We report a survey of molecular emission from cometary volatiles using the James Webb Space Telescope (JWST) toward interstellar object 3I/ATLAS carried out on UT 2025 December 22 and 23 at a heliocentric distance (rH) of 2.37−2.41 au. These measurements of CO, CO2, H2O, CH3OH, and CH4 sampled molecular chemistry in 3I/ATLAS as it receded from its encounter with our Sun and entered the vicinity of the H2O ice line -- the region between rH = 2−3 au where the temperature becomes too low for H2O to vigorously sublime and CO and CO2 begin to control the overall activity. CO was the most abundant molecule, followed by H2O and CO2, whose molecular abundances with respect to CO were (40.5±3.1)% and (41.6±0.3)%, respectively. This work presents spatial-spectral maps of column density and rotational temperature as a function of distance from the nucleus for all detected species. The spatial distributions of both quantities were highly anisotropic for the apolar species in the coma of 3I/ATLAS, yet were more nearly symmetric for the polar molecules. These results demonstrate how volatiles were segregated in the nucleus ices of 3I/ATLAS and reveal heating and cooling mechanisms in its coma. Derived maps of the ortho-to-para ratio (OPR) for H2O were flat with increasing distance from the nucleus and consistent with a coma-averaged value OPR=2.7±0.2, slightly less than the expected equilibrium value of three.
@article{arxiv.2603.20460,
title = {Coma Physics of an Interstellar Object: JWST Spatial-Spectral Mapping of 3I/ATLAS},
author = {Nathan X. Roth and Martin A. Cordiner and Stefanie N. Milam and Geronimo L. Villanueva and Steven B. Charnley and Nicolas Biver and Dominique Bockelee-Morvan and Dennis Bodewits and Steven J. Bromley and Jacques Crovisier and Maria N. Drozdovskaya and Sara Faggi and Davide Farnocchia and Kenji Furuya and Michael S. P. Kelley and Marco Micheli and John W. Noonan and Cyrielle Opitom and Megan E. Schwamb and Cristina A. Thomas},
journal= {arXiv preprint arXiv:2603.20460},
year = {2026}
}