Interstellar Visitor 3I/ATLAS Shows Unexpected Methane Signature in JWST Observations

The object designated 3I/ATLAS traversed the inner solar system in late 2022, passing close to several planets before exiting on a hyperbolic trajectory. Its passage offered a unique opportunity to study material that originated beyond the Sun’s gravitational influence.

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Ground-based telescopes first identified 3I/ATLAS as an interstellar object based on its hyperbolic orbit. Subsequent monitoring captured its approach to perihelion in early December 2022, after which it receded toward the outer solar system. The object’s composition remained largely unknown until observations with the James Webb Space Telescope (JWST) were obtained.

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Using the Mid-Infrared Instrument (MIRI) aboard JWST, astronomers recorded thermal emission spectra in mid- and late-December, roughly two months after perihelion. The spectra revealed distinct emission features of methane (CH4), carbon dioxide (CO2), and water (H2O). Quantitatively, the methane-to-water ratio was markedly higher than that measured in typical solar-system comets, while CO2 was also abundant. The spatial distribution indicated that the gases were released after perihelion, suggesting a subsurface source beneath an insulating icy mantle. The analysis was published in *The Astrophysical Journal Letters*.

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The detection of methane constitutes the first confirmed identification of this volatile in an interstellar object. Because methane sublimates readily at low temperatures, its presence implies that it was trapped deep within the nucleus and only liberated as the object warmed. The elevated methane abundance, together with the strong CO2 signal, points to a formation environment with distinct chemical pathways compared with most comets that formed within the Solar System.

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The European Space Agency (ESA) highlighted the novelty of the observation, emphasizing that the methane detection “sniffed methane” from the object. ESA further noted that the combined findings “point to a very different formation environment and chemistry than the vast majority of comets that formed within our Solar System.” NASA’s commentary underscored methane’s volatility, explaining that its release after perihelion is consistent with a buried volatile layer beneath a thick ice shell.

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• “sniffed methane” — European Space Agency, press release

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• “Both these findings point to a very different formation environment and chemistry than the vast majority of comets that formed within our Solar System,” — European Space Agency, press release

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The current data set does not contain contradictory measurements of the methane abundance. However, the limited temporal coverage—observations only after perihelion—leaves uncertainties about the object’s pre-perihelion volatile inventory.

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The research team plans to model the thermal evolution of 3I/ATLAS to constrain the depth of volatile reservoirs. Additional spectroscopic studies of future interstellar visitors are expected to test whether the high methane content observed in 3I/ATLAS is typical of extrasolar planetesimals.

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