ALMA observations revealed an unusually high ratio of methanol to hydrogen cyanide, suggesting that some of the methanol was also likely emitted from ice grains in the comet's mantle. The finding adds another unusual chemical signature for a body formed in another planetary system.
Every interstellar object that passes through the solar system is a rare opportunity to examine material formed around another star. Now observations with the ALMA radio telescope array in Chile have revealed an unusual chemical feature in comet 3I/ATLAS: it is very rich in methanol, an organic molecule from the alcohol family that is also found in local comets, but does not usually appear in such high proportions with hydrogen cyanide.
"Looking at 3I/ATLAS is like taking a fingerprint from another solar system," said Professor Nathan Roth of American University, the lead author of the study. He said the comet's chemical composition is different from that of most comets formed in our solar system.
Chemical fingerprint from another system
3I/ATLAS is only the third interstellar object confirmed to have arrived in the solar system, after 1I/Oumuamua and 2I/Borisov. Unlike a long-period comet originating at the edge of the solar system, it formed in another planetary system, was ejected from it, and passed here on an open orbit on its way back to interstellar space.
The researchers used the ALMA compact array to observe the comet on several occasions during 2025, as it approached the Sun. Heating of its icy surface released gas and dust, creating a coma around the nucleus—a thin envelope in which the spectral signatures of different molecules can be detected.
The team focused on two molecules: methanol (CH₃OH) and hydrogen cyanide (HCN). Both are familiar to cometary researchers, but the ratio between them in 3I/ATLAS was unusual. Two measurements yielded methanol-to-hydrogen cyanide ratios of about 79 and 124. These values place the comet among the richest bodies in methanol relative to HCN ever studied by radio methods.
Lots of methanol — but no evidence of life
Methanol is defined as an organic molecule because it contains carbon, but its presence alone does not indicate life. Its importance here is as an indicator of the chemical and thermal processes that the icy material from which the comet was formed underwent.
The unusual ratio may indicate that the ice in 3I/ATLAS was formed, or later modified, under conditions different from those that shaped most comets in the solar system. Previous observations with the James Webb Space Telescope showed that when the comet was farther from the sun, its atmosphere was particularly rich in carbon dioxide. The ALMA results add methanol to the list of unusual features in its composition.
Ice grains behave like mini-comets
ALMA's high-resolution imaging showed that the methanol and hydrogen cyanide are not emitted in the same way. The hydrogen cyanide appears to come primarily directly from the comet's nucleus—a pattern also known from comets in the Solar System.
The methanol, on the other hand, appears to be emitted both from the nucleus and from a more extended source within the comet. The preferred explanation is that small ice grains were emitted from the nucleus and continued to heat up afterwards. Each such grain essentially behaves like a mini-comet: the ice that the mirror passes through releases methanol into the gas envelope surrounding the comet.
Similar activity has been observed in local comets before, but this is the first time that this kind of detailed outgassing physics has been mapped in an interstellar body.
A brief window into building planetary systems
Because 3I/ATLAS formed outside the solar system, any measurements of it serve as a remote sample of material from another planetary system. Astronomers do not yet know whether the unusual composition reflects an exceptionally cold formation region, a different chemical makeup, prolonged processing by cosmic rays, or a combination of factors.
It is also impossible to conclude from just three objects what a “typical interstellar comet” is. 3I/ATLAS may also be an anomaly in relation to the comet population in the galaxy. As more interstellar visitors are discovered in the future, it will be possible to compare them and learn how different planetary systems are from one another.
The study was published inThe Astrophysical Journal LettersFor researchers, the comet is not just a temporary guest, but a chemical messenger that carries information about the place where it was formed — a place that is currently inaccessible by spacecraft.
Questions and Answers
What is 3I/ATLAS?
This is the third interstellar object confirmed to have passed through the solar system. Its trajectory indicates that it came from outside the solar system and will continue outward.
What did ALMA observations reveal?
They discovered an unusually high ratio of methanol to hydrogen cyanide, as well as a difference in the way the two molecules are emitted from the comet.
Why is the abundance of methanol important?
It may reflect different formation or processing conditions than those of most solar system comets, and therefore provides information about a different planetary system.
Does methanol indicate life?
No. Methanol is an organic molecule, but its discovery on a comet is not evidence of life.
Where does methanol come from?
The data indicate emission from both the nucleus and small ice grains in the chamber, which behave like mini-comets.
Why is it difficult to know whether 3I/ATLAS represents interstellar comets at all?
So far, only three interstellar objects have been confirmed, so the sample is too small to determine typical characteristics.
For sources: NRAO Announcement; The scientific article.
More of the topic in Hayadan:
- The deuterium-rich water of
- Webb Detects Methane in Interstellar Comet
- Color images reveal greenish glow of interstellar comet 3I/ATLAS
- What does the interstellar comet 3I/ATLAS teach us about other solar systems?
- NASA Sums Up Interstellar Comet 3I/ATLAS Passage: A “Frozen Fossil” from an Alien Solar System
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