Abstract
Hexagonal ice (ice Ih) is the only form of ice that is known to occur naturally on the Earth. Recently it has been suggested that small droplets of water in the upper atmosphere may often freeze first to cubic ice (ice Ic), which is metastable relative to ice Ih. The evidence is Scheiner's halo, a rare halo that occurs at ∼28° from the Sun or the Moon1–3, and the observation that dendritic snowflakes often have their c-axes at about 70° to one another4,5. Here we report the formation of cubic ice from liquid water at ≤200 K, using a method for vitrifying pure liquid water, namely rapid quenching of aqueous aerosol droplets ∼3 µm in diameter on a cryoplate6,7. Cubic ice made from liquid water transforms much more slowly to the stable form than sample prepared by deposition of the vapour. Speculations about naturally occurring ice Ic (refs 1–5) are supported by our results.
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