Shafts of snow sculpted by sun

From Baltimore, at a meeting of the American Physical Society

On the surfaces of many glaciers high in the Andes Mountains, towering spikes of snow called penitentes crowd the terrain like legions of white ghosts. Now, experiments on miniature, laboratory versions of such spikes suggest that those remarkable pillars start to form when strong sunlight, bitter cold, and snow-surface irregularities conspire to cause snow to evaporate first from low spots.

Understanding how penitentes arise and then vanish may clarify the expected impact of global warming on the snow pack in the Andes and elsewhere, says Meredith D. Betterton of the University of Colorado at Boulder. Previous simulations by other scientists had suggested that fields of penitentes might slow glacier disappearance because the towers cast shadows that diminish the amount of sunlight absorbed by the glacier surface.

To create lab-grown penitentes, Betterton, Vance Bergeron of the École Normale Supérieure in Lyon, France, and Charles Berger of École Normale Supérieure in Paris made artificial snow in a bathtub-size freezer by mixing water vapor and air that had been chilled by liquid nitrogen. The team then illuminated a block of snow with a flood lamp, representing the sun. Within a few hours, spikes up to 5 centimeters tall formed.

The experiments confirm prior notions that budding penitentes enlarge by reflecting and concentrating light into the valleys between them, Betterton says. The illumination initially makes the snow in those valleys sublime, or evaporate directly from its solid form, the experiments indicate. However, changes in the patterns of spike growth revealed that once the penitentes reach larger sizes and capture more light, melting contributes to further stages of penitente formation.

When the researchers tested the effect of particulate pollution on penitentes by using snow dirtied by a thin layer of carbon powder, they were surprised to find that the dirt accelerated spike formation. Ironically, if penitentes actually do preserve glaciers, pollution may enhance glacier survival by speeding penitentes development, Betterton notes.

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