Webb telescope finds dusty ice on interstellar comet 3I/ATLAS, echoing bodies beyond Neptune

A dust-rich form of water ice coats the grains streaming off the interstellar comet 3I/ATLAS, according to a study led by the Southwest Research Institute using NASA's James Webb Space Telescope. The composition sets the object apart from most comets native to the solar system and instead resembles icy bodies orbiting far beyond Neptune, the researchers report.

The analysis, published in The Astrophysical Journal Letters, is based on three separate Webb observations of the comet's coma, the cloud of gas and dust surrounding its nucleus, taken between August 2025 and April 2026, bracketing the object's closest approach to the sun. 3I/ATLAS is only the third interstellar object ever confirmed passing through the solar system, after 1I/'Oumuamua in 2017 and 2I/Borisov in 2019.

Dust-heavy grains, not pure ice

Water ice typically leaves three telltale absorption features in infrared light, at wavelengths near 1.5, 2.0 and 3 microns. Using Webb's Near-Infrared Spectrograph, the team found the 3-micron band strongly present in 3I/ATLAS's coma, but the shorter-wavelength bands were very weak or missing entirely, a pattern rarely seen in comets that formed within the solar system, most of which show all three features clearly.

Modeling the spectra, the researchers concluded the ice is locked up in aggregates roughly a micron or smaller, mixed with enough dusty, refractory material to suppress the weaker absorption bands while leaving the dominant 3-micron feature intact. By the final observation in April 2026, as the comet receded from the sun, the data pointed to an additional population of slightly larger, more ice-rich grains, still on the order of a micron. Lead author Silvia Protopapa, a planetary scientist at SwRI, said the object initially stood out from familiar comets because of how much dust is mixed into its ice.

"At first glance, 3I/ATLAS looks different from the icy comets we know," said Silvia Protopapa of the Southwest Research Institute, the study's lead author.

What makes the finding notable is where else that spectral signature turns up. The shape of 3I/ATLAS's 3-micron band resembles ice signatures seen on several classes of trans-Neptunian objects, icy bodies orbiting in the Kuiper Belt and beyond, many of which also carry carbon dioxide ice and organic compounds and likewise lack strong 1.5- and 2.0-micron features. The authors propose that the physical form water ice takes, from relatively pure to heavily dust-laden, may vary in a continuum across the planet-forming disks that produce objects like comets and trans-Neptunian bodies, with 3I/ATLAS sitting toward the dustier end.

Why an interstellar comet's ice matters

3I/ATLAS was first spotted on July 1, 2025, by the ATLAS survey telescope in Río Hurtado, Chile, and its hyperbolic, markedly eccentric orbit confirmed within days that it did not originate in the solar system. Unlike 'Oumuamua, which showed no detectable gas or dust activity, 3I/ATLAS quickly displayed a visible coma, giving astronomers a rare chance to study the frozen material of a planetary system other than our own with modern space telescopes.

Each interstellar object studied so far has turned up something chemically distinctive. Borisov's coma was found to be unusually rich in carbon monoxide relative to water, enriched by roughly an order of magnitude compared with typical solar system comets, according to radio observations with the Atacama Large Millimeter/submillimeter Array and ultraviolet data from the Hubble Space Telescope. That earlier work and the new ice-grain study are part of a broader, continuing effort by largely the same research groups to compare volatile and ice chemistry across all three known interstellar visitors.

That opportunity has already produced a run of findings about the comet's origins. Earlier Webb observations, reported by a NASA Goddard-led team in a June 2026 paper in Nature, found the comet's coma dominated by carbon dioxide rather than water vapor, along with deuterium ("heavy hydrogen") levels roughly 30 times higher than in any solar system comet measured to date, and carbon isotope ratios indicating very little chemical processing over time. Taken together, that team estimated the comet could have formed 10 to 12 billion years ago, during a period of intense star formation astronomers call "cosmic noon," more than twice the age of the sun. The new ice-grain study builds directly on that picture: if 3I/ATLAS's volatile and dust record was effectively frozen in place in a distant, ancient disk, its ice structure offers a rare physical sample of planet-building material from outside the solar system.

Caveats the researchers flagged themselves

The study involved researchers at SwRI, NASA's Goddard Space Flight Center, the Catholic University of America and the University of Maryland. Martin Cordiner of NASA Goddard and Catholic University, the principal investigator of the Webb observing program and a co-author on both the new ice study and the earlier origin paper, said the program was designed specifically to respond quickly to objects like this one.

"We designed this program to be ready the moment an active interstellar comet was found and to follow it over time," Cordiner said, according to the SwRI release.

The authors are careful to flag what the data cannot yet settle. Co-author Michael Kelley of the University of Maryland noted that the team cannot yet tell whether the ice population observed in April 2026 changed because sunlight was heating the grains differently at each distance from the sun, or because the comet's eroding surface was exposing fresh material as it approached and passed perihelion, both remain plausible explanations. The broader claim that icy planetesimals form along a continuum of dust-to-ice ratios across different planet-forming disks is presented by the researchers as a proposed interpretation requiring confirmation from additional objects, not a settled conclusion.

What comes next

3I/ATLAS is now continuing outward on its one-way hyperbolic trajectory, leaving the solar system permanently, which limits the window for further direct observation even as Webb and ground-based telescopes continue tracking it while it remains bright enough to study. Testing the proposed continuum of ice properties will require comparable spectra from future interstellar visitors, as well as continued cross-checking against trans-Neptunian objects within the solar system. Only two previous interstellar objects have been confirmed, so a fourth detection, something astronomers expect to become more frequent as new wide-field survey telescopes come online, would offer the first real test of whether 3I/ATLAS's dusty ice is typical of material arriving from other stellar systems or a quirk of this one object.

For now, the findings add another data point to an unusually well-documented interstellar visit: a comet whose chemistry suggests a birthplace in a star-forming era long before the sun existed, and whose ice grains, oddly, look dustier than scientists initially expected.