Coma Physics of an Interstellar Object: JWST Spatial-Spectral Mapping of 3I/ATLAS
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| Natura: | Preprint |
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2026
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| author | Roth, Nathan X. Cordiner, Martin A. Milam, Stefanie N. Villanueva, Geronimo L. Charnley, Steven B. Biver, Nicolas Bockelee-Morvan, Dominique Bodewits, Dennis Bromley, Steven J. Crovisier, Jacques Drozdovskaya, Maria N. Faggi, Sara Farnocchia, Davide Furuya, Kenji Kelley, Michael S. P. Micheli, Marco Noonan, John W. Opitom, Cyrielle Schwamb, Megan E. Thomas, Cristina A. |
| author_facet | Roth, Nathan X. Cordiner, Martin A. Milam, Stefanie N. Villanueva, Geronimo L. Charnley, Steven B. Biver, Nicolas Bockelee-Morvan, Dominique Bodewits, Dennis Bromley, Steven J. Crovisier, Jacques Drozdovskaya, Maria N. Faggi, Sara Farnocchia, Davide Furuya, Kenji Kelley, Michael S. P. Micheli, Marco Noonan, John W. Opitom, Cyrielle Schwamb, Megan E. Thomas, Cristina A. |
| contents | 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 ($r_H$) of $2.37-2.41$ au. These measurements of CO, CO$_2$, H$_2$O, CH$_3$OH, and CH$_4$ sampled molecular chemistry in 3I/ATLAS as it receded from its encounter with our Sun and entered the vicinity of the H$_2$O ice line -- the region between $r_H$ = $2-3$ au where the temperature becomes too low for H$_2$O to vigorously sublime and CO and CO$_2$ begin to control the overall activity. CO was the most abundant molecule, followed by H$_2$O and CO$_2$, whose molecular abundances with respect to CO were $(40.5\pm3.1)\%$ and ($41.6\pm0.3)\%$, 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 H$_2$O were flat with increasing distance from the nucleus and consistent with a coma-averaged value $\mathrm{OPR}=2.7\pm0.2$, slightly less than the expected equilibrium value of three. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_20460 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Coma Physics of an Interstellar Object: JWST Spatial-Spectral Mapping of 3I/ATLAS Roth, Nathan X. Cordiner, Martin A. Milam, Stefanie N. Villanueva, Geronimo L. Charnley, Steven B. Biver, Nicolas Bockelee-Morvan, Dominique Bodewits, Dennis Bromley, Steven J. Crovisier, Jacques Drozdovskaya, Maria N. Faggi, Sara Farnocchia, Davide Furuya, Kenji Kelley, Michael S. P. Micheli, Marco Noonan, John W. Opitom, Cyrielle Schwamb, Megan E. Thomas, Cristina A. Earth and Planetary Astrophysics Astrophysics of Galaxies 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 ($r_H$) of $2.37-2.41$ au. These measurements of CO, CO$_2$, H$_2$O, CH$_3$OH, and CH$_4$ sampled molecular chemistry in 3I/ATLAS as it receded from its encounter with our Sun and entered the vicinity of the H$_2$O ice line -- the region between $r_H$ = $2-3$ au where the temperature becomes too low for H$_2$O to vigorously sublime and CO and CO$_2$ begin to control the overall activity. CO was the most abundant molecule, followed by H$_2$O and CO$_2$, whose molecular abundances with respect to CO were $(40.5\pm3.1)\%$ and ($41.6\pm0.3)\%$, 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 H$_2$O were flat with increasing distance from the nucleus and consistent with a coma-averaged value $\mathrm{OPR}=2.7\pm0.2$, slightly less than the expected equilibrium value of three. |
| title | Coma Physics of an Interstellar Object: JWST Spatial-Spectral Mapping of 3I/ATLAS |
| topic | Earth and Planetary Astrophysics Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2603.20460 |