Observations from the NASA/ESA/CSA James Webb Space Telescope indicate that water ice in the interstellar comet 3I/ATLAS is mixed with small particles of dark, rocky material. The spectral properties of these particles differ from those of ice particles in Solar System comets but resemble those found on some icy worlds beyond Neptune, offering clues about how planetary building blocks formed around different stars.
Webb observations of 3I/ATLAS
“An interstellar comet is a small icy object that formed within a disk of gas or dust around another star and is currently transiting our Solar System,” said Dr. Sylvia Protopapa, an astronomer at the Southwest Research Institute.
“As 3I/ATLAS warms up, gas escaping from its interior carries ice and dust grains into the surrounding cloud, or coma. These particles contain clues to the conditions under which comets were born.”
Protopapa and her colleagues observed 3I/ATLAS with Webb’s Near-Infrared Spectrograph (NIRSpec) on August 6, 2025, December 22, 2025, and April 1, 2026. The observations spanned about eight months and included the period around the comet’s closest approach to the Sun on October 29, 2025.
The spectrum shows strong water-ice absorption, along with outgassing from water, carbon dioxide, and carbon monoxide. However, the weaker water-ice signatures typically seen in icy comets were faint or absent.
According to the researchers’ modeling, the most likely explanation is that the comet contains particles smaller than a few micrometers, with each particle composed of water ice mixed with a refractory material.
“The particles in 3I/ATLAS are about the same size as the particles in our comet,” Protopapa said. “What seems to differentiate them is how much dust is mixed into the ice.”
The spectrum obtained in April 2026 also suggests the presence of a second population of larger, micron-sized, ice-rich particles.
“We still don’t know whether the changes over time are the result of the Sun heating the particles differently in each observation, or whether they are the result of new material exposed as the comet’s surface erodes near the Sun,” said University of Maryland astronomer Dr. Michael Kelly.
“Both possibilities exist, and distinguishing between them is one of the open questions we’re trying to solve.”
Comparing interstellar and Solar System comets
The researchers compared 3I/ATLAS with other comets and several types of transsolar objects. They suggest that icy planetesimals formed around different stars may share a range of ice-grain properties, from relatively pure ice to ice mixed with refractory material. Their findings place 3I/ATLAS toward the refractory-rich end of that range.
“At first glance, 3I/ATLAS looks different from the icy comets we know,” Protopapa said. “But when we put it next to the distant icy worlds of our Solar System, it no longer looked out of place.”
“This suggests that the icy components of planets here and around other stars may share ice in the form of small, micron-scale particles, ranging from relatively pure ice to dust-laced ice. 3I/ATLAS is towards the dusty end.”
“If confirmed, this range of common particle properties could indicate that the physical form of water ice was established early in the planet-forming disk and is at least partially preserved within the ice bodies that form there.”
The results will appear in Astrophysical Journal Letters.
Sylvia Protopapa et al. 2026. “JWST reveals refractory-rich water ice in interstellar comet 3I/ATLAS: Evidence for continuity of particle properties across the protoplanetary disk.” APJL 1011, L11; doi: 10.3847/2041-8213/aea60d.
Source: www.sci.news
