Interstellar Ice: The Chemical Fingerprint of 3I/ATLAS

Observations with the Atacama Large Millimeter/submillimeter Array reveal that the interstellar comet 3I/ATLAS contains an unusually high fraction of semi-heavy water—over 30× typical solar-system levels. This isotopic anomaly points to formation in extreme cold (below ~−406°F), implying a very different birth environment.

By reading these molecular ratios, astronomers treat such visitors as preserved records of distant planetary systems, offering direct constraints on how chemistry varies across the galaxy.

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