Hubble image of 3I/ATLAS. Credit: NASA/ESA
All (or at least most) astronomical eyes are on 3I/ATLAS, our most recent interstellar visitor that was discovered in early July. Given its relatively short observational window in our solar system, and especially its impending perihelion in October, a lot of observational power has been directed toward it.
That includes the most powerful space telescope of them all—and a recent paper published on the pre-print server arXiv describes what the James Webb Space Telescope (JWST) discovered in the comet’s coma. It wasn’t like any other it had seen before.
3I/ATLAS’s coma, which is the material surrounding its nucleus, is primarily made up of carbon dioxide (CO2), according to the paper first authored by Martin Cordiner of NASA’s Goddard Space Flight Center and the Catholic University of America.
It also contains water, carbon monoxide and carbonyl sulfide, all of which are expected to be in a comet’s coma. But the ratio of carbon dioxide to water is 8:1, the highest ever seen in a comet, and six standard deviations above the typical value. Strangely, the carbon monoxide (CO) ratio with water is more in line with previous observations, at 1.4.
NASA Explains what we know about 3I/ATLAS so far.
To detect these chemicals, JWST used its NIRSpec infrared camera to observe 3I/ATLAS on August 6th, when it was 3.32 AU from the sun. Other indications, which weren’t quite as surprising, include that the coma does have a bunch of water and dust scattered around it, as well as a higher dust concentration facing the sun, which is typical for higher outgassing on the side the sun heats.
Another finding was that the ratio of two types of carbon isotopes, Carbon-12 and Carbon-13, was broadly similar to that found on Earth, suggesting the material was created in an environment with similar carbon species. However, there are a couple of features of 3I/ATLAS’s creation that could have caused the lopsided CO2/H2O ratio.
One is extremely high levels of ultraviolet radiation in the host star system the object was created in. Another could be that it was created beyond the CO2 “ice line,” where carbon dioxide ice is relatively abundant compared to water.
Other explanations have to do with how heat from the sun is able to affect the nucleus—if it is harder to heat up, then CO2, which has a lower melting point than water, would be sublimated first, accounting for the lopsided ratio despite having plenty of water stored in the nucleus waiting to be released as it gets closer to the sun.
Either way, more observations are needed. This is only the third interstellar visitor we have confirmed, and the first (‘Oumuamua) wasn’t bright enough to capture its coma’s spectra, though even if it was, it didn’t appear to have a coma anyway. That leaves the second interstellar visitor 2I/Borisov, as our only other point of comparison for the coma spectra of an interstellar comet.
It actually had a higher carbon monoxide to water ratio, even compared to 3I/ATLAS’s, so it seems of the two we have collected so far, each interstellar visitor’s coma hides new insights.
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This undoubtedly won’t be the last paper examining 3I/ATLAS’s coma—it probably won’t even be the last one from JWST. We still have a few weeks of observational time before it passes too close to the sun to be detectable, and then reaches its perihelion in early October, which it is still obscured from our view, though there is a chance some probes at Mars might be able to catch a glimpse of it during that time.
When it finally becomes visible again in December, it will already be on its way out of our solar system, and would likely have shed most of the material it was going to.
Sometimes astronomical events are fleeting, and astronomers have to try to capture them as they’re happening. At least with this one they’ll have a little bit of warning—we’ll see what they find as they continue to observe our newest interstellar visitor.
More information:
Martin A. Cordiner et al, JWST detection of a carbon dioxide dominated gas coma surrounding interstellar object 3I/ATLAS, arXiv (2025). DOI: 10.48550/arxiv.2508.18209
Journal information:
arXiv
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Universe Today
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JWST reveals 3I/ATLAS’s coma is largely carbon dioxide (2025, September 1)
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