Isotopic Signature of Organic Molecules from Beyond the Solar System: An Enriched Methane D/H Ratio in the Interstellar Object 3I/ATLAS

Abstract

Interstellar objects are interlopers from other planetary systems, and their volatile compositions provide a glimpse into planet formation around their host star. We present near-infrared spectra of the coma of interstellar object 3I/ATLAS measured with the James Webb Space Telescope. Our results demonstrate an unexpectedly high $\mathrm{D}/\mathrm{H} = (3.31\pm0.34)\%$ for methane and represent an exceedingly rare detection of deuterated organic molecules in an interstellar object. This D/H ratio is a factor of $14\pm2$ higher than that measured in comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft, the only other comet for which CH$_3$D has been detected, yet the ratio of deuteration in methane compared with water is consistent for both comets within $1.2σ$. The D/H ratio in methane is observationally unconstrained in extrasolar sources to date, but the enriched ratio in 3I/ATLAS is most similar to those measured in other organic molecules toward primitive environments. The high D/H ratios of water and methane in 3I/ATLAS are a natural consequence of formation in a high D/H elemental ratio environment as a result of locally cold conditions in the protoplanetary disk and prior interstellar cloud. Thus, 3I/ATLAS formed in an environment very different from that in which our Sun and planets originated.

Figures (5)

• Figure 1: Upper Panel. 3I/ATLAS CH4 $+$CH3D residual spectrum, generated by extracting a spectrum in a $1".5$ diameter aperture centered on the nucleus position and subtracting forward models of CH3OH, H2CO, C2H6, and OH* emission. The best-fit CH4 model is overplotted. Lower Panel. 3I/ATLAS CH3D residual spectrum generated by subtracting the best-fit CH4 model and the spectral baseline from the upper panel. The best-fit CH3D model and the $1\sigma$ instrumental noise envelope are also shown. The gray shaded regions denote the positions of CH4 and blended CH3D lines, which were masked when fitting CH3D (see Methods).
• Figure 1: Upper Panel. Observed 3I/ATLAS spectrum and second-order polynomial baseline, fit independently and before the fitting of PSG molecular emission models. Lower Panel. Best-fit PSG CH4 model fit to the baseline-subtracted spectra, demonstrating that a baseline fitted separately from the emission models does not provide improvement to the fits of the higher-$J$CH4 lines (and simultaneously degrades the fit near the lower-$J$ lines), yet consistent $Q(\ce{CH4})$ and Trot(CH4) are retrieved when considering only the lower-$J$ lines.
• Figure 2: D/H ratios in solar system and extrasolar sources. Ratios are for methane except when noted otherwise. Sources for which the D/H ratio in $c-$C3H2 was used to estimate the D/H in CH4 are denoted with a $\star$. References and values for each object are given in Extended Data Table \ref{['tab:dh']}.
• Figure 2: Left Panels. Upper. 3I/ATLAS 2.7 $\mu$m H2O spectrum on 2025 December 22 with best-fit model and spectral baseline overplotted. Lower. Baseline-subtracted 3I/ATLAS spectrum with best-fit individual molecular emission model(s) shown. Middle Panels. As in the left panels, but for CO on 2025 December 23. Right Panels. As in the left panels, but for H2O and CN on 2025 December 23. The gray shaded regions show the strong CO and OCS emission masked when retrieving $Q(\ce{H2O})$.
• Figure 3: Left Panels. Upper. 3I/ATLAS CH3OH and H2CO spectrum on 2025 December 23 with best-fit model and spectral baseline overplotted. This was used to forward model both species when isolating C2H6, CH4, and CH3D. Lower. Baseline-subtracted 3I/ATLAS spectrum with best-fit individual molecular emission model(s) shown. Middle Panels. Upper. Forward modeled CH3OH, C2H6, H2CO, and OH* spectra used to generate a CH4 $+$CH3D residual spectrum of 3I/ATLAS. Lower. Comparison of observed 3I/ATLAS spectrum with the CH4 $+$CH3D residual spectrum generated after subtracting the forward models in the upper panel. Right Panels. Observed 3I/ATLAS 3.3 $\mu$m spectrum with total emission model and baseline overplotted. Bottom. Baseline-subtracted 3I/ATLAS spectrum with individual molecular emission models for all detected species shown.