Comet chemistry is sometimes regarded as some form of forensic documentation: hot a nucleus, record which of its ices vaporizes first, and guess how and where it was made. That ranking is no longer acting like a reliable manual with interstellar comet 3I/ATLAS.
Few such visitors have ever been closely examined, and 3I/ATLAS came with the added benefit of current infrared and ultraviolet telescopes to observe it with, on a variety of angles. Initial spectra showed that the coma contained more carbon dioxide than water and the CO2/H2O ratio was approximately 8.0+-1.0 in the JWST/NIRSpec as the comet was still distant. The importance of that one number was that it was well out of the trend lines constructed over decades of observation of the Solar System comets, where water normally assumes the frontline position as soon as it can start heating.
The more profound surprise would follow the perihelion, when the MIRI instrument of JWST introduced a new item in the list of volatile substances: methane. Mechanized emission was detected in post-perihelion spectra together with water and carbon dioxide, with the rate of methane production as high as 13.7% and 27% of the rate of molecular production of water in two epochs of the month of December 2025, using JWST/MIRI spectroscopy in mid-to-late December 2025. The emergence of methane placed a strain on the simple fact that more volatile species were supposed to appear earlier, not later. But none of the previous campaigns due in August 2025 had mentioned methane in the same manner, though other volatiles were evident.
Such a mismatch does not merely rearrange a catalog of molecules; it makes one pay attention to the structure and history of the comet. The outermost layers are not an unaltered surface but one interpretation in the literature is that the rind is chemically altered. With a more detailed analysis, it is suggested that carbon monoxide can be processed by galactic cosmic ray to form carbon dioxide and crust over an organic-rich surface, and those who sample an irradiated region instead of the primordial core. In such framing, “composition” is now partially an exposure narrative: what the sunlight can reach with thus crustal insulation and what has been already changed throughout the years by time in interstellar space.
We have signs that 3I/ATLAS too does not dare to await the time when the heat of the Sun in the inner system will start acting like a comet. In one study, observations at ultraviolet revealed the presence of hydroxyl an echo of water at the distance of 2.9 AU, suggesting the release of water way out in the Solar System well beyond the area in which water ice is generally the dominant activity in many comets of the Solar System. Individually, SPHEREx probed a large scale coma of carbon dioxide revealing that the CO2 was not just detectable but specifically formed spatially, which is in line with a larger CO2 coma seen in August 2025.
Then there is the physical body who is performing all this chemistry. Post-perihelion observations with Hubble suggested an effective nucleus diameter of 2.6+-0.4 kilometers, which scales 3I/ATLAS into another regime than the previous interstellar visitors and opens the scale of potential reservoir of layered ices and radiation-processed materials. A more massive nucleus is able to maintain richer cold-reservoirs and yet lose the modified surface products, meaning the CO2 dominance, early water, late methane signals are able to be co-occurring without any simple template.
With comet science, it is not that the old rules were bad but that they were locally educated. A pattern of volatility is not complete at all, but is a thermal history, irradiation chemistry, and stratification pattern, which can be produced in an interstellar comet.
