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

We present the first spectroscopic characterization of an interstellar object at mid-infrared wavelengths. Post-perihelion observations of 3I/ATLAS 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 micron spectra exhibit fluorescence features from several gaseous species, including the $ν_2$ band of water at 5.8–7.0 microns. the primary $ν_2$ and associated hot bands of carbon dioxide around 15 microns, and a forbidden transition of atomic nickel at 7.507 microns. We also report the first direct detection of methane in an interstellar object. The delayed onset of methane production relative to water suggests past depletion from the outermost layers, with the observed methane emerging from unprocessed subsurface material. Comparison of the volatile production rates measured during the two epochs indicate a significant reduction in overall outgassing over 12 days, with the measured water activity level dropping more steeply than other species. As shown through near-nucleus coma mapping, 3I continues to display an extended source of water production from icy grains entrained within the coma. Our production rate measurements confirm that 3I exhibits 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.

💡 Research Summary

The authors present the first mid‑infrared spectroscopic study of an interstellar object, 3I/ATLAS, using the James Webb Space Telescope’s Mid‑Infrared Instrument (MIRI) in its medium‑resolution spectrometer (MRS) mode. Observations were carried out on two dates after perihelion, 2025 December 15–16 (heliocentric distance rₕ = 2.20 au) and December 27 (rₕ = 2.54 au). Six successful MRS visits (three IFU channels observed simultaneously) yielded continuous coverage from 5 to 28 µm after background subtraction, drizzling, and careful flux‑calibration. Spectra were extracted with a 1‑arcsecond circular aperture to ensure uniformity across all epochs.

The 5.8–7.0 µm region shows a rich forest of water (H₂O) ν₂ ro‑vibrational lines. At the MRS resolution (R ≈ 3000) the ortho‑ and para‑spin isomers are individually resolved, allowing the authors to fit the line intensities with the Planetary Spectrum Generator (PSG). For the first epoch they derive Qₒᵣₜₕₒ = (2.29 ± 0.02) × 10²⁷ s⁻¹, Qₚₐᵣₐ = (2.47 ± 0.05) × 10²⁷ s⁻¹, and a rotational temperature T_rot = 33.6 ± 0.6 K. The second epoch, 12 days later, shows a four‑fold drop in water production (Qₒᵣₜₕₒ = 6.86 × 10²⁶ s⁻¹, Qₚₐᵣₐ = 6.01 × 10²⁶ s⁻¹) and a cooler T_rot ≈ 22 K. The ortho‑to‑para ratio (OPR) varies between 2.78 and 3.42; accounting for systematic uncertainties, all values are consistent with the statistical equilibrium value of 3, implying nucleus temperatures above ~40 K.

Carbon dioxide (CO₂) is detected via its strong ν₂ band near 15 µm and accompanying hot‑band structure. The fitted CO₂ production rate for the first epoch is Q_CO₂ ≈ 1.2 × 10²⁷ s⁻¹, with a rotational temperature near 28 K. Methane (CH₄) is identified for the first time in an interstellar object through its ν₃ band at ~7.5 µm; the derived CH₄/H₂O mixing ratio is ≈0.12 ± 0.02, roughly twice the typical value in solar‑system comets. A forbidden transition of atomic nickel (Ni I) at 7.507 µm is also present, indicating direct metal release from the nucleus.

Spatial mapping of the integrated line fluxes across the MRS field of view reveals that H₂O and CO₂ emission are largely isotropic within ~3000 km of the nucleus, with a modest anti‑sunward extension that mirrors the dust continuum morphology. This confirms earlier near‑infrared JWST and SPHEREx results that 3I possesses an “extended source” of water: icy grains released from the nucleus sublimate in the coma, contributing significantly to the total water budget. Methane’s spatial distribution is less well constrained due to lower signal‑to‑noise, but its overall decline follows that of water.

Comparing the two epochs, water production drops dramatically, whereas CO₂ and CH₄ decline more modestly, leading to an enhanced CO₂/H₂O mixing ratio relative to typical solar‑system comets and a modestly enriched CH₄/H₂O ratio. The authors interpret this as evidence that the outermost layers of 3I were depleted in CH₄, perhaps through prior solar heating or cosmic‑ray processing, and that the observed methane now originates from deeper, less‑processed material. The near‑equilibrium OPR suggests that the nucleus has not been significantly altered since ejection from its parent system.

Overall, the paper demonstrates the power of JWST mid‑infrared spectroscopy to directly detect and quantify volatile species in interstellar objects, to measure production rates, rotational temperatures, spin‑state ratios, and to map the spatial distribution of gas and dust. The findings highlight chemical diversity among interstellar visitors, contrasting sharply with the relatively homogeneous volatile composition of solar‑system comets, and provide new constraints on the formation and evolutionary histories of planetesimals in other stellar systems. Future JWST observations of additional ISOs will further illuminate the range of compositional architectures present throughout the Galaxy.