Astronomers analyzing data from the James Webb Space Telescope have discovered that the water ice in interstellar comet 3I/ATLAS is intimately mixed with dark, rocky material in tiny, micrometer-scale grains. The findings, published in the Astrophysical Journal Letters, offer fresh analytical insight into the early formation of planetary building blocks across different star systems.
NIRSpec Observations Map the Interstellar Visitor
Dr. Silvia Protopapa from the Southwest Research Institute led a research team utilizing the Near-Infrared Spectrograph (NIRSpec) instrument aboard the NASA/ESA/CSA James Webb Space Telescope. The team targeted the interstellar comet across three distinct observation windows: August 6, 2025, December 22, 2025, and April 1, 2026. These dates bracketed the comet’s perihelion, or closest approach to the Sun, which occurred on October 29, 2025.
As 3I/ATLAS traversed the inner Solar System, solar heating volatilized interior gases, dragging both ice and dust particles outward into its surrounding coma. Spectral analysis of this material revealed a strong water-ice absorption band near 3 micrometers, alongside gas emissions from water, carbon dioxide, and carbon monoxide. However, the weaker water-ice signatures typically observed in icy comets near 1.5 and 2.0 micrometers were notably faint or entirely missing.
Southwest Research Institute: Study Uses JWST to Compare Interstellar Comet 3I/ATLAS with Icy Bodies in Our Solar System https://t.co/ju8nJFv12R
“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,” Dr. Protopapa stated. “As 3I/ATLAS warms up, gas escaping from its interior carries ice and dust grains into the surrounding cloud, or coma. Those grains hold clues to the conditions where the comet was born.”
Contrasting Spectra With Trans-Neptunian Objects
When modeling the spectral data, researchers concluded that the best fit involves grains smaller than a few micrometers, where each individual grain represents a blend of water ice and refractory material. While the physical size of these grains matches those found in our own comets, the high concentration of embedded dust fundamentally sets 3I/ATLAS apart from standard local cometary bodies.
Further comparative analysis linked the interstellar visitor’s spectrum to a specific class of icy worlds residing beyond Neptune. These trans-Neptunian objects feature spectra that also show carbon dioxide ice and organics. Consequently, researchers suggest that icy planetesimals formed across different stars may share a continuum of ice grain characteristics, positioning 3I/ATLAS toward the refractory-rich end.
“At first glance, 3I/ATLAS looks different from the icy comets we know,” Dr. Protopapa noted. “But when we placed it alongside distant icy worlds in our own Solar System, it no longer looked out of place.”
Scientists Debate Causes of Ice-Rich Grain Variations
Spectroscopic data gathered during the April 2026 observation run introduced an additional variable by hinting at a secondary population of larger, micron-sized, ice-rich grains. Scientists currently debate whether this shift stems from differential solar heating across the observational timeline or from the exposure of new material as the comet’s surface eroded near the Sun.
“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,” said Dr. Michael Kelley, an astronomer at the University of Maryland. “Both are possible, and telling them apart is one of the open questions we aim to address.”
Ultimately, these findings imply that the physical form of water ice may be established early within planet-forming disks, retaining its fundamental properties even after ejection into interstellar space.
