“It’s very slow, but over billions of years, it’s a very strong effect.” That is the main surprise of the observations of the interstellar comet 3I/ATLAS by the James Webb Space Telescope: the chemistry we can see today could be a record of deep radiation exposure over time rather than an imprint of the origin object.
According to JWST data, which were combined with simulations, 3I/ATLAS survived billions of years of cosmic-ray bombardment in the galaxy in an estimated 7-billion-year lifetime. During those spans, the high-energy particles are able to ionize and break CO-rich ice allowing oxygen to reprocess back into CO2. This processing does not lie in the surface veneer of the interstellar medium, beyond the heliospheric shielding of the Sun. The models put the modified “crust” 15 to 20 meters thick and the ices remaining pure are virtually buried unless some of them are dug up by subsequent erosion.
Such thickness alters the habit of interpretation of comet science. In the case of solar system comets, near surface volatiles can still be treated, with caution, as partially preserved material of early epochs. The outermost layers of an interstellar object which has spent the bulk of its history in regions not enclosed by protecting magnetic bubbles, become a long-acting chemical reactor, in which gradual progressive changes predominate that which is first sampled by telescopes.
The notion is consistent with the multi-layer comet ice chemistry models, where the radiation dose is absorbed by the outer strata, and deeper matter retains an older molecular inventory. The peculiarity of 3I/ATLAS is the scale: when the area under processing is tens of meters, even active processes around the Sun will be able to evacuate gases that are primarily constituents of the altered layer, rather than the environment of birth.
Perihelion provided an unique natural experiment due to the fact that the activity of the object varied significantly once it had passed the solar. Comparison of August and December observations had water signatures becoming very intense with the water emission being approximately 20 times higher after perihelion and further organics were observed that had not been previously seen, which was in line with volatiles being trapped in water ice or being covered by it. The broad volatile palette of the comet, consisting of water, CO 2, CO, and nitrogen bearing species and its continuing CO 2 richness, with a strong extension of carbon dioxide gas around the comet, were also highlighted by the same monitoring.
Postperihelion independent optical spectroscopy provided another point of view: traditional comet radicals and metals were still visible, with CN and the observance of carbon-chain molecules C2 and C3, as well as Fe and Ni. The measured production rates were QFe=(9.55±3.96)×1025 atoms s -1 and QNi = (6.61 0.274) ×1025 atoms s -1, with a ratio between them, log(QNi/QFe) = -0.16 0.03. The characteristic e-folding radii increased significantly relative to pre-perihelion levels, and a geometrical shift in the C3 emission structure indicated that various species may be able to map onto the various areas of source in the coma.
The contrasting richness of observation is an adverse engineering-type limitation on inference: the instruments do not sample “the comet,” but this particular columns of gas and dust, the composition of which is determined by the distribution of where and how the material is emitted. Suppose cosmic-ray processing has already reduced much of the available CO to CO2 throughout a thick mantle in the first place then the very fact that makes 3I/ATLAS so spectacular, its high CO2, is also a red flag about interstellar sources being read at near-surface chemistry.
Only the third known interstellar object after 2I/Borisov and ’Oumuamua, 3I/ATLAS has a hyperbolic orbit, and therefore its extrasolar origin is not disputed. The remaining question is methodological: how to isolate primordial signals out of slow and inexorable rewrite of galactic radiation on an object, which spent eons of its history before it ever saw the Sun.
