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

TL;DR

3I/ATLAS offers a rare look at an extrasolar small body’s volatile makeup. The authors use JWST/MIRI MRS to obtain 5–28 µm mid-infrared spectra at two outbound epochs and apply non-LTE fluorescence modeling with PSG to extract gas production rates $Q$ and rotational temperatures $T_{ m rot}$ for H$_2$O, CO$_2$, CH$_4$, and Ni, including extended-source and optical-depth considerations. They find pronounced CO$_2$ enrichment relative to H$_2$O and a first mid-IR detection of CH$_4$ in an ISO, with a sharp decline in $Q_{ m H_2O}$ between epochs and CH$_4$/H$_2$O rising, suggesting subsurface reservoirs and thermal processing; Ni emission traces dust and is consistent with production from organometallic precursors. The results imply a CO$_2$-type, possibly CO$_2$-rich KBO-like origin for 3I and demonstrate JWST’s capability to characterize ISO volatiles, informing models of planetesimal formation and processing in other stellar systems.

Abstract

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.

Figures (5)

• Figure 1: Top panels: median-stacked images derived from the fully calibrated Channel 2 data cubes for the six successful JWST/MIRI observations of 3I. A logarithmic stretch has been applied to accentuate the extended dust coma. The sunward and target velocity directions are indicated. The panels are labeled with the corresponding date and spectral grating setting. Bottom panel: spectra of 3I extracted using a $1"$-diameter circular aperture. Observations 2, 4, and 6 (blue) are combined into a single spectrum by normalizing individual segments to align across their overlapping wavelengths. The spectra from Observations 13 and 15 (orange) are unaltered, illustrating the significant decrease in flux as the comet passed from a heliocentric distance of 2.20 au to 2.54 au. The main H$_{2}$O, CO$_{2}$, CH$_{4}$, and Ni fluorescence features are marked. The inset panel provides a zoomed-in view of the CO$_{2}$ primary and hot bands.
• Figure 2: Top panels: continuum-subtracted JWST/MIRI spectra of 3I in the $\nu_2$ band of H$_{2}$O from both epochs, with the best-fit PSG coma fluorescence models overlaid. The contributions from ortho- and para-H$_{2}$O are shown separately in blue and green, respectively. The line intensities are significantly lower in the second epoch, corresponding to a sizable decrease in H$_{2}$O production. Bottom left panels: analogous fits to the $\nu_2$ band of CO$_{2}$. Bottom right panels: CH$_{4}$ fluorescence model and data. The emission feature highlighted in purple at 7.5066 µm is from the ground-state forbidden transition of Ni I. To illustrate the systematic decrease in production between the two epochs, the vertical scale is identical within each set of panels.
• Figure 3: Coma maps of H$_{2}$O, CO$_{2}$, and CH$_{4}$, computed as the integrated emission flux across the corresponding fluorescence bands for Observations 6, 15, and 13, respectively. The sunward and target velocity directions are denoted by the white arrows. The target centroids, computed as the photocenter in the median-stacked images (\ref{['fig:fig1']}), are marked with the black points. For H$_{2}$O and CO$_{2}$, the white contours correspond to emission levels of 75%, 50%, and 25% relative to the maximum value and illustrate the slight anti-sunward extension of the respective comae. The precise spatial distribution of CH$_{4}$ in the near-nucleus region is poorly constrained due to the low signal-to-noise ratio of the data.
• Figure 4: Radial flux profiles of fluorescing gas-phase species normalized to the azimuthally averaged dust radial profile, where the dust profile is measured from wavelength slices adjacent to the emission features. The shading demarcates the $1\sigma$ uncertainty regions. Values of 1 indicate that the gas and dust emission follow the same radial dependence, while elevated values suggest an extended source. Only H$_{2}$O exhibits a significant deviation from the dust profile, indicating distributed sublimation from icy grains within the coma.
• Figure 5: Left panel: a compilation of H$_{2}$O, CO$_{2}$, and CH$_{4}$ production rates for 3I published in Combi2025arXiv, Cordiner2025, Lisse2025arXiv, Xing2025ApJL, Lisse2026, and this work, plotted as a function of heliocentric distance. The JWST epochs (both NIRSpec and MIRI) are marked with vertical dashed lines, and the JWST/MIRI measurements are shown with larger symbols. Left-facing triangles denote pre-perihelion measurements, and vice versa. Right panel: CO$_{2}$-to-H$_{2}$O production ratios versus heliocentric distance for various active objects, sorted by dynamical class, with 3I's measurements marked with the star symbols. This plot is a reproduction of Figure 6 in Cordiner2025 (see reference therein), with the new JWST/MIRI-derived values added. The solid and dashed black lines indicate the best-fit log-linear trend and associated $1\sigma$ bounds for the population of long-period comets (LPCs) and Jupiter-family comets (JFCs), excluding the exceptionally CO$_{2}$-rich object C/2016 R2. The measured CO$_{2}$:H$_{2}$O mixing ratios for 3I are significantly enriched relative to the trend, though these values may be artificially enhanced due to the limited field of view of the JWST observations. See the discussion for details.