The interstellar comet 3I/ATLAS is releasing a remarkable amount of carbon-based chemicals, with new observations showing unusually high levels of methanol – a molecule central to early prebiotic chemistry and never previously detected in an object arriving from outside our solar system.
3I/ATLAS, only the third known interstellar visitor, is turning out to be strikingly different from the comets born in our own cosmic neighbourhood. As it moved toward the sun, scientists observed a rapidly expanding cloud of water vapour and gas around it, packed with far more carbon dioxide than is normally seen in typical solar system comets. Its light appeared noticeably redder as well, hinting at an unusual surface composition. Even more surprising, the comet began shedding gas while still far from the sun, suggesting it may not have approached another star for hundreds of millions of years – possibly not since it left its original star system.
A research team led by Martin Cordiner of NASA’s Goddard Space Flight Center used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to investigate the comet’s chemistry in detail. They found that 3I/ATLAS is emitting substantial amounts of hydrogen cyanide, along with even greater amounts of methanol. “Hydrogen cyanide and methanol usually appear only in small traces in our own comets,” Cordiner said. “But in this interstellar object, they seem to be unusually abundant.”
According to the team’s findings, hydrogen cyanide is emerging close to the comet’s rocky nucleus, at rates between 250 and 500 gram per second. Methanol is also present in the nucleus, but is being produced far more heavily throughout the comet’s coma — the extended halo of dust and gas that can stretch many kilometres from the core.
Methanol production is especially striking, clocking in at roughly 40 kilogram per second and accounting for about 8 per cent of all vapour coming from the comet. By comparison, typical solar system comets produce only around 2 per cent. The differing locations of these molecules suggest the nucleus of 3I/ATLAS may be compositionally uneven, offering scientists rare clues about how such an object might have formed in another star system.
Although methanol is a simple carbon-based compound, it acts as a crucial building block for more complex molecules tied to the origins of life. Cordiner said that such high methanol levels usually point to other important chemical reactions happening within the comet. “It’s very unlikely you can reach high chemical complexity without producing methanol along the way,” he added.
Researchers, including Josep Trigo-Rodríguez of the Institute of Space Sciences in Spain, have previously predicted that a metal-rich comet – one containing abundant iron – would generate large quantities of methanol. In such a scenario, heat from the sun would melt subsurface water, allowing it to react with iron-bearing minerals inside the nucleus and form methanol. Detecting methanol in the comet’s coma could, therefore, indicate that 3I/ATLAS is rich in metals, offering another clue to its ancient, alien origins.
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