We may never figure out where interstellar comet 3I/ATLAS came from

3I/ATLAS, an interstellar comet passing through our solar system, may have been radically transformed by cosmic rays over billions of years, changing its appearance so thoroughly that we may never be able to understand where it came from.

Since astronomers first discovered 3I/ATLAS in July, they have detailed some of its mysterious properties. These include levels of carbon dioxide in its coma – a plume of gas and dust – that are at least 16 times higher than those of typical comets in our solar system, making it one of the richest CO2-rich comets ever observed.

Some astronomers hoped that this might be an indication of the exotic star system from which 3I/ATLAS originated (or, more unlikely, extraterrestrial participation), but there could be a much simpler explanation.

Romain Maggiolo from the Royal Belgian Institute of Space Aeronomy in Uccle and colleagues argue that the high CO2 levels are best explained by the fact that the outer part of 3I/ATLAS has been radically altered by high-energy particles known as cosmic rays over billions of years.

“Somehow this process has been a little overlooked or perceived as a secondary process because it is very slow. But in the end, for objects like comets or interstellar objects, it has a strong effect,” says Maggiolo.

The researchers compared the 3I/ATLAS observations to laboratory studies in which cosmic rays hit ice made of water and carbon monoxide, which is believed to be similar to the ice that forms on comets. These studies show that this process creates large amounts of CO2 and also leaves behind a reddish, carbon-rich residue that astronomers also observed on the comet.

“Very slowly [cosmic rays] will break down molecules and produce reactive radicals, fragments of molecules that will recombine, and so they will slowly change their chemical composition. [comet’s] ice,” says Maggiolo.

This would be a major blow to our hopes of understanding where these comets come from, he said, since cosmic rays could destroy important evidence. Astronomers previously believed that interstellar comets like 3I/ATLAS were extremely well preserved and acted as cold fossils containing key information about other star systems, but we may have to be more careful about how much information we can glean from them.

The possibility of a satellite visiting the comet to directly sample material was ruled out due to its high speed through our solar system. But there is a glimmer of hope for recognizing the true nature of 3I/ATLAS.

The comet is currently passing close to the Sun, out of sight from Earth, but should reappear in December. This close passage could melt enough ice in the outer layer to expose the material underneath, protected from cosmic rays, Maggiolo said. But that depends on how much ice it has already lost since it entered our solar system, and on the thickness of its icy crust, which we don't currently know about, he says.

Cyriel Opitom at the University of Edinburgh, UK, say upcoming observations, both from the James Webb Space Telescope and ground-based telescopes, will be critical to finding more pristine material beneath the comet's surface. “We have a very interesting few months ahead of us,” she says.

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