Uranus’s Small Moons Are Dark, Red, and Water-Poor

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By Kimberly M. S. Cartier, Eos/AGU. 

Excerpt: 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. ...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. ...In 1986, Voyager 2 flew past Uranus in humanity’s only visit to the system. ...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. ...the infrared powerhouse James Webb Space Telescope (JWST)...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. .....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.... 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. Did the two moons encounter each other sometime during Uranus’s chaotic past?... 

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