Would a comet in interstellar space come with a chemical record that largely tells us about its journey, but not its origin? The results of the observations of comet 3I/ATLAS using the James Webb Space Telescope are indicative of that. The coma of the object, its out-pouring halo of gas and dust, presents an excessive preponderance of carbon dioxide as compared to water, a feature which is typical of ice and which corresponds with ice that has been reprocessed by radiation through enormous distances of space over very long periods of time. As opposed to being a closed system of another planetary system, 3I/ATLAS will act like an extended experiment in deep space chemistry.
The main process is gradual and progressive. Beyond the protective heliosphere of the Sun, galactic cosmic rays hit cold surface ice and break bonds, causing reactions to rearrange molecules to new formations. This processing transforms carbon monoxide into carbon dioxide in CO-rich ice by radiolysis routes in laboratory analogs. An extended stay in the interstellar space gives the effect time to accumulate until the modified material ceases to be a thin veneer. This means that the processed zone can be projected to reach 15-20 meters into the nucleus- sufficient to dominate what is covered by telescopes during early activity despite the comet still being a long way off the intense heat of the sun.
JWST observed the comet at a distance of approximately 3.3 astronomical units of the Sun but the outgassing of the coma was already well-developed. Detailed spectroscopy was used to measure the extent to which the volatile mix is abnormal; the signal after averaging over the coma showed a CO2/H2O mixing ratio of 7.6 +- 0.3, and there was substantial carbon monoxide. That pairing matters. The fact that CO is not always consumed to zero, both through laboratory and modeling experiments demonstrates that it may have a persistence state or be regenerated as the more complex organics are irradiated and more broken down later, keeping the CO- CO2 balance constant though still driving the overall chemistry to CO2 dominance.
One of the implications is interpretive: the word “interstellar” does not necessarily create an image of something “pristine”. 3I/ATLAS is one of only a very few confirmed extrasolar visitors-1I/Oumuamua and 2I/Borisov-but it contains a more accurate warning that chemistry at the surfaces can be an evolved surface. The crust which has been processed by the cosmic rays is a sort of blanket of shielding where material that is much deeper is deposited and that is more reflective of the conditions under which the comet was formed.
That is an effective observation tactic. With the erosion of the comet becoming severe enough around the perihelion to cut through tens of meters of altered ice, the ratios of the volatiles in the coma can change in such a way that suggests the existence of deeper layers that can become exposed. This comet went through perihelion at 1.36 au, just far enough to be heated by the sun to a greater degree that sublimation and the release of dust became actively increased, yet not so far that the wholesale removal of these nucleus is assured at all possible sizes and activity levels.
The bigger engineering teaching is concerning the covering and exposure to geologic time. The cosmic rays are a very regular (commonplace), and inevitable processing instrument within the interstellar medium, and 3I/ATLAS demonstrates the extent to which that processing instrument can be used to carve out materials. Even the interstellar objects can include information concerning distant planetary systems, it cannot be certain but in most of the times the story of the voyage is the first signal removed.
