The Inner Ring Thickens
JWST observed the occultation event simultaneously across two near-infrared channels, at wavelengths of 1.5 micrometers and 3.2 micrometers. This marked the first time astronomers had ever observed a minor body’s ring system at wavelengths beyond three micrometers—a infrared spectral region completely obscured to ground-based telescopes by the absorption of Earth’s atmosphere.
When the team analyzed the incoming light curves, the inner ring, C1R, registered with dramatic clarity. It created abrupt, sharp-edged drops in starlight during both ingress (as the star passed behind the ring system entering the event) and egress (as it emerged on the other side). However, the ring blocked far more starlight than it ever had in prior observations.
In astronomy, optical depth or opacity describes the proportion of light blocked by a structure. Averaged across roughly ten previous ground-based occultation observations recorded since 2013, the baseline opacity of C1R stood at 0.303. JWST’s instruments, however, measured C1R’s opacity at 0.431—a massive increase indicating that the inner ring had become far denser or more opaque.
"We didn’t believe it at the beginning, so we fought a lot with the data," Santos-Sanz said.
Initial skepticism led the research team to investigate whether the unexpected darkness of the ring could be explained by local clumping rather than a systemic change. Rings are rarely perfectly uniform ribbon structures; gravitational instabilities or embedded boulders can create localized regions of higher density. If JWST had simply happened to cut through an unusually dense knot of ring material, the high opacity measurement might be a localized fluke rather than proof of a changing system.
To evaluate that possibility, the team created a detailed computer model of a lumpy, non-uniform ring and conducted 10 million simulated occultations to test how often a random line of sight would yield such high opacity values. The statistical results were striking. The probability of hitting a clump dense enough to yield an opacity of 0.431 was roughly 1 in 1,000 at the 1.5-micrometer wavelength, and a vanishingly small 4 in 100,000 at 3.2 micrometers for a single measurement pass.
Crucially, JWST observed the inner ring twice—once on ingress and once on egress—and detected the heightened opacity on both sides of the target. The odds of randomly striking isolated dense clumps on opposite sides of the ring system simultaneously were so infinitesimally small that the team ruled out local clumping. The conclusion was clear: C1R as a whole had physically thickened over the intervening nine years.
A Fading Outer Structure and Mass Disparity
While the inner ring had grown noticeably denser, the outer ring, C2R, presented the opposite behavior. In the JWST data, C2R barely produced a detectable dip at 1.5 micrometers and was completely absent at 3.2 micrometers, despite the space telescope recording the exact same region of space across both spectral bands at the same time.
"At the beginning we didn’t even see the outer ring in the light curve," Santos-Sanz explained. "We had to use models. It was really barely visible, so we said, ‘What is happening here?’"
The researchers evaluated two competing hypotheses to explain the outer ring’s disappearance. The first was an optical effect: because JWST was observing in infrared wavelengths never before tested on small-body rings, particle scattering characteristics at 1.5 and 3.2 micrometers might make the ring appear invisible without any physical change having occurred. The second hypothesis was far more profound: the outer ring had physically lost material and was actively dissipating.
To distinguish between the two scenarios, the team performed extensive radiative transfer modeling. Previous visible-light observations gathered from Earth were well described by standard compositional models consisting of a balanced mixture of water ice and dark silicate minerals. However, when the new JWST infrared data points were integrated into these models, no static combination of grain sizes or material mixtures could account for the observed light curve across both wavelengths.
"We are witnessing a real evolution of the rings with time," Santos-Sanz said. "Of course it is not a certainty, but for me it is the preferred explanation."
Intriguingly, the data revealed that the changes were not simply the result of material migrating inward from the outer ring to feed the inner ring. By measuring the rings’ equivalent width—a structural metric that quantifies the total amount of light-blocking material present—the team discovered a significant mass imbalance. The material gained by the inner ring C1R was roughly ten times greater than the total amount lost by the fading outer ring C2R. The inner ring was receiving substantial material from an external source.
"We don’t know where the extra material is coming from, but there are some hypotheses," Santos-Sanz said.
