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.

银河系中的气体和尘埃云是恒星和行星系统诞生的温床。在从弥漫的星际介质到原行星盘的旅程中，分子固体通过吸积和表面化学作用积聚在冷尘埃颗粒上。这些所谓的冰颗粒将不断演化，特别是通过碰撞和聚集过程，改变它们的尺寸。我们的“冰河时代”詹姆斯·韦伯太空望远镜对致密变色龙 I 云的观测表明，这种生长很早就开始了，在原恒星阶段之前，大大改变了光谱中的冰带轮廓。光谱分析证实，颗粒达到微米尺寸，这意味着局部微观物理发生了无数变化，包括从小颗粒到大颗粒的质量传递、可用于化学的颗粒表面的减少以及辐射场的穿透和传播的改变。观察到的轮廓的变形使化学丰度的确定变得复杂。观察稠密云中广泛的冰晶颗粒生长，定量地限制了恒星和行星形成之前的晶粒尺寸演化。