Comet 3I/ATLAS is continuing to surprise astronomers. New observations from the Atacama Large Millimeter/submillimeter Array (ALMA), of which the U.S. National Science Foundation National Radio Astronomy Observatory (NSF NRAO) is a partner, show that the interstellar visitor contains an unusually large amount of methanol. In fact, its methanol abundance exceeds that seen in almost all known comets from our own solar system.
“Observing 3I/ATLAS is like taking a fingerprint from another solar system,” shares 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.”
A Chemical Fingerprint From Another Solar System
The research team used ALMA’s Atacama Compact Array in Chile to observe 3I/ATLAS on several dates in late 2025 as it moved closer to the Sun. As sunlight heated the comet’s icy surface, gas and dust escaped and created a bright cloud (or coma) around its core.
By studying the molecules in this coma, astronomers could identify the chemical makeup of material carried by the comet. Because 3I/ATLAS originated outside our solar system, those measurements provide a rare opportunity to investigate how small icy bodies may form in another planetary system without traveling beyond our own.
The scientists concentrated on the faint submillimeter signatures of two molecules: methanol (CH3OH), a type of alcohol, and hydrogen cyanide (HCN), a nitrogen-bearing organic molecule commonly seen in comets.
ALMA revealed that 3I/ATLAS contains an unusually high amount of methanol relative to hydrogen cyanide. On two observation dates, researchers measured methanol-to-HCN ratios of roughly 70 and 120. Those values place 3I/ATLAS among the most methanol-rich solar system comets ever studied.
Unusual Chemistry Points to Different Origins
The measurements suggest that the ice inside 3I/ATLAS formed under, or was later exposed to, conditions very different from those experienced by most comets in our solar system.
Earlier observations with the James Webb Space Telescope found that 3I/ATLAS had a coma dominated by carbon dioxide while it was still far from the Sun. The new ALMA measurements now add abundant methanol to the growing list of unusual features in the comet’s chemical makeup.
ALMA’s high resolution for imaging also allowed astronomers to track how different molecules are released and travel away from the comet. The observations revealed a striking contrast between methanol and hydrogen cyanide.
Hydrogen cyanide appears to originate mostly from the comet’s central body, or nucleus. That behavior is common among comets in our solar system. Methanol, however, appears to come from both the nucleus AND from ice particles floating within the coma.
Tiny Ice Grains Act Like Mini-Comets
These small icy grains effectively behave like miniature comets. As 3I/ATLAS approaches the Sun and temperatures rise, ice within the grains turns into gas and releases additional methanol into the surrounding coma.
Astronomers have seen similar behavior in some comets from our own solar system. However, this is the first time researchers have traced the detailed physics of this type of outgassing in an object that originated in interstellar space.
Comet 3I/ATLAS is only the third confirmed object ever observed entering our solar system from interstellar space, following 1I/’Oumuamua and 2I/Borisov. Studies of those earlier visitors also uncovered unusual characteristics.
As astronomers find and examine more interstellar objects, each one offers another opportunity to compare our solar system with planetary systems elsewhere in the galaxy. The unusual chemistry of 3I/ATLAS is adding another intriguing piece to the broader picture of how planets, comets, and other small bodies form around distant stars.
