Interstellar comet 3I/ATLAS contains exceptionally high levels of methanol, suggesting it formed under conditions different from most comets in our solar system. Observations by the Atacama Large Millimeter/submillimeter Array (ALMA) revealed this unusual chemical composition. The comet's methanol abundance exceeds that of nearly all known comets from our own solar system. This provides a rare opportunity to study the formation of icy bodies in another planetary system.
Astronomers used ALMA's Atacama Compact Array to observe 3I/ATLAS in late 2025 as it approached the Sun. Sunlight heated the comet's icy surface, releasing gas and dust that formed a bright cloud, or coma, around its core. By analyzing molecules in this coma, researchers identified the comet's chemical makeup. They focused on methanol (CH3OH) and hydrogen cyanide (HCN).
ALMA data showed that 3I/ATLAS has an unusually high methanol-to-hydrogen cyanide ratio. Ratios of approximately 70 and 120 were measured on two observation dates. These values place 3I/ATLAS among the most methanol-rich comets ever studied. This suggests the ice within 3I/ATLAS formed or was exposed to conditions unlike those experienced by most solar system comets.
Earlier observations with the James Webb Space Telescope indicated that 3I/ATLAS's coma was dominated by carbon dioxide when it was far from the Sun. The new ALMA measurements add abundant methanol to the comet's list of unusual chemical features. ALMA's high-resolution imaging also allowed scientists to track how different molecules are released from the comet.
Hydrogen cyanide primarily originates from the comet's central body, or nucleus. This behavior is typical for comets in our solar system. However, methanol appears to come from both the nucleus and from ice particles within the coma. These small icy grains act like miniature comets. As 3I/ATLAS nears the Sun, ice within these grains turns into gas, releasing additional methanol into the surrounding coma.
This is the first time researchers have traced the detailed physics of this type of outgassing in an object from interstellar space. Comet 3I/ATLAS is the third confirmed object from interstellar space to enter our solar system. The other two were 1I/'Oumuamua and 2I/Borisov. Each interstellar object offers insights into planetary systems beyond our own.
Related stories
New Giant Dinosaur Species Discovered in Brazil
A new giant dinosaur species, *Dasosaurus tocantinensis*, discovered in Brazil, reveals ancient connections between South America and Europe.
Earth's Magnetic Field Used to Search for Dark Matter Signals
Scientists used Earth's magnetic field and atmosphere as a giant detector to search for ultralight dark matter particles, improving axion limits and finding mysterious dark photon signals.
Earth Used as Dark Matter Detector, Reveals Mysterious Signals
Scientists used Earth's magnetic field as a giant dark matter detector, setting new limits on axion interactions and finding mysterious dark photon signal candidates.