ALMA Finds Interstellar Comet 3I/ATLAS Loaded With Methanol

The ALMA radio telescope array in Chile’s Atacama desert has measured an unusually high methanol content in interstellar comet 3I/ATLAS, an organic compound from the alcohol family, in an amount rarely seen in comets. The measurement was made by a team using a section of the array known as the Atacama Compact Array, and the findings were published by the National Radio Astronomy Observatory (NRAO), one of the institutions that operates ALMA. The paper’s lead author is Nathan Roth, a professor at American University in Washington.

The finding builds on an earlier discovery about the same comet. The James Webb Space Telescope, while 3I/ATLAS was still far from the Sun, recorded a coma dominated by carbon dioxide. ALMA’s new measurements now add methanol as another unusual feature of the object’s chemical makeup. The comet was discovered on July 1, 2025, and is only the third confirmed interstellar visitor to the Solar System, after 1I/Oumuamua in 2017 and comet 2I/Borisov in 2019.

How the Measurement Was Made

The team observed 3I/ATLAS on several dates during late 2025, as the comet approached the Sun and solar heat vaporized material from its icy surface, forming a coma, the cloud of gas and dust that surrounds a comet’s nucleus. The measurements targeted faint submillimeter signatures from two molecules: methanol (CH3OH) and hydrogen cyanide (HCN), a compound commonly found in comets within our own Solar System.

The ratio of methanol to hydrogen cyanide came out at roughly 70 on one observation date and roughly 120 on another. That places 3I/ATLAS among the comets with the highest methanol ratio ever measured, Solar System comets included.

ALMA’s high resolution let the team trace where each gas was coming from. Hydrogen cyanide was released mainly from the comet’s nucleus itself, typical behavior for a comet. Methanol, however, came from both the nucleus and tiny icy grains scattered through the coma. When heated, these grains act like miniature comets in their own right and become sources of gas themselves. Similar behavior has been observed before in some Solar System comets, but this is the first time the physics of that kind of vaporization has been tracked in detail for an interstellar object.

Observing 3I/ATLAS is like taking a fingerprint from another planetary system. The details reveal what it's made of, and it's full of methanol.

Nathan Roth, the paper’s lead author, said this in a statement from the National Radio Astronomy Observatory.

What the High Methanol Content Means for the Comet’s Origin

This ratio of methanol to hydrogen cyanide suggests that the ice in 3I/ATLAS formed, or was later exposed to, conditions substantially different from those that shaped most comets in our own Solar System. Scientists expect the Vera Rubin Observatory, which is just beginning operations, to turn up more interstellar comets in the coming years, letting researchers compare these measurements across more objects and pin down more precisely what counts as an exception and what counts as the rule.

The paper was published in The Astrophysical Journal Letters, volume 999, issue 2, article number L32, identifier 10.3847/2041-8213/ae433b.

The measurements alone, however, do not prove exactly where within 3I/ATLAS’s parent planetary system its ice formed, nor what temperatures or radiation characterized the star it formed around. The ratio of the two molecules on just two observation dates shows a difference in chemical composition compared with Solar System comets, but it does not support a conclusion about the exact formation mechanism without further observations as the comet keeps moving away from the Sun.