ALMA Finds 3I/ATLAS Is Bursting With Methanol

Interstellar comet 3I/ATLAS just handed astronomers a chemical surprise. New observations from the Atacama Large Millimeter/submillimeter Array (ALMA) reveal that this visitor from another star system is loaded with methanol — far more than nearly any comet ever studied in our solar system. The findings, published in The Astrophysical Journal Letters by Nathan X. Roth and colleagues, offer the first detailed chemical fingerprint of a comet born around an alien star, and it looks nothing like what we see at home.

How ALMA Mapped an Interstellar Comet

3I/ATLAS was discovered in July 2025 by the Asteroid Terrestrial-impact Last Alert System, making it only the third confirmed object to enter our solar system from interstellar space — after 1I/'Oumuamua in 2017 and 2I/Borisov in 2019. Unlike 'Oumuamua, which was small and gas-poor, 3I/ATLAS is an active comet that began releasing gas and dust as it warmed on its approach to the Sun.

Roth's team used ALMA's Atacama Compact Array (also known as the Morita Array) in Chile to observe 3I/ATLAS on four dates between August and October 2025 — specifically UT August 28, September 18, September 22, and October 1. These observations spanned pre-perihelion heliocentric distances of roughly 2.6 to 1.7 au, catching the comet as solar heating progressively intensified its outgassing.

By tuning to the specific radio frequencies where methanol (CH₃OH) and hydrogen cyanide (HCN) emit, the team was able to map the distribution of both molecules across the comet's coma — the glowing halo of gas and dust surrounding its nucleus.

A Record-Breaking Methanol Abundance

The headline result: 3I/ATLAS is extraordinarily rich in methanol. On two dates when both molecules were detected, the team measured methanol-to-HCN production rate ratios of approximately 124 and 79. To put that in perspective, most solar system comets show ratios between about 1 and 30.

These values place 3I/ATLAS among the most methanol-enriched comets ever studied. Only one solar system comet, the anomalous C/2016 R2 (PanSTARRS), has shown higher methanol-to-HCN ratios — and R2 is itself a deeply unusual object with an atmosphere dominated by carbon monoxide rather than water.

Methanol is not rare in space. It forms on the surfaces of icy dust grains in cold interstellar clouds as carbon monoxide reacts with hydrogen atoms. It is commonly incorporated into comets during planet formation. What makes 3I/ATLAS remarkable is not that it contains methanol, but how much it contains relative to other volatile species like HCN.

Two Molecules, Two Outgassing Patterns

Beyond the sheer abundance of methanol, ALMA's spatial maps revealed that the two molecules behave very differently as they escape the comet.

Hydrogen cyanide appears to originate mainly from the nucleus, which is typical for solar system comets. Its emission was depleted in the sunward hemisphere of the coma, suggesting that the HCN-producing ice on the sun-facing surface may have been partly exhausted or buried under a lag deposit.

Methanol, on the other hand, was enhanced in the sunward direction. The team interprets this as evidence that methanol is released not only from the nucleus but also from icy grains in the coma itself. As sunlight warms these tiny particles — essentially miniature comets drifting through the coma — the ice they contain sublimates and releases methanol into the surrounding gas. This "extended source" of methanol boosts the overall production rate and helps explain the extreme ratios.

This dual outgassing behavior has been seen in some solar system comets, but the degree of asymmetry in 3I/ATLAS is striking and adds another piece to the puzzle of how this object differs from comets born closer to home.

A Chemical Fingerprint From Another Solar System

Every comet is a frozen archive of the conditions under which it formed. The ice, dust, and volatile molecules locked inside a comet's nucleus preserve a record of the temperature, radiation environment, and chemical inventory of the protoplanetary disk where it was assembled — billions of years ago.

For solar system comets, astronomers have spent decades cataloguing these chemical fingerprints. The relative abundances of molecules like water, carbon monoxide, methanol, HCN, and others vary from comet to comet, but they cluster within a recognizable range that reflects the conditions in our own protoplanetary disk.

3I/ATLAS breaks that pattern. Its extreme methanol enrichment suggests it formed under conditions distinctly different from those that shaped most comets in our solar system. The icy material may have been processed in a colder, more CO-rich, or more irradiation-shielded environment than the regions where our Oort Cloud and Kuiper Belt comets originated.

Comparing 3I/ATLAS with the previous interstellar comet, 2I/Borisov — which showed unusually high carbon monoxide levels — astronomers are beginning to see that interstellar visitors carry genuinely diverse chemistry. Each one is a probe into a different planetary system's formation history, and no two have looked alike so far.

What Comes Next for 3I/ATLAS

3I/ATLAS continues its journey through the inner solar system, and astronomers worldwide are racing to gather as much data as possible before it fades from view. ALMA and other facilities are expected to continue monitoring the comet's outgassing as it passes perihelion and begins its outbound journey.

Future observations may reveal additional molecular species — such as formaldehyde, carbon monoxide, or more complex organic molecules — that would further refine the chemical portrait of this interstellar traveler. The James Webb Space Telescope (JWST) is also expected to contribute infrared spectroscopy that complements ALMA's radio-wavelength view.

For anyone wanting to follow 3I/ATLAS in real time, our live orbit tracker shows the comet's current position in the solar system, and our observing guide provides tips on when and where to look for it in the night sky. The science page has the latest data on the comet's physical properties and trajectory.

The methanol discovery is more than a curiosity — it is a concrete demonstration that the chemistry of planet formation varies across the galaxy. As survey telescopes like the Vera C. Rubin Observatory come online, astronomers expect to find many more interstellar visitors. Each one will carry its own chemical story, and together they will build a map of how planetary systems — and the building blocks of life — differ from star to star.