Summary
NASA’s James Webb Space Telescope used its MIRI Mid-Infrared Instrument to observe ATLAS on two post-perihelion dates (December 15–16 and December 27, 2025) as the object receded from the Sun. The observations yielded the first direct detection of methane gas on any interstellar object, along with confirmation of anomalously high carbon dioxide relative to water. Both methane and CO₂ abundances depart significantly from typical solar-system comets, pointing to a chemically distinct formation environment. A sharp decline in gas production with increasing heliocentric distance — most pronounced for water — was also observed, consistent with expected thermal behavior. Results were published in The Astrophysical Journal Letters.
Key Claims
- First direct detection of methane on an interstellar object; methane was buried in the subsurface until solar heat reached deeper icy layers after perihelion. (Fisher / NASA Science Blog)
- Methane-to-water ratio is “surprisingly high” with few analogs among solar-system comets. (Fisher / NASA Science Blog)
- 3I/ATLAS confirmed to be unusually rich in carbon dioxide relative to water compared to typical solar-system comets, consistent with earlier observations. (Fisher / NASA Science Blog)
- Gas production dropped sharply as the comet moved away from the Sun; water showed the steepest decline due to lower volatility than methane or CO₂. (Fisher / NASA Science Blog)
- MIRI’s Medium Resolution Spectrometer — an integral field unit — simultaneously measured gas composition and spatial distribution around the nucleus. (Fisher / NASA Science Blog)
Notable Quotes
“Both these findings point to a very different formation environment and chemistry than the vast majority of comets that formed within our solar system.” — NASA/Webb team conclusions, as reported by Alise Fisher