Scanning electron microscopy of impurity structures in snow

We examined well-sintered, mid-winter snow from Mammoth Mountain, California using an environmental scanning electron microscope equipped with a cold stage. Ring-like filaments were observed at all grain boundaries in a highly sublimated, sputter-coated sample 2 days after collection, though filaments were much harder to find in uncoated samples. Filaments appeared in uncoated specimens from the same snow sample five months later, though X-ray microanalysis failed to detect soluble impurities in the filaments, grain boundary grooves or on ice grain surfaces above background noise levels. After 8 months in storage and under forced sublimation in high vacuum, the same sample produced not only grain boundary filaments but complex, web-like structures, nodules and films that, in extreme cases, formed castings of completely sublimated snow grains. Energy dispersive X-ray spectrometry reveals that the filaments and webs are composed of the same set of light elements typical of the soluble impurities in Sierra snow. The spectra also reveal significant spatial variability in element ratios. Impurities probably concentrate in a quasi-liquid layer at the air/ice interface and in the disordered grain boundary region where crystal lattices are misoriented. If concentrated at grain boundaries these light elements could act as dopants, reducing grain boundary energy, reducing melting temperatures and promoting the sintering process.