A JWST Study of Stardust. I. Infrared Spectroscopy of Comet 81P/Wild 2 and Overall Composition

We report observations of comet 81P/Wild 2, target of the Stardust sample return mission, on UT 2023 March 20 and 24 at a heliocentric distance (r H) of 1.85 au using the NIRSpec and MIRI integral field unit spectrographs on board the James Webb Space Telescope (JWST). This study is the first compositional comparison between JWST remote-sensing spectroscopy of a solar system object against terrestrial analysis of its returned samples. We securely detected molecular emission from H 2 O, CH 4, C 2 H 6, CH 3 OH, CO, CO 2, 13 CO 2, OCS, HCN, and CN and find molecular abundances consistent within 2 sigma with those reported during previous perihelion passages. The water ortho-to-para ratio was 2.76 plus or minus0.05, and the 12 CO 2 / 13 CO 2 ratio was 85 plus or minus4. Thermal emission from the nucleus and dust was detected and modeled, providing an effective nucleus radius of 1.77 plus or minus0.04 km and a dust composition (relative mass fraction of the submicron grains) of about36% amorphous carbon, about25% amorphous Mg:Fe olivine, about23% Mg-rich crystalline olivine, and about15% amorphous Mg:Fe pyroxene. The crystalline mass fraction of the sub-micron grains in the coma was 0.362 plus or minus0.003. Comparison of the JWST-derived thermal model against the fine-grained materials in Stardust returned samples demonstrates complementarity between the missions, with each most sensitive to a different population of the coma dust grains.