3I/ATLAS Update: Webb Telescope Observes Comet Over Eight Months Finding 'How Much Dust Is Mixed' in Its Ice

Scientists studying the interstellar comet 3I/ATLAS with the James Webb Space Telescope have found evidence that its water-ice grains contain substantial amounts of rocky material.

Southwest Research Institute researchers announced the findings on 26 October 2026, following three observations of the comet in our Solar System.

The comet originated beyond our Solar System. As sunlight warms it, escaping gas carries ice and dust into a surrounding cloud, called a coma. Studying that material allows astronomers to investigate conditions in another planetary system.

The team, led by Dr Silvia Protopapa, examined an unusual pattern in the comet's infrared light. Their findings suggest that the ice's composition, rather than merely its presence, sets 3I/ATLAS apart from many familiar comets.

Webb's 3I/ATLAS Measurements Point to Dust-Rich Ice

Researchers used Webb's Near-Infrared Spectrograph, or NIRSpec, on 6 August and 22 December 2025 and 1 April 2026. These were three separate observing sessions spanning approximately eight months, including the comet's closest approach to the Sun on 29 October 2025.

The study in The Astrophysical Journal Letters reports water-ice absorption alongside gas emissions from water, carbon dioxide and carbon monoxide. Each produces features in the spectrum that help reveal the material around the comet.

Water ice normally absorbs infrared light near 1.5, 2.0 and 3 micrometres. Webb recorded a strong feature near 3 micrometres, but the two shorter-wavelength signatures were weak or missing.

That difference challenged expectations based on relatively pure ice grains in other comets. The researchers tested models involving different grain sizes and compositions to explain the measurements.

Their preferred models favour aggregates around one micrometre or smaller, containing water ice mixed with dust. One model used pyroxene, a silicate mineral, to reproduce the unusual absorption pattern.

Dust changes the distance infrared light travels through ice, weakening some spectral features more than others. The result supports an interpretation of dust-rich ice, rather than a direct measurement of each grain's ingredients.

Although Webb observed 3I/ATLAS three times, the detailed ice-grain modelling focused on the August and April spectra. Stronger gas and organic emissions, together with thermal radiation from dust, complicated the December analysis.

What 3I/ATLAS Shares With Icy Worlds Beyond Neptune

The team compared the comet's spectrum with those of Solar System comets and trans-Neptunian objects, icy bodies orbiting beyond Neptune.

Several groups of those distant objects show water-ice absorption patterns resembling the comet's. The closest similarities include objects whose spectra also display signatures linked to carbon dioxide ice and organic compounds.

Importantly, the resemblance concerns the shape of the water-ice absorption feature. It does not establish that the objects contain identical amounts of carbon dioxide, organics or rocky material.

The researchers suggest that tiny icy grains may exist across a range of compositions, from relatively pure ice to ice containing more dust. Their models place 3I/ATLAS towards the dust-rich end.

Such similarities could offer clues to how icy bodies form around different stars. Protopapa's team proposes that some properties develop in planet-forming disks and survive as smaller bodies take shape.

That possibility remains an interpretation to be tested, not proof that the comet and distant Solar System objects formed under identical conditions.

April Observations Suggest Changes in the Comet's Ice

Webb's final observation on 1 April 2026 also suggested that the particles surrounding 3I/ATLAS had changed since August.

The later spectrum favoured an additional population of larger, more ice-rich grains, although these remained around a micrometre in size. It also contained a subtle feature consistent with crystalline water ice.

Researchers could not determine whether the ice had changed its physical structure or whether differences in grain size and dust content explained the signature.

Dr Michael Kelley of the University of Maryland identified two further possibilities: sunlight may have altered the released grains, or erosion may have exposed fresh material beneath the comet's surface.

Neither explanation has been established. The material visible in the coma may also differ from what remains deeper inside the comet.

Further observations of interstellar visitors and familiar Solar System comets could help determine whether dust-rich ice is unusual or part of a broader pattern in planetary formation.