The Volatile Inventory of 3I/ATLAS as Seen with JWST/MIRI

Abstract We present the first spectroscopic characterization of an interstellar object at mid-infrared wavelengths. Postperihelion observations of 3I/ATLAS (3I) using the JWST/MIRI medium-resolution spectrometer were obtained on 2025 December 15–16 and 27, when the object was at heliocentric distances of 2.20 and 2.54 au, respectively. Our 5–28 μ m spectra exhibit fluorescence features from several gaseous species, including the ν 2 band of H 2 O at 5.8–7.0 μ m, the primary ν 2 and associated hot bands of CO 2 around 15 μ m, and a forbidden transition of atomic nickel at 7.507 μ m. We also report the first direct detection of CH 4 in an interstellar object. The sharp rise in CH 4 production relative to H 2 O suggests past CH 4 depletion from the outermost layers, with the observed CH 4 emerging from unprocessed subsurface material. Comparison of the volatile production rates measured during the two epochs indicates a significant reduction in the overall outgassing over 12 days, with the measured H 2 O activity level dropping more steeply than other species. As shown through near-nucleus coma mapping, 3I continues to display an extended source of H 2 O production from icy grains entrained within the coma. Our production rate measurements confirm that 3I has a strongly enhanced CO 2 :H 2 O mixing ratio relative to typical solar system comets, as well as a somewhat enriched CH 4 :H 2 O value.