Polar Mesospheric Clouds (PMCs) form in the summer high latitude mesosphere at temperatures below 150 K and are comprised of water ice. Water ice within PMCs is often assumed to exist as the cubic form of ice I. However, under the extreme temperatures at which PMCs form, historic laboratory work indicates that ice I could be forming in the hexagonal, metastable stacking disordered or even the amorphous forms. However, many historic studies were performed under conditions not necessarily relevant for PMCs, hence it is not clear which phases form. Knowledge of the specific structure of ice occurring in PMCs is crucial for understanding the microphysics of PMCs as a whole. Differences in crystal structure would affect a range of PMC particle properties including crystal shape and vapour pressure.

In order to investigate the structure of water ice forming under mesospheric temperature conditions an environmental chamber was used. Ice was deposited in a series of experiments at deposition temperatures ranging from 115 to 145 K using a humidified nitrogen gas flowed over a temperature controlled stage within the environmental chamber. The specific structure of the ice formed was probed using X-ray Diffraction (XRD). XRD patterns show that the ice exists in several different crystal structures across the temperature range studied and the specific structure that forms is extremely sensitive to the deposition temperature. We observe amorphous, stacking disordered and hexagonal ice depositing at mesospheric temperatures. Notably, we also observe hexagonal ice depositing at temperatures at and above 140 K, where samples of ice deposited at much lower temperatures and warmed to this temperature persist in a metastable stacking disordered phase. Since hexagonal ice and stacking disordered ice are expected to produce crystals with large aspect ratios, while cubic and amorphous ice are not, the results help explain the observed high aspect ratios of ice crystals in PMCs and also inform modelling studies.