ALMA Detects Extremely Abundant Alcohol in Interstellar Comet 3I/ATLAS
Comet 3I/ATLAS continues to make astonishing headlines, thanks to new findings from astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA), of which the U.S. National Science Foundation National Radio Astronomy Observatory (NSF NRAO) is a partner. This new research reveals that 3I/ATLAS is packed with an unusually large amount of the organic molecule methanol – more than almost all known comets in 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.”
Using ALMA’s Atacama Compact Array in Chile, on multiple dates in late 2025, the team observed 3I/ATLAS as it approached our Sun. As sunlight warmed its icy surface, 3I/ATLAS released gas and dust, forming a glowing halo (or coma) around its core. By analyzing this coma, astronomers revealed the chemical fingerprints of the material it is composed of, allowing them to study how objects might be made in another planetary system, without leaving our own.
The team focused on the faint submillimeter fingerprints of two molecules: methanol (CH₃OH), a type of alcohol, and hydrogen cyanide (HCN), a nitrogen-bearing organic molecule commonly seen in comets. The ALMA data reveal that 3I/ATLAS is heavily enriched in methanol compared to hydrogen cyanide, far beyond what is typically seen in comets born in our own solar system. On two observing dates, the team measured methanol‑to‑HCN ratios of about 70 and 120, placing 3I/ATLAS among the most methanol‑rich solar system comets ever studied.
These measurements imply that the icy material from 3I/ATLAS was formed by (or experienced) very different conditions than those that shape most comets in our own solar system. Previous work with the James Webb Space Telescope has shown that 3I/ATLAS had a coma dominated by carbon dioxide when it was far from the Sun, and these new ALMA results add methanol as another unusual detail in its chemical inventory.
ALMA’s high resolution for imaging also allowed the team to see how different molecules move away from the comet, revealing surprising differences between methanol and hydrogen cyanide. Hydrogen cyanide appears to come, for the most part, directly from the comet’s core, or nucleus, which is typical for comets in our solar system. Methanol, on the other hand, appears to come from both the nucleus AND from ice particles in the coma. These tiny, icy grains act like mini-comets: as the object moves closer to the Sun, where ice turns into gas, they also release methanol. Similar behavior has been observed in some solar system comets, but this is the first time the physics of such detailed outgassing has been traced in an interstellar object.
Comet 3I/ATLAS is only the third confirmed object ever seen passing through our solar system from interstellar space, after 1I/‘Oumuamua and 2I/Borisov. Observations of these objects also revealed unusual properties. As astronomers continue to discover and study more interstellar objects, our understanding of planet formation in other planetary systems continues to grow more interesting.
About NRAO
The National Radio Astronomy Observatory (NRAO) is a facility of the U.S. National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
About ALMA
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
Astrobiology, Astrochemistry,
