Spectroscopic sizing of interstellar icy grains with JWST

Clouds of gas and dust in the Galaxy are nurseries in which stars and planetary systems are born. During their journey from the diffuse interstellar medium to the protoplanetary disks, molecular solids accumulate on cold dust grains by accretion and surface chemistry. These so-called icy grains will continuously evolve, notably by collision and aggregation processes, modifying their sizes. Our 'Ice Age' James Webb Space Telescope observations of the dense Chamaeleon I cloud reveal that this growth starts early, before the protostellar phase, substantially modifying the ice band profiles in the spectra. Spectral analysis confirms that the grains reach micrometre sizes, implying myriad changes in local microphysics, including mass transfer from small to large grains, reduction in the grain surface available for chemistry and modification of the penetration and propagation of radiation fields. Deformation of the observed profiles complicates the determination of chemical abundance. Observing the extensive icy grain growth in dense clouds quantitatively constrains the grain size evolution before star and planet formation. Cold ice-covered dust grains grow during their journey from the interstellar medium to protoplanetary disks. JWST observations show that this growth begins before the protostellar phase and provide quantitative insights into the grain growth process.