PHYSICIST Tom Tiedje, out hiking on a snowy mountainside, began wondering
what caused the metre-wide hollows in the snow called “sun cups” that can give a
clean snowfield the pitted texture of a giant golf ball. Back in Tiedje’s lab at
the University of British Columbia in Vancouver, graduate student Eric Nodwell
came up with an explanation, inspired by the physics of semiconductor
surfaces.
Looking at the sun cups, Tiedje had already realised that there was something
familiar about the depressions and the ridges between them. They were like
hugely magnified impressions of the surfaces of semiconductors, such as silicon
and gallium arsenide, which are covered with billows separated by v-shaped
grooves. “These grooves were the inverse of the ridges you get in snow,” he
says. “It’s the same phenomenon, only in reverse.”
Tiedje and Nodwell started kicking the problem around, and Nodwell found that
the key was sunlight diffusing into the snow. This eliminates some natural
fluctuations in its surface and accentuates others. In depressions with small
diameters, the light diffuses into the ridges, which makes them melt away. In
larger depressions, the centres tend to catch and hold the light, which makes
them grow deeper.
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Overall, the effect is that small depressions get flattened while wider ones
get deepened. That explains why all the sun cups tend to be the same size,
Nodwell says. “There is a definite minimum sun cup diameter.”
The wind or some other factor probably causes very slight ripples in a
snowfield, Tiedje says. There then has to be some way of making the larger ones
grow into sun cups. “Eric has identified that mechanism,” he says.
