James Webb Telescope Finds Methane on Interstellar Comet 3I/ATLAS

The James Webb Space Telescope has captured a mid-infrared chemical signature from comet 3I/ATLAS, marking the first time methane has been detected on an interstellar visitor. These findings, published in The Astrophysical Journal Letters, reveal a chemical composition that sets the object apart from typical solar system comets.

Observations conducted by Webb's Mid-Infrared Instrument (MIRI) occurred in December as the comet exited the solar system. By analyzing light at distances of 205 million and 236 million miles from the Sun, researchers identified methane gas beneath the comet's surface. Because methane is highly volatile, its delayed release suggests it remained protected as solid ice until solar heat penetrated the comet's outer layers.

The data also highlights an unusually high ratio of carbon dioxide to water compared to local comets. This chemical profile points to a distinct origin, suggesting 3I/ATLAS formed in an environment unlike those that created the objects within our own solar system. As the comet moved away from the Sun, gas production dropped sharply, with water emissions cooling and declining more rapidly than the more volatile gases. By utilizing the Medium Resolution Spectrometer, the team mapped the distribution of these gases around the nucleus, providing a rare look at the complex chemistry of an interstellar traveler.