Webb Detects Methane on Interstellar Comet 3I/ATLAS, Confirms Unusually High CO2
Updated
Updated · European Space Agency · Jun 1
Webb Detects Methane on Interstellar Comet 3I/ATLAS, Confirms Unusually High CO2
6 articles · Updated · European Space Agency · Jun 1
Two December Webb observations captured the first direct methane detection on an interstellar visitor and showed 3I/ATLAS remained unusually rich in carbon dioxide.
Methane appeared only after the comet’s close pass by the Sun, suggesting heat penetrated its icy outer shell and released gas that had been buried below the surface.
At 330 million km and then 380 million km from the Sun, Webb’s MIRI spectrometer mapped water, carbon dioxide and methane, finding water spread through the coma while CO2 and methane stayed concentrated near the nucleus.
The methane-to-water ratio is rare among Solar System comets, and the comet emits far more CO2 relative to water than typical local comets, pointing to a very different formation environment and chemistry.
The findings, published in The Astrophysical Journal Letters, give Webb its first chemical fingerprint of an interstellar object as 3I/ATLAS heads back out of the Solar System.
With its layered interior and strange gases, does this visitor reveal a different way to build a solar system?
This interstellar comet's water is unlike anything in our solar system. What kind of star could have created it?
We have its chemical fingerprint, but why did this ancient interstellar traveler lack a prominent tail during its visit?
3I/ATLAS: Unveiling the Largest and Brightest Interstellar Comet and Its Implications for Planetary Science
Overview
In July 2025, astronomers discovered 3I/ATLAS, the third confirmed interstellar object, which quickly showed clear signs of being a comet with a bright coma and tail. After its formal designation as C/2025 N1 (ATLAS), it reached its closest point to the Sun in October 2025 and passed Jupiter before heading back into interstellar space by June 2026. The arrival of 3I/ATLAS sparked public interest and speculation, but scientific observations have consistently shown it is a natural comet, with no evidence suggesting otherwise. This discovery highlights the excitement and importance of studying objects from beyond our solar system.