Astronomers using the NASA/ESA/CSA James Webb Space Telescope have found that water ice in the interstellar comet 3I/ATLAS is mixed with dark, rocky material in tiny grains, sci.news reported. The findings offer new clues about the early formation of planetary building blocks across different star systems.
Webb Telescope Observations Track Comet 3I/ATLAS
Dr. Silvia Protopapa and colleagues at the Southwest Research Institute observed the interstellar visitor using the NIRSpec instrument on three distinct dates: August 6, 2025, December 22, 2025, and April 1, 2026. These observations spanned an eight-month window centered around the comet’s closest approach to the Sun on October 29, 2025. As the comet warmed during its perihelion passage, escaping interior gas carried ice and dust grains into the surrounding coma.
Did you know? Interstellar comets form in the gas and dust disks surrounding other stars before embarking on journeys that occasionally carry them through our Solar System.
Spectral Analysis Reveals Refractory-Rich Ice Grains
While the Webb telescope detected strong water-ice absorption alongside gas emissions of water, carbon dioxide, and carbon monoxide, the weaker water-ice signatures typical of icy comets were faint or entirely missing. Computer modeling by the research team indicates that the grains measure less than a few micrometers and consist of an intimate blend of water ice and refractory material. What seems to set them apart is how much dust is mixed into the ice,
Dr. Protopapa stated regarding the distinction between 3I/ATLAS and local comets.
Comparing Interstellar Comets With Trans-Neptunian Objects
When researchers placed the spectrum of 3I/ATLAS alongside distant icy worlds beyond Neptune, the interstellar object closely matched classes of trans-Neptunian objects rich in carbon dioxide ice and organics. Additional observations from April 2026 also hinted at a secondary population of larger, micron-sized ice-rich grains.
Pro Tip: Researchers suggest that icy planetesimals across different star systems share a continuum of ice grain properties, placing 3I/ATLAS near the refractory-rich end of that spectrum.
Open Questions Regarding Surface Evolution
Scientists remain divided on whether temporal changes observed in the comet stem from varying solar heating angles or newly exposed material eroding from the surface. 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,
noted Dr. Michael Kelley of the University of Maryland. Resolving this mechanism remains a primary goal for the research team following their study published in the Astrophysical Journal Letters.
Frequently Asked Questions About Interstellar Comet 3I/ATLAS
When did 3I/ATLAS make its closest approach to the Sun?
The interstellar comet reached its closest point to the Sun on October 29, 2025, during an eight-month observation campaign conducted by the James Webb Space Telescope.
How do the ice grains of 3I/ATLAS differ from solar system comets?
While solar system comets generally feature relatively pure ice grains separate from dust, 3I/ATLAS contains tiny micron-scale grains where water ice is intimately mixed with dark, refractory material.
Which telescope instrument captured the spectral data?
Astronomers gathered the spectral data using the Near-Infrared Spectrograph (NIRSpec) instrument aboard the NASA/ESA/CSA James Webb Space Telescope.
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