Webb Detects Methane on Interstellar Comet 3I/ATLAS as Carbon Dioxide Levels Run Exceptionally High
Updated
Updated · ScienceDaily · Jun 4
Webb Detects Methane on Interstellar Comet 3I/ATLAS as Carbon Dioxide Levels Run Exceptionally High
3 articles · Updated · ScienceDaily · Jun 4
Summary
NASA's Webb telescope captured the first mid-infrared chemical fingerprint of an interstellar object, directly detecting methane on comet 3I/ATLAS and confirming unusually high carbon dioxide.
Two observing runs on Dec. 15-16 and Dec. 27 found methane appearing only after the comet passed the Sun, suggesting solar heating reached deeper icy layers that had hidden the volatile gas.
The methane-to-water ratio was far above what is typical for solar-system comets, and carbon dioxide relative to water also greatly exceeded normal levels, pointing to formation in a very different chemical environment.
At 205 million and then 236 million miles from the Sun, Webb also saw gas production drop sharply as 3I/ATLAS cooled, with water declining fastest because it is less volatile than methane or carbon dioxide.
The results, published in The Astrophysical Journal Letters, show how Webb's MIRI spectrometer can identify and map gases around rare visitors from other star systems.
What secrets of a long-dead star system does this 12-billion-year-old comet's unique chemistry reveal about the galaxy's past?
Could ancient interstellar comets like this one be carrying the chemical blueprints for life from across our galaxy?
3I/ATLAS—A Third Interstellar Comet with Unique Methane and Carbon Dioxide Ratios Challenges Planetary Formation Models
Overview
In December 2025, astronomers confirmed 3I/ATLAS as the third interstellar object to pass through our Solar System. These rare visitors, originally formed around other stars and ejected by dynamic interactions, offer scientists unique opportunities to study extrasolar small bodies. The James Webb Space Telescope quickly revealed that 3I/ATLAS has a chemical composition unlike anything seen before, sparking excitement in the scientific community. Its brief journey through our Solar System provides crucial insights into how planetesimals form across the galaxy, allowing researchers to compare and contrast the building blocks of planetary systems beyond our own.