A comet from another star system just surprised astronomers with hidden methane and exotic chemistry unlike almost anything seen in our solar system.
NASA's James Webb Space Telescope captured the first mid-infrared chemical fingerprint of an interstellar object, revealing that comet 3I/ATLAS contains methane gas and exceptionally high levels of carbon dioxide. The findings were published in The Astrophysical Journal Letters.
Methane emerged only after the comet passed the Sun
For the first time, scientists directly identified methane gas on a visitor from another star system. Methane is extremely volatile, meaning it can quickly turn from solid ice into gas. Its appearance only after the comet had already passed close to the Sun suggests the methane was buried beneath the surface.
The comet's upper layers likely shielded the methane ice until solar heating penetrated deeper into the icy interior. The ratio of methane to water also surprised researchers. It is much higher than what is typically seen in comets from our own solar system, with only a handful of known examples showing similar characteristics.
Carbon dioxide levels far exceed solar system comets
The observations also confirmed another unusual feature of 3I/ATLAS. The comet releases exceptionally large amounts of carbon dioxide relative to water, far exceeding the levels commonly measured in solar system comets.
Together, the methane and carbon dioxide measurements point to a formation history that differs significantly from that of most comets that originated around our Sun. The results suggest that 3I/ATLAS formed in a very different chemical environment before beginning its journey through interstellar space.
Researchers used Webb's MIRI instrument during two observing sessions after the comet passed its closest point to the Sun. The first set of observations took place on December 15 and 16, when 3I/ATLAS was about 205 million miles from the Sun. A second round followed on December 27, with the comet about 236 million miles away.
Webb also tracked how the comet's activity changed as it moved farther from the Sun. Scientists observed a sharp decline in the production of gases, with water showing the steepest decrease. This behavior is expected as the comet receives less solar energy. As temperatures fall, less ice vaporizes from the surface and near-surface layers. Water is less volatile than methane or carbon dioxide, which means its gas production shuts down more quickly as the comet cools.
The chemical fingerprint of 3I/ATLAS now stands as the most detailed ever obtained for an interstellar object, offering a rare direct look at material formed in another planetary system.
