James Webb Telescope Reveals Unique Ice Grain Composition in Interstellar Comet 3I/ATLAS

A Southwest Research Institute team used NASA's James Webb Space Telescope to analyze ice grains emitted by interstellar comet 3I/ATLAS, the third known object from outside the solar system. Observations made with the telescope's Near-Infrared Spectrograph between August 2025 and April 2026 captured the comet before and after its closest approach to the Sun.

"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 of Southwest Research Institute, lead author of the study published in The Astrophysical Journal Letters.

The spectra showed a strong water-ice absorption band near 3 microns but weak or absent bands at 1.5 and 2.0 microns, a pattern unlike most solar-system comets. Modeling indicated that the grains are about a micron or smaller and consist of water ice mixed with dust, which suppresses the shorter-wavelength bands while preserving the 3-micron feature.

"We designed this program to be ready the moment an active interstellar comet was found and to follow it over time," said Dr. Martin Cordiner of NASA's Goddard Space Flight Center and the Catholic University of America, principal investigator of the Webb program.

Comparisons with spectra of trans-Neptunian objects revealed similar 3-micron band shapes, suggesting that the comet’s icy grains share properties with distant icy worlds in our own system. The researchers noted that the grain composition appears to range from relatively pure ice to ice heavily mixed with dust, with 3I/ATLAS representing the dustier end of this spectrum.

Temporal changes were observed: later measurements indicated a modest increase in larger, ice-rich grains as the comet receded from the Sun. The cause of these variations—whether due to solar heating or exposure of fresh material as the comet’s surface eroded—remains uncertain.

"Both are possible, and telling them apart is one of the open questions we aim to address," said Dr. Michael Kelley of the University of Maryland, co-author of the paper.

The findings imply that the physical form of water ice may be established early in planet-forming disks and preserved in icy bodies across different planetary systems. Ongoing and future Webb observations of both solar-system comets and newly discovered interstellar objects will test these conclusions.