JWST detection of a carbon dioxide dominated gas coma surrounding interstellar object 3I/ATLAS (2508.18209v1)

Abstract: 3I/ATLAS is the third confirmed interstellar object to visit our Solar System, and only the second to display a clear coma. Infrared spectroscopy with the James Webb Space Telescope (JWST) provides the opportunity to measure its coma composition and determine the primary activity drivers. We report the first results from our JWST NIRSpec campaign for 3I/ATLAS, at an inbound heliocentric distance of $r_H=3.32$ au. The spectral images (spanning 0.6-5.3 $\mu$m) reveal a CO2 dominated coma, with enhanced outgassing in the sunward direction, and the presence of H2O, CO, OCS, water ice and dust. The coma CO2/H2O mixing ratio of $8.0\pm1.0$ is among the highest ever observed in a comet, and is 6.1-sigma above the trend as a function of heliocentric distance for long-period and Jupiter-family comets (excluding the outlier C/2016 R2). Our observations are compatible with an intrinsically CO2-rich nucleus, which may indicate that 3I/ATLAS contains ices exposed to higher levels of radiation than Solar System comets, or that it formed close to the CO2 ice line in its parent protoplanetary disk. A low coma H2O gas abundance may also be implied, for example, due to inhibited heat penetration into the nucleus, which could suppress the H2O sublimation rate relative to CO2 and CO.

• The paper reports that JWST NIRSpec data revealed a CO2-dominated coma with a CO2/H2O ratio of 8.0±1.0, far exceeding typical Solar System values.
• The paper details spatially resolved spectral analyses and Q-curve modeling techniques that quantified production rates and mapped dust and gas distributions in the coma.
• The paper discusses implications for interstellar planetesimal formation, proposing that unique volatile inventories and suppressed H2O sublimation signal non-traditional formation histories.

JWST NIRSpec Detection of a CO$_2$-Dominated Gas Coma in Interstellar Object 3I/ATLAS

Introduction

The detection and characterization of interstellar objects (ISOs) traversing the Solar System provide unique constraints on the physical and chemical conditions of planetesimal formation in extrasolar environments. The third confirmed ISO, 3I/ATLAS, presents an opportunity to probe the volatile inventory of a body formed outside the Solar System. This paper reports JWST NIRSpec IFU observations of 3I/ATLAS at $r_H=3.32$ au, revealing a coma dominated by CO$_2$ gas, with significant implications for the understanding of volatile processing and retention in interstellar planetesimals.

Observational Strategy and Data Reduction

JWST NIRSpec IFU observations were conducted with a $3''\times3''$ field of view, providing spatially resolved spectroscopy from 0.6 to 5.3 $\mu$m at $R\sim30$–300. The data were acquired in four dithered positions, with background subtraction performed using offset sky exposures. The resulting data cubes enabled the extraction of both spatially integrated and spatially resolved spectra, facilitating the mapping of dust and gas emission features across the coma.

Spectral Characterization of the Coma

The spatially integrated NIRSpec spectrum of 3I/ATLAS displays prominent emission features attributable to CO$_2$, H$_2$O, and CO, as well as solid-state absorption bands from water ice and dust scattering.

Figure 1: JWST NIRSpec prism spectrum of 3I/ATLAS, showing strong CO$_2$ emission at 4.3~$\mu$m and additional features from H$_2$O, CO, and water ice.

Spatially resolved flux maps reveal the distribution of dust and volatiles in the coma.

Figure 2: JWST NIRSpec flux maps for 3I/ATLAS: (a) dust-scattered light at 1.2~$\mu$m, (b) CO$_2$ at 4.3~$\mu$m, (c) H$_2$O at 2.7~$\mu$m, (d) CO at 4.7~$\mu$m. Insets show spatially averaged, continuum-subtracted spectra for each gas.

The dust coma exhibits a pronounced sunward enhancement, while the gas species display more symmetric distributions, with CO$_2$ being the most spatially extended and intense. The $1/\rho$-enhanced maps further highlight the anisotropy in dust and subtle asymmetries in gas emission.

Figure 3: $1/\rho$-enhanced 1.2~$\mu$m dust map, emphasizing the sunward plume structure in the coma.

Quantitative Spectral Modeling

Production rates and rotational temperatures for CO$_2$, CO, H$_2$O, and OCS were derived using the Planetary Spectrum Generator (PSG) and optimal estimation routines. The modeling accounted for continuum contributions, line opacity, and spatial variations in the coma.

Figure 4: NIRSpec IFU spectra within a 0.625''-radius aperture centered on the nucleus, with best-fit models for continuum and gas emission components.

