Webb finds dust-rich water ice in 3I/ATLAS that resembles icy worlds beyond Neptune

Webb's infrared spectra of 3I/ATLAS show water ice locked in grains about a micrometre across, heavily mixed with dust. The shape of the ice feature resembles that of icy bodies orbiting beyond Neptune, Silvia Protopapa's team reported in The Astrophysical Journal Letters on 26 October 2026.

What Webb measured

Astronomers spotted 3I/ATLAS entering the solar system in the summer of 2025. It is only the third object found arriving from outside, after 1I/'Oumuamua in 2017 and 2I/Borisov in 2019. Its home star is unknown, though it appears to have come from the direction of the constellation Sagittarius. Researchers think it formed in a planet-forming disk possibly 10 to 12 billion years ago, more than twice the age of the solar system.

Webb's measurements back the age estimate. The deuterium-to-hydrogen ratio in the comet's water is near 1%, tens of times higher than in solar system comets, and its carbon-12 to carbon-13 ratio sits well above values seen in local comets and in nearby interstellar gas. That combination points to ice that formed colder than about 30 kelvin, in a metal-poor corner of the early Milky Way.

Webb's Near-Infrared Spectrograph, NIRSpec, observed 3I/ATLAS on 6 August 2025, 22 December 2025 and 1 April 2026. The three visits spanned about eight months and bracketed perihelion, the comet's closest approach to the Sun, on 29 October 2025. The instrument splits light into wavelengths. That shows what a material is made of and its physical state.

Water ice normally leaves a deep absorption band near 3 micrometres and two weaker bands near 1.5 and 2.0 micrometres. Most icy comets show all three. In 3I/ATLAS the 3-micrometre band was strong, while the shorter-wavelength bands were weak or missing. "That combination was a puzzle," said Silvia Protopapa of the Southwest Research Institute, who led the study.

Model fits pointed to grains about a micrometre across or smaller, each a blend of water ice and dust. Webb also detected water vapour, carbon dioxide and carbon monoxide around the comet. The April 2026 spectrum favoured a second population of ice-rich grains, larger than the earlier ones but still around a micrometre, and showed a faint feature consistent with crystalline ice. The team could not tell whether the ice had changed structure or whether grain size and dust content explained the signal.

Martin Cordiner of NASA's Goddard Space Flight Center and the Catholic University of America leads the Webb program and is a co-author of the study. "We designed this program to be ready the moment an active interstellar comet was found and to follow it over time," he said. The team wanted to learn what the comet's ice, dust and gas say about planetesimals, the building blocks of planets, around other stars.

Why dust changes the spectrum

Light travels only a short distance through such grains. That weakens the 1.5 and 2.0 micrometre bands and leaves the 3 micrometre band strong.

Size is not the difference. Protopapa said the grains of 3I/ATLAS are about the same size as those in our own comets. "What seems to set them apart is how much dust is mixed into the ice," she said.

One model used pyroxene, a silicate mineral, to reproduce the unusual absorption pattern. The researchers compared the comet with solar system comets and with several classes of trans-Neptunian objects, the icy bodies that orbit beyond Neptune. The comet's spectrum falls between the two groups and matches neither exactly. The closest TNO matches are objects whose spectra also show carbon dioxide ice and organic compounds.

What the comparison does and does not show

The resemblance concerns the shape of the water-ice feature. It does not establish identical amounts of carbon dioxide, organics or rocky material.

The team suggests icy planetesimals around different stars may share a continuum of grain properties, from nearly pure ice to ice mixed with dust, with 3I/ATLAS toward the dusty end. "But when we placed it alongside distant icy worlds in our own Solar System, it no longer looked out of place," Protopapa said. The study, "JWST Reveals Refractory-rich Water Ice in Interstellar Comet 3I/ATLAS: Evidence for a Continuum of Grain Properties across Protoplanetary Disks," appeared in The Astrophysical Journal Letters (doi: 10.3847/2041-8213/aea60d).

The authors describe the shared range as a possibility to be tested. They do not call it proof that the comet and distant solar system objects formed under identical conditions.

What remains open

One question is what changed between observations. "We can't yet tell whether the changes over time are the result of the Sun heating the grains differently at each observation, or of new material exposed as the comet's surface eroded near the Sun," said Michael Kelley of the University of Maryland, the paper's second author. "Both are possible, and telling them apart is one of the open questions we aim to address."

The December analysis was harder than the other two. Stronger gas and organic emissions, plus thermal radiation from dust, complicated the modelling of ice grains.

Webb's last look came on 1 April 2026, when the comet was 5.7 astronomical units from the Sun. On the first visit it had been about 494 million kilometres out. By the last it was about 853 million kilometres away and heading back out. The comet is now leaving our view.