How unusual is this object? Two separate observing sessions in late 2025 supplied the number: methanol-to-hydrogen-cyanide ratios of roughly 70 and 120.
Those aren’t borderline figures. They place 3I/ATLAS in the company of the most methanol-rich comets on record, and the benchmark group covers essentially every comet a radio dish has ever been aimed at within our own solar system.
The data came out of the Atacama Large Millimeter/submillimeter Array in Chile, using its Atacama Compact Array in particular. ALMA counts the U.S. National Science Foundation National Radio Astronomy Observatory among its partners.
What a fingerprint from another star actually looks like
"Observing 3I/ATLAS is like taking a fingerprint from another solar system," said Nathan Roth, lead author on this research and a professor with American University. "The details reveal what it’s made of, and it’s bursting with methanol in a way we just don’t usually see in comets in our own solar system."
The underlying idea is simple; pulling it off is not. Through late 2025, as the comet closed in on the Sun, solar heat worked on its icy exterior. Gas and dust streamed away, building the bright envelope that surrounds the core — the coma.
Decode the molecules sitting in that coma and you have the comet’s recipe. Since 3I/ATLAS originated beyond our solar system, that recipe belongs to something put together in an entirely different place. And no probe had to go fetch it.
What the team hunted for were the faint submillimeter signatures of two specific molecules: methanol, a form of alcohol, and hydrogen cyanide, a nitrogen-bearing organic compound that shows up in comets as a matter of course.
The ice didn’t form the way ours did
An imbalance that severe narrows the possibilities. Either the ice locked inside 3I/ATLAS came together in conditions unlike anything most solar system comets went through, or something subjected it to those conditions after the fact.
Nor is this the comet’s only quirk. Previous work with the James Webb Space Telescope caught a coma ruled by carbon dioxide while 3I/ATLAS still sat far from the Sun. Plentiful methanol is the newest entry on that list.
Two molecules, two very different exits
The ratio is striking, but this next detail is the one I keep coming back to. ALMA’s imaging resolution allowed astronomers to pin down not only which molecules were there but where each was escaping from — and the two behave in completely different ways.
Hydrogen cyanide largely streams off the nucleus, the body at the comet’s center. That’s by the book, exactly what solar system comets do.
Methanol breaks the pattern. Its source appears to be the nucleus plus ice particles drifting inside the coma itself.
Mini-comets inside the comet
Think of those icy grains as comets in miniature. With 3I/ATLAS drawing nearer to the Sun and temperatures rising, ice within the grains vaporizes and releases still more methanol into the coma.
The process itself isn’t novel — astronomers have seen it play out in comets native to our solar system. The novelty is mapping the fine-grained physics of that outgassing on an object born between the stars. Nobody had managed that until now.
Only three interstellar objects have been confirmed entering our solar system, and 3I/ATLAS is the latest, following 1I/’Oumuamua and 2I/Borisov. Those first two visitors each showed peculiarities of their own, a track record that increasingly reads as a pattern in need of explanation rather than chance.
Each interstellar arrival offers another opportunity to measure our solar system against a planetary system elsewhere in the galaxy. Three data points hardly constitute a sample. Still, if the question is whether the chemistry behind our comets is standard or an outlier, the methanol figures from 3I/ATLAS are exactly the sort of measurement that begins to answer it.