A $Q$-curve analysis was performed to extract production rates as a function of nucleocentric distance, mitigating optical depth effects near the nucleus and enabling the determination of terminal coma mixing ratios.

Figure 5: Gas production rates ($Q$) for CO$_2$, CO, H$_2$O, and OCS as a function of distance from the nucleus. CO$_2$ dominates the volatile output at all radii.

The terminal production rates are $Q({\rm CO_2})=(1.76\pm0.02)\times10^{27}$ s$^{-1}$, $Q({\rm CO})=(3.0\pm0.2)\times10^{26}$ s$^{-1}$, $Q({\rm H_2O})=(2.19\pm0.08)\times10^{26}$ s$^{-1}$, and $Q({\rm OCS})=(4.3\pm0.9)\times10^{24}$ s$^{-1}$. The derived CO$_2$/H$_2$O mixing ratio is $8.0\pm1.0$, a value that is $6.1\sigma$ above the trend for Solar System comets at similar heliocentric distances.

Comparative Analysis with Solar System Comets

The CO$_2$/H$_2$O ratio in 3I/ATLAS is anomalously high compared to the established distribution for both long-period and Jupiter-family comets.

Figure 6: CO$_2$/H$_2$O mixing ratios as a function of heliocentric distance for comets of various dynamical classes. 3I/ATLAS (red star) is a strong outlier, with a ratio 16$\times$ higher than the Solar System trend at $r_H=3.3$ au.

This elevated ratio is only rivaled by the hypervolatile-rich comet C/2016 R2, but 3I/ATLAS does not exhibit a similarly extreme CO/H$_2$O ratio. The data suggest either an intrinsically CO$_2$-rich nucleus or suppressed H$_2$O sublimation, potentially due to a low thermal conductivity crust or high surface albedo inhibiting heat penetration.

Coma Structure and Outgassing Morphology

The spatial analysis of the coma reveals heterogeneous outgassing patterns. The dust is strongly enhanced in the sunward direction, while the gas species show more modest asymmetries, consistent with their respective sublimation temperatures. The $S/S'$ ratios (sunward/antisunward) for dust, CO$_2$, CO, and H$_2$O are 1.37, 1.07, 1.04, and 1.20, respectively, indicating that the less volatile species (H$_2$O) are more sensitive to solar heating.

Isotopic Constraints

A lower limit on the $^{12}$C/$^{13}$C ratio in CO$_2$ was established ($>77$ at $3\sigma$), consistent with terrestrial values, but higher SNR and spectral resolution are required for a definitive measurement.

Rotational Temperatures

Rotational temperatures for CO$_2$, CO, and H$_2$O were retrieved as a function of nucleocentric distance, providing constraints on coma excitation conditions.

Figure 7: Rotational temperatures for CO$_2$, CO, and H$_2$O as a function of projected distance from the nucleus.

Implications for Interstellar Planetesimal Formation

The observed CO$_2$-dominated volatile inventory in 3I/ATLAS is inconsistent with the bulk ice compositions of most Solar System comets, where CO$_2$/H$_2$O ratios are typically $\lesssim$0.2. Theoretical models of protoplanetary disk chemistry indicate that CO$_2$-rich ices can form in regions exposed to elevated UV/X-ray irradiation or near the CO$_2$ ice line, especially in low-metallicity or dynamically old stellar populations. The high CO$_2$/H$_2$O ratio in 3I/ATLAS may thus reflect formation in such an environment, or extensive radiolytic processing during a prolonged interstellar sojourn.

Future Prospects

Further JWST and ground-based observations of 3I/ATLAS at smaller heliocentric distances are essential to determine whether the high CO$_2$/H$_2$O ratio persists as H$_2$O sublimation becomes more efficient. The combined capabilities of JWST and the Vera C. Rubin Observatory will enable systematic studies of volatile inventories in both interstellar and Solar System comets, improving the statistical basis for comparative planetology.

Conclusion

JWST NIRSpec IFU observations of 3I/ATLAS reveal a coma with a CO$_2$/H$_2$O mixing ratio of $8.0\pm1.0$, a value that is a significant outlier relative to Solar System comets at comparable heliocentric distances. This result suggests either an intrinsically CO$_2$-rich nucleus or suppressed H$_2$O outgassing, with implications for the formation and evolutionary history of interstellar planetesimals. The findings underscore the diversity of volatile compositions among ISOs and highlight the diagnostic power of high-sensitivity infrared spectroscopy for constraining the chemical evolution of small bodies beyond the Solar System.