested in, like water ice, occur at wavelengths that JWST can observe.
The researchers observed Uranus at several infrared wavelengths in February and got a deep look at the inner portions of the planetary system. They wanted to characterize the known small moons and search for new ones. They did discover a previously unknown moon, temporarily named S/2025 U1, orbiting just outside the epsilon ring.
Those observations also provided the first information on the infrared brightnesses of the smallest moons, many of which have remained elusive since the Voyager flyby.
“Most of the rings and inner moons show very similar properties,” Hedman said. They tend to be much redder, darker, and more water-poor when compared with the larger outer moons Miranda, Ariel, Umbriel, Titania, and Oberon.
“And then there’s Mab,” Hedman added.
The new spectra show that Mab’s surface is bluer and more water-rich than the other inner moons, said Jacob Herman, a physics graduate student at the University of Idaho and lead author on the research. In fact, its surface spectrum looks very similar to Miranda’s, the major moon that orbits closest to the rings and to Mab. Miranda’s jigsaw surface suggests a messy history.
“There is still much to be discovered about Uranus’s small inner moons, particularly regarding their origin, composition, and long-term orbital stability.”
Did the two moons encounter each other sometime during Uranus’s chaotic past? Could that encounter be related to Uranus’s mu ring, which is likely generated by material sloughing off Mab? Hedman hopes that future observations or a long-term mission to Uranus will provide those answers.
“These new measurements significantly expand our current knowledge, revealing, for instance, striking variations in the composition and reflectivity of the surfaces of moons such as Mab, Cupid, and Perdita,” said Jadilene Xavier, an astrophysicist at São Paulo State University in Guaratinguetá, Brazil, who was not involved with this research.
“There is still much to be discovered about Uranus’s small inner moons, particularly regarding their origin, composition, and long-term orbital stability,” Xavier said. “More precise data on their density, three-dimensional shape, and surface properties would be essential to determine whether these moons are fragments produced by collisions, captured objects, or primordial remnants associated with the formation of Uranus’s ring system.”
Just a Little Bit Off
Because Voyager 2 spent only a short time visiting Uranus, it could provide only limited information about the small moons’ orbital periods and distances, sometimes with large uncertainties. When the researchers compared the moons’ current positions with the positions predicted by Voyager 2 data, some of the moons were not where they seemingly should have been.
“Perdita was quite a bit off,” Herman said. “And there’s also Cupid, which was surprising.” The positions of Cordelia, Ophelia, Cressida, and Desdemona were also off, but not by much. The team is still trying to figure out whether the differences are just a matter of having more precise observations of these tiny objects or if there are unknown dynamics in play.
“These new observations are quite useful for improving our understanding of the inner Uranian system, especially its orbital dynamics.”
“These new observations are quite useful for improving our understanding of the inner Uranian system, especially its orbital dynamics,” said Matija Ćuk, who researches solar system dynamics at the SETI Institute in Mountain View, Calif.
Ćuk, who was not involved with this research, pointed out that Cordelia and Ophelia shepherd Uranus’s epsilon ring, Cressida and Desdemona are part of a pack of moons with chaotic orbits, and Perdita is known to interact with another moon, Belinda. “So the fact that these [five] moons are not in their predicted positions is valuable for understanding the system, but I wouldn’t say it’s unexpected,” Ćuk said.
These observations hint at just how many mysteries Uranus is still hiding.
“For a dynamicist like me,” Ćuk said, “knowing the precise masses of these moons would be ideal, because then we could predict their future interactions and also estimate with some confidence how stable they are on long timescales.”
Hedman and their team plan to observe the Uranian system again with JWST, are looking through archived and technical images, and hope to establish long-term monitoring to better understand the moons’ dynamics and possibly estimate their masses. The researchers are also leaning on their colleagues who simulate planetary orbits to better understand how Uranus’s moons and rings might be influencing each other.
“It’s a very dynamic and interconnected system,” Herman said.
—Kimberly M. S. Cartier (@astrokimcartier.bsky.social), Staff Writer
Citation: Cartier, K. M. S. (2025), Uranus’s small moons are dark, red, and water-poor, Eos, 106, https://doi.org/10.1029/2025EO250442. Published on 25 November 2025.
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