A very common dust possesses a surprising ability to make ice appear in clouds.
In a cold cloud, water droplets do not necessarily freeze as soon as the temperature drops below 0 °C. They can remain liquid well below this threshold. However, the presence of tiny particles changes the situation: some provide a surface on which the first water molecules can organize themselves to form ice, initiating the change of state.
Clouds seen from an airplane.
Image Wikimedia
Among these particles, microcline has intrigued researchers for a long time. This mineral belongs to the feldspar family, which is widespread in rocks. Once torn from the ground, tiny grains can be carried through the atmosphere. Microcline is particularly effective at promoting ice formation, although the reason has not been clearly established until now.
An international team examined its surface at the scale of a few billionths of a meter. The researchers compared microcline with sanidine, a closely related mineral. On sanidine, ice appears mainly near small steps on the surface, as previous observations had predicted.
Microcline behaves differently. The experiments show that ice can begin to form on flat areas, without any particular defect serving as a starting point. This result indicates that the mineral’s ability does not depend solely on cracks, steps, or other microscopic irregularities.
Simulation of an ice nanocrystal in contact with microcline and sanidine surfaces.
Credit: Schneider et al. / Nature Communications — CC BY 4.0
To understand this behavior, the team supplemented its observations with simulations at the atomic scale. A particular arrangement of molecules in ice matches the organization of atoms on the surface of microcline quite well. This correspondence facilitates the formation of the first icy structures.
In other words, the geometry of the mineral itself can help water adopt the organization necessary for freezing. The researchers emphasize that several possible orientations of ice must be considered to identify this correspondence. The simplest models may therefore miss the observed mechanism.
This phenomenon matters beyond the laboratory. Ice in clouds contributes to their evolution and to the formation of precipitation. Understanding why some dust particles are more effective than others could therefore improve the representation of these processes in atmospheric models.
The researchers must now determine how this mechanism withstands real atmospheric conditions. A natural dust grain may carry defects, molecules, or other deposits absent from a surface prepared in the laboratory. Measuring their influence will help bridge the gap between these nanometric observations and the behavior of actual clouds.