NASA's James Webb Space Telescope Detects Methane On Interstellar Comet For First Time, Reveals Unusual Chemistry

NASA's James Webb Space Telescope has collected its first mid-infrared chemical fingerprint of an interstellar object during a recent revisit to comet 3I/ATLAS. The findings were recently published in The Astrophysical Journal Letters. The observations were carried out using Webb's Mid-Infrared Instrument (MIRI) on two separate dates as the comet traveled back out of the solar system after passing around the Sun. The first observation took place from December 15 to 16, when the comet was about 205 million miles (329 million kilometres) from the Sun. A second observation followed on December 27, when the comet was about 236 million miles (379 million kilometres) from the Sun, reported NASA.

For the first time on an interstellar visitor, Webb directly detected methane gas. Scientists said methane is highly volatile and easily changes from solid ice into gas. Its delayed appearance in comet 3I/ATLAS suggests that it was buried beneath the comet's surface layer and remained protected from sublimation until heat from the comet's close approach to the Sun reached deeper icy regions. Researchers also found that the amount of methane relative to water was surprisingly high, with very few similar examples known in our solar system.

The observations further confirmed that comet 3I/ATLAS remains unusually rich in carbon dioxide. The comet was found to release much more carbon dioxide relative to water than is typically seen in comets from the solar system.

According to the researchers, both findings indicate a formation environment and chemical composition that differ from those of most comets formed within our solar system.

Webb also recorded a sharp decline in gas production as comet 3I/ATLAS moved farther away from the Sun. Water showed the most noticeable drop. Scientists said this is expected because the comet receives less heat as it travels away from the Sun, causing its surface to cool and reducing the amount of ice that turns into gas. Water, which is less volatile than methane or carbon dioxide, stops producing gas more quickly under these conditions.

The observations were made using MIRI's Medium Resolution Spectrometer, an instrument designed to separate infrared light into its component wavelengths. The spectrometer functions as an integral field unit, providing a spectrum at every point within a small area of the sky. This allowed the team to identify the gases present and map their distribution around the comet's nucleus at the same time.