3I/ATLAS is chemically unusual, with methanol raging not just in the nucleus

Scientists have discovered a strange chemical signature in 3I/ATLAS, different from anything ever seen in another comet. Using the Atacama Compact Array of the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile, a group of astrophysicists spotted methanol (CH3OH) and later hydrogen cyanide (HCN) being released from the interstellar comet. The observations were made from late August through October 2025. While both elements are commonly found in the frigid conditions of space, what stands out here is the ratio of methanol to HCN. The researchers note in their study that hydrogen cyanide appeared to be coming directly from the comet’s nucleus. This is typical of outgassing seen in comets. But methanol was spread towards a greater area, and was not just coming from the nucleus, but was also seemingly being produced farther out in the comet’s coma.

Methanol was also increasing as 3I/ATLAS moved closer to the Sun. From about 2.6 to 1.7 AU from the Sun, the chemical shot up rapidly. "The molecules showed outgassing patterns distinct from one another, with HCN production being depleted in the sunward hemisphere of the coma, whereas CH3OH was enhanced in that direction," they wrote in the study available on the arXiv server.

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3I/ATLAS chemical makeup is an anomaly

When measured at radio wavelengths, the methanol to hydrogen cyanide ratio was extremely high, the biggest seen in any comet to date. The authors wrote that this is much higher than what is commonly seen in the solar system comets. The only other comet that has had a higher ratio of these two chemicals was C/2016 R2 (PanSTARRS). It is also classified as an anolamlous comet because of being rich in carbon monoxide (CO) but having less hydrogen cyanide, which caused its coma to turn blue.

The study suggests that methanol is likely not just being released from the nucleus of 3I/ATLAS, but is possibly being made in the coma itself, something not seen in regular comets. Besides, the presence of more methanol as compared to HCN makes 3I/ATLAS chemically unusual, an anomaly.