SOLAR SYSTEM — The James Webb Space Telescope directly detected methane gas on interstellar Comet 3I/ATLAS for the first time. The detection indicates chemical differences compared to comets originating in the Solar System.
The telescope collected a mid-infrared chemical fingerprint of the comet using its Mid-Infrared Instrument (MIRI) on December 15-16 and December 27. During the first observation, the comet was approximately 330 million kilometers from the Sun, and during the second observation, it was approximately 380 million kilometers from the Sun.
Scientists attribute the methane detection to gas that was buried beneath the comet's surface. This gas remained protected until solar heating penetrated the outer icy layers. The ratio of methane to water detected in the comet is higher than that typically found in comets from the Solar System. The comet also releases carbon dioxide at a higher rate relative to water than typical Solar System comets. These measurements of methane and carbon dioxide suggest a chemical formation environment that differs from most comets formed within the Solar System.
MIRI's Medium Resolution Spectrometer separated infrared light into wavelengths to identify gases present around the object. The spectrometer captured a spectrum at every point in a small patch of sky. This process mapped the gas distribution around the nucleus of the comet. Carbon dioxide and methane were concentrated closer to the comet's nucleus than water vapor. Water vapor observed around the comet extends beyond the nucleus due to release from icy grains in the surrounding coma. Gas production decreased as the comet moved farther from the Sun. Water vapor showed the steepest reduction in production compared to methane and carbon dioxide as temperatures dropped.
The observational results were published in The Astrophysical Journal Letters. The study details the first direct detection of methane gas on an interstellar comet. The published study's authors include Matthew Belyakov, Ian Wong, Bryce T. Bolin, M. Ryleigh Davis, Steven J. Bromley, Carey M. Lisse, and Michael E. Brown.