Transfigured crystals arise from changes that occur after snowflakes land. Fluctuating temperatures, pressures, air currents, and sunlight can resculpture a crystal, disintegrate it, or reconfigure it into an entirely different shape. Over a span of days to months, the hexagonal geometry of a freshly fallen crystal might repeatedly deform, partially melt, refreeze, and fuse with nearby crystals, resulting in a minuscule ice carnation. More extreme environmental effects yield a shape akin to a cluster of grapes. One type of metamorphosis is of particular interest because it can produce the conditions that enable avalanches to occur.
When the temperature beneath a layer of snow crystals is significantly higher than the temperature above, ice from crystals lower in the snowpack sublimes—that is, vaporizes directly without melting—and then refreezes onto overlying crystals. In time, this redistribution of mass leads to large and blocky crystals known as depth hoar. A layer of depth hoar tends to make the snowpack unstable. When safety managers in ski areas find such layers in snow pits dug during routine inspections, they issue warnings, close off vulnerable areas, and sometimes fire mortars into the snowpack to provoke an avalanche before it can catch anyone off guard.
The labyrinthine interiors of depth hoar crystals also cause problems for researchers like the U.S. Department of Agriculture’s Al Rango, who uses microwave-sensing satellites to measure the amount of water locked away in the winter snow cover. Tiny passageways within the crystals are great at scattering microwaves, thereby fooling satellite-based sensing systems into reading six inches of snow as six feet of snow. Low-temperature electron microscopy images of depth hoar are leading to better models for converting the raw satellite readings into accurate measurements of snowfall.
Among the thousands of snowflakes examined by Wergin and Erbe over the past decade are many unusual specimens that do not fit into any of the standard classifications. Often the bizarre forms result from the exotic ways that frozen water interacts with living things.
Ice worms: The surface of a glacier contains a willy-nilly assemblage of irregularly shaped ice grains joined together by necks of ice. Threadlike ice worms often make a home in the crevices between the grains. Erbe remembers searching all day for such worms on the South Cascade Glacier in Washington State. Finally, at quitting time, he noticed small moving black lines on the thermal pad he had been lying on. “They were attracted by the heat,” he says. This quarter-inch-long worm is typical of the specimens he found there.
Frost: On a night when the temperature dipped below the frost point, this blade of grass from Bearden Mountain, West Virginia, served as an organic post onto which ice crystals nucleated and grew. The result was a tightly clustered bunch of needle-shaped crystals.
Red snow: When snow repeatedly melts partially and then refreezes, the stage is set for a red-pigmented alga, Chlamydomonas nivalis, to take up residence in the thin films of water around the snow particles. This fractured and strangely eroded sample of summertime snow, collected at Loveland Pass in Colorado, contains several spherical algal cells, including two that have been split apart.
Red Planet snow: To explore the conditions that future Mars probes might encounter, Wergin, Erbe, and a group of NASA researchers created imitation Martian snow. They caused carbon dioxide gas to condense onto a sample plate cooled to a Mars-like –240°F. Images from the low-temperature scanning electron microscope show the resulting carbon dioxide frost consists largely of octahedral crystals, quite unlike those of water ice. “These crystals are believed to be similar to those in the seasonal polar caps of Mars,” Wergin says. The snows there contain a mix of water and carbon dioxide crystals. Such studies will help researchers make better models of the climate cycle on Mars.