Coma Physics of an Interstellar Object: JWST Spatial-Spectral Mapping of 3I/ATLAS

Abstract

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 $(44.4\pm0.7)\%$ and ($42.4\pm0.9)\%$, 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 $\mathrm{OPR}=2.7\pm0.1$, slightly less than the expected equilibrium value of three.

Figures (8)

• Figure 1: JWST spectrum of 3I/ATLAS extracted in a $1".5$ diameter aperture centered on the nucleus position. Major species are labeled.
• Figure 2: Upper Panels. Maps of $N\times\rho$ for CO, CO2, CH4, CH3OH, and H2O. The white arrow shows the projected direction of the Sun. Lower. Maps of molecular rotational temperatures.
• Figure 3: Upper Panels Maps of $N\times\rho$ for ortho-H2O and para-H2O, the derived OPR, and Trot(H2O). Lower Panels. Signal-to-noise ratio (SNR) for each of the respective quantities in the upper panels.
• Figure 4: Left. Observed azimuthally averaged Trot for CO, CO2, CH4, CH3OH, and H2O. Right. Modeled Trot as a function of nucleocentric distance for CO, CH3OH, and H2O using the SUBLIME radiative transfer code. The black dashed vertical line shows the projected radial distance of the $1".5$ diameter nucleus-centered aperture from which representative $Q$'s were extracted for each molecule (Table \ref{['tab:qs']}). The $x$-axis extends to approximately the field of view of our spatial-spectral maps.
• Figure 5: Histogram of spaxel-by-spaxel OPR values for H2O drawn within a 10-spaxel radius of the comet photocenter.