The Interstellar Comet Carried Dirty Water Ice

JWST/NIRSpec observed interstellar comet 3I/ATLAS at three epochs spanning eight months around perihelion. Its spectra show a broad three-micrometer absorption band together with water, carbon-dioxide and carbon-monoxide gas emission, but the shorter water-ice bands common in Solar System comae were weak or absent. Modeling favors submicron-to-micron water-ice-bearing aggregates mixed with refractory material. The final epoch also favors a second population of larger ice-rich grains and a subtle structure consistent with crystalline ice, though the authors say changing grain size and mixing can explain the evolution without requiring crystallization during the visit.