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 μ m) reveal a CO 2 -dominated coma, with enhanced outgassing in the sunward direction and the presence of H 2 O, CO, water ice, dust, and a tentative detection of OCS. The coma CO 2 /H 2 O mixing ratio of 7.6 ± 0.3 is among the highest ever observed in a comet, and is 4.5 σ above the trend as a function of r H for long-period and Jupiter-family comets (excluding the outlier C/2016 R2). Our observations are compatible with an intrinsically CO 2 -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 CO 2 ice line in its parent protoplanetary disk. A relatively low coma H 2 O gas abundance may also be implied, for example, due to inhibited heat penetration into the nucleus, which could suppress the H 2 O sublimation rate relative to CO 2 and CO.
Cordiner et al. (Fri,) studied this question.