JWST Detection Of A Carbon Dioxide Dominated Gas Coma Surrounding Interstellar Object 3I/ATLAS

3I/ATLAS is the third confirmed interstellar object to visit our Solar System, and only the second to display a clear coma.

Infrared spectroscopy with the James Webb Space Telescope (JWST) provides the opportunity to measure its coma composition and determine the primary activity drivers. We report the first results from our JWST NIRSpec campaign for 3I/ATLAS, at an inbound heliocentric distance of rH=3.32 au. The spectral images (spanning 0.6-5.3 μm) reveal a CO₂ dominated coma, with enhanced outgassing in the sunward direction, and the presence of H₂O, CO, OCS, water ice and dust.

The coma CO₂/H₂O mixing ratio of 8.0±1.0 is among the highest ever observed in a comet, and is 6.1-sigma above the trend as a function of heliocentric distance for long-period and Jupiter-family comets (excluding the outlier C/2016 R2).

Our observations are compatible with an intrinsically CO₂-rich nucleus, which may indicate that 3I/ATLAS contains ices exposed to higher levels of radiation than Solar System comets, or that it formed close to the CO2 ice line in its parent protoplanetary disk.

A low coma H₂O gas abundance may also be implied, for example, due to inhibited heat penetration into the nucleus, which could suppress the H₂O sublimation rate relative to CO₂ and CO.

Martin A. Cordiner, Nathaniel X. Roth, Michael S. P. Kelley, Dennis Bodewits, Steven B. Charnley, Maria N. Drozdovskaya, Davide Farnocchia, Marco Micheli, Stefanie N. Milam, Cyrielle Opitom, Megan E. Schwamb, Cristina A. Thomas

Comments: Submitted to ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2508.18209 [astro-ph.EP] (or arXiv:2508.18209v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.18209
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Submission history
From: Martin Cordiner PhD
[v1] Mon, 25 Aug 2025 17:10:10 UTC (398 KB)
https://arxiv.org/abs/2508.18209
Astrobiology, Astrochemistry,