JWST Finds Refractory-Rich Water Ice in Interstellar Comet 3I/ATLAS
Astronomers using the NASA/ESA/CSA James Webb Space Telescope have discovered that water ice in the interstellar comet 3I/ATLAS is blended with dark, rocky material in tiny, micron-scale grains. The findings, published in The Astrophysical Journal Letters, reveal that while the comet’s overall ice signatures differ from those of ice grains in solar system comets, they closely resemble the spectra of some icy worlds beyond Neptune.
Interstellar comets are small icy bodies that formed in the disks of gas and dust around other stars and are now passing through our Solar System,
said Dr. Silvia Protopapa, an astronomer at the Southwest Research Institute.
Observations Across Closest Approach to the Sun
Dr. Silvia Protopapa, an astronomer at the Southwest Research Institute, and her colleagues observed 3I/ATLAS using the Webb telescope’s NIRSpec instrument on three distinct dates: August 6, 2025, December 22, 2025, and April 1, 2026. These observations spanned about eight months surrounding the comet’s closest approach to the Sun, which occurred on October 29, 2025. The resulting spectra displayed strong water-ice absorption alongside gas emissions from water, carbon dioxide, and carbon monoxide. However, the weaker water-ice signatures, which are usually seen in icy comets, remained faint or missing.
According to the research team’s modeling, the best explanation points to grains smaller than a few micrometers, each a blend of water ice and refractory material. The grains of 3I/ATLAS are about the same size as those in our own comets,
Dr. Protopapa stated. What seems to set them apart is how much dust is mixed into the ice.
Data from the April 2026 observation further hinted at a secondary population containing larger, micron-sized ice-rich grains.
As 3I/ATLAS warms up, gas escaping from its interior carries ice and dust grains into the surrounding cloud, or coma,
Dr. Protopapa noted. Those grains hold clues to the conditions where the comet was born.
Unresolved Questions Regarding Surface Changes
Dr. Michael Kelley, an astronomer at the University of Maryland, noted that the shifts could stem from the Sun heating the grains differently during each observation or from fresh material becoming exposed as the comet’s surface eroded near the Sun. Both are possible, and telling them apart is one of the open questions we aim to address,
Dr. Kelley explained.
When researchers compared 3I/ATLAS against other comets and several classes of trans-Neptunian objects, they suggested that icy planetesimals formed around different stars may share a continuum of ice grain properties. This continuum stretches from pure ice to ice mixed with refractory material, placing 3I/ATLAS toward the refractory-rich end.
Implications for Early Planetary Formation
At first glance, 3I/ATLAS looks different from the icy comets we know,
Dr. Protopapa said. But when we placed it alongside distant icy worlds in our own Solar System, it no longer looked out of place.
This suggests that the icy building blocks of planets, here and around other stars, may share ice in the form of tiny, micron-scale grains, ranging from relatively pure ice to ice mixed with dust, with 3I/ATulas toward the dusty end,
Dr. Protopapa added.
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