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

TL;DR

This study presents the first mid-infrared spectroscopic analysis of interstellar object 3I/ATLAS using JWST/MIRI, revealing volatile compositions, including water, carbon dioxide, methane, and atomic nickel, and their temporal variations.

Contribution

First spectroscopic characterization of an interstellar object at mid-infrared wavelengths, detecting methane and analyzing volatile composition and outgassing behavior.

Findings

Detected water, CO2, methane, and atomic nickel in 3I/ATLAS.

Observed a decrease in outgassing rates over 12 days.

Found an enhanced CO2:H2O ratio compared to solar system comets.

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 $\nu_2$ band of water at 5.8--7.0 microns. the primary $\nu_2$ 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.