JWST detection of a carbon dioxide dominated gas coma surrounding interstellar object 3I/ATLAS

3I/ATLAS is the third confirmed interstellar object to visit our Solar System, and only the second to 19 display a clear coma. Infrared spectroscopy with the James Webb Space Telescope (JWST) provides 20 the opportunity to measure its coma composition and determine the primary activity drivers. We 21 report the first results from our JWST NIRSpec campaign for 3I/ATLAS, at an inbound heliocentric 22 distance of rH = 3.32 au. The spectral images (spanning 0.6–5.3 µm) reveal a CO2 dominated coma, 23 with enhanced outgassing in the sunward direction, and the presence of H2O, CO, OCS, water ice and 24 dust. The coma CO2/H2O mixing ratio of 8.0 ± 1.0 is among the highest ever observed in a comet, 25 and is 6.1σ above the trend as a function of rH for long-period and Jupiter-family comets (excluding 26 the outlier C/2016 R2). Our observations are compatible with an intrinsically CO2-rich nucleus, which 27 may indicate that 3I/ATLAS contains ices exposed to higher levels of radiation than Solar System 28 comets, or that it formed close to the CO2 ice line in its parent protoplanetary disk. A low coma H2O 29 gas abundance may also be implied, for example, due to inhibited heat penetration into the nucleus, 30 which could suppress the H2O sublimation rate relative to CO2 and CO.