Ascertaining the morphology and composition of the icy mantles covering dust grains in dense, cold regions of the interstellar medium is essential to developing accurate astrochemical models, determining conditions for ice formation, constraining chemical interactions in and on icy grains and understanding how ices withstand space radiation. The widely observed infrared spectroscopic signature of H₂O ice at ~3 μm discriminates crystalline from amorphous structures in interstellar ices. Weaker bands seen only in laboratory ice spectra at ~2.7 μm, termed ‘dangling OH’ (dOH), are attributed to water molecules not fully bound to neighbouring water molecules and are often considered as tracing the degree of ice compaction. We exploit the high sensitivity of JWST NIRCam to detect two dOH features at 2.703 and 2.753 μm along multiple lines of sight probing the dense cloud Chamaeleon I, attributing these signatures to unbound dOH in cold water ice and dOH in interaction with other molecular species. These detections open a path to using the dOH features as tracers of the formation, composition, morphology and evolution of icy grains during the star and planet formation process.

确定星际介质致密寒冷区域中覆盖尘埃颗粒的冰幔的形态和成分对于开发精确的天体化学模型、确定冰形成条件、限制冰颗粒内部和表面的化学相互作用以及了解冰如何承受空间辐射至关重要。广泛观察到的约 3 μm H ₂ O 冰的红外光谱特征可区分星际冰中的结晶结构和非晶结构。仅在实验室冰光谱中看到约 2.7 μm 处的较弱谱带，称为“悬空 OH”(dOH)，归因于水分子未与邻近水分子完全结合，通常被认为是追踪冰压实程度。我们利用 JWST NIRCam 的高灵敏度，沿多条视线探测 2.703 和 2.753 μm 处的两个 dOH 特征，探测稠密云 Chamaeleon I，将这些特征归因于冷水冰中未结合的 dOH 以及与其他分子物种相互作用的 dOH。这些探测结果开辟了一条利用 dOH 特征作为恒星和行星形成过程中冰晶颗粒的形成、成分、形态和演化示踪剂的途径。