James Webb Detects Carbon Dioxide–Dominated Coma in Interstellar Object 3I/ATLAS

The entry of a third known object named 3I/ATLAS into our solar system has been confirmed on July 1, 2025 by the astronomers. On August 6, 2025, The James Webb Space Telescope (JWST) trained its infrared vision and its Near-Infrared Spectrograph instrument (NIRspec) on the interstellar visitor. As the third object ever discovered floating through our solar system, 3I/ATLAS was identified by the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope and is thought to have come from around another star. This observation revealed a coma dominated by carbon dioxide (CO₂), accompanied by unexpectedly low levels of water and carbon monoxide. The unusual composition suggests that 3I/ATLAS possesses either a CO₂-rich interior or an insulating outer layer suppressing water outgassing.

A CO₂-Dominated Coma Unveiled

According to a preprint paper, spectroscopic imaging from JWST's NIRSpec instrument—at a heliocentric distance of about 3.32 AU—has revealed 3I/ATLAS's coma is overwhelmingly rich in CO₂, with a CO₂-to-H₂O mixing ratio of around 8, one of the highest ever recorded in a comet. The mass-loss measurements further underscore the dominance of CO₂: approximately 130 kg/s of CO₂, compared to just 6.6 kg/s of H₂O and 14 kg/s of CO. These results strongly suggest that 3I/ATLAS's nucleus may be intrinsically rich in CO₂, or alternatively, that surface properties—perhaps a highly reflective or insulating crust—limit water sublimation.

Possible origin

The abundance of carbon dioxide coma of 3I/ATLAS is indicative of its intrinsically carbon dioxide rich core and raises compelling questions about its origin and formation environment. One possibility is that it formed near a CO₂ frost line within its parent star's protoplanetary disk, or that long-term exposure to cosmic radiation altered its volatile inventory. Alternatively, an insulating outer layer could be masking water beneath the surface.

Notably, other observations—including water activity detected via OH emissions and ultraviolet imaging, as well as emission lines of cyanide and nickel—point to a diverse and evolving coma composition.