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The interstellar comet 3I/ATLAS is belching out carbon-rich chemical compounds at higher rates than almost any other comet in our solar system. One of these compounds is methanol, a key ingredient in prebiotic chemistry that hasn’t been seen in other interstellar objects.

3I/ATLAS, which is only the third visitor to our solar system from elsewhere in the galaxy, appears to be quite unlike any comet from our own galactic neighbourhood. As it travelled towards the sun, an envelope of water vapour and gas rapidly grew around it, which also contained much greater amounts of carbon dioxide than we see in typical solar system comets. The comet’s light also appeared to be much redder than is typical, indicating a possible unusual surface chemistry, and it began releasing its gases while relatively far away from the sun, an indication that it might not have passed close to another star for hundreds of millions of years, or since it left its home star system.

Now, at NASA’s Goddard Space Flight Center in Maryland and his colleagues have used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to discover that 3I/ATLAS is producing significant amounts of hydrogen cyanide gas, and even larger amounts of gaseous methanol. “Molecules like hydrogen cyanide and methanol are at trace abundances and not the dominant constituents of our own comets,” says Cordiner. “Here we see that, actually, in this alien comet they’re very abundant.”

Cordiner and his team found the hydrogen cyanide gas was coming from relatively close to the rocky core of the comet, and was being produced in quantities of around a quarter to a half a kilogram per second. Methanol was also found in the core, but it also appeared to be produced in significant quantities in the comet’s coma, which is the long tail of dust and gas that is many kilometres away from the comet itself.

Methanol appeared in much greater quantities than the hydrogen cyanide – around 40 kilograms per second – and makes up around 8 per cent of the total vapour coming from the comet, compared with around 2 per cent for standard solar system comets. The differences in location for these two molecules also suggests that the comet’s nucleus is not uniform, which could eventually tell us about how it formed, says Cordiner.

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While methanol is a relatively simple carbon-containing compound, it is a key stepping stone to producing more complex molecules essential for life, says Cordiner, and would likely be produced in high quantities when other chemical reactions that produce these molecules are occurring. “It seems really chemically implausible that you could go on a path to very high chemical complexity without producing methanol,” says Cordiner.

at the Institute of Space Sciences in Spain and his colleagues have predicted that a comet high in metals like iron should also produce relatively large amounts of methanol, because liquid water, freed up by the sun’s heat, would begin pushing through the comet’s nucleus and chemically reacting with its iron compounds – a process that should create methanol. As such, finding evidence of methanol in the comet’s coma could be a sign that the comet is relatively metal rich, he says.