JWST Reveals Uranus’s Tiny Inner Moons Are Darker, Redder and Poorer in Water — With One Surprising Outlier

New infrared observations from the James Webb Space Telescope (JWST) show that Uranus’s small inner moons are generally much darker, redder, and poorer in surface water than the planet’s larger outer satellites. The first JWST spectra for many of these tiny moons — results scheduled for presentation on 18 December at the 2025 AGU Annual Meeting in New Orleans — also revealed a newly detected temporary object and unexpected orbital offsets for several moons.

Observations and Key Findings

Uranus was visited only once by a spacecraft: Voyager 2 flew past in 1986, discovering additional moons and rings and providing the first size and orbit estimates. Since then, telescopes on the ground and in space have increased Uranus’s confirmed moon count to 29, ranging from Sycorax at roughly 150 km across to tiny bodies such as Mab and Cupid at about 10 km.

Because these small moons orbit close to the bright planet and within or near its rings, they are difficult to study in visible light. JWST’s infrared capability is an advantage: Uranus appears relatively faint in the infrared, making faint nearby objects easier to detect, and key spectral markers such as water-ice absorption bands fall squarely within JWST’s wavelength range.

In a deep infrared survey conducted in February, the research team characterized the inner moons’ spectra and searched for new satellites. They discovered a previously unknown, temporary moon designated S/2025 U1, orbiting just outside Uranus’s epsilon ring. For many of the smallest moons, these are the first robust infrared brightness and spectral measurements since Voyager 2.

Surfaces: Redder, Darker, and Less Watery

Most of the rings and inner moons examined show broadly similar surface properties: they are redder in color, have lower reflectivity, and display fewer signs of surface water ice than the larger outer moons Miranda, Ariel, Umbriel, Titania and Oberon. These results point to compositional differences between inner and outer satellite populations and hint at distinct evolutionary histories or surface processing mechanisms.

Mab stands out as an exception. Its spectrum is bluer and shows clearer water-ice features, resembling the surface of Miranda. Miranda’s fractured, jigsaw-like terrain implies a violent or complex past, leading researchers to ask whether Mab and Miranda might have interacted long ago — and whether that history is tied to the mu ring, which may be supplied by material escaping from Mab.

Orbital Surprises and System Dynamics

When the team compared current positions with predictions based on Voyager-era orbital data, several small moons were displaced from their expected locations. Perdita was notably far from its predicted position; Cupid was also surprising. Smaller offsets were measured for Cordelia, Ophelia, Cressida and Desdemona. The differences may reflect improved measurement precision, active dynamical interactions, or a mix of both.

"These new observations are quite useful for improving our understanding of the inner Uranian system, especially its orbital dynamics," said Matija Ćuk of the SETI Institute, who was not involved with the study.

Some moons play specific dynamical roles: Cordelia and Ophelia shepherd the epsilon ring; Cressida and Desdemona belong to a group with chaotic orbits; and Perdita interacts with nearby Belinda. Precise measurements of the moons’ masses, shapes and densities would greatly improve dynamical models and long-term stability estimates.

What’s Next?

The research team plans further JWST observations, is mining archived and technical images for additional data, and hopes to establish long-term monitoring to refine orbital solutions and possibly estimate masses. They are also collaborating with colleagues who run orbital simulations to probe how Uranus’s rings and moons influence one another.

"It's a very dynamic and interconnected system," said Jacob Herman, the study's lead author. Additional observations and, eventually, a dedicated mission to Uranus would provide deeper insight into the origins, compositions and evolution of this intriguing planetary system.