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Could a sprinkling of dirt save the glaciers?

February 2, 2006 By Philip Ball This article courtesy of Nature News.

Knowing how spiky glaciers form could give clues about how to slow ice melt.

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Researchers have recreated miniature versions of curious spiky glaciers found in the Andes. By studying how these structures form in the lab, they conclude that inducing such spikes in glaciers should help to slow their melting, and perhaps provide a way to preserve glaciers that are under threat from global warming.

The spikes appear naturally in the high-altitude snowfields of the Andes, where glaciers can be moulded into a forest of ice spires. The spikes, which are typically 1–4 metres high, are known as ‘penitentes’ because of their resemblance to a procession of white-hooded monks. They form only in dry air, when intense sunlight burning into ice transforms it straight into water vapour.

Physicist Vance Bergeron of the higher teacher training school in Lyon and his co-workers made ‘micro-penitentes’ in their laboratory by exposing blocks of snow or ice to a bright spotlight. After a few hours of illumination, tiny peaks just a few centimetres tall appeared. Structures of this size have also been seen naturally, and are thought to be the precursors of full-scale penitentes.

The formation of the ice pinnacles is a self-amplifying process, Bergeron explains. First, the light evaporates small patches of snow at random, creating small dimples in the flat surface. “The little cavities then act like lenses, focusing energy into their centre,” Bergeron says. This makes the troughs get deeper, and eventually pinnacles of ice are left standing between the troughs.

Once the spikes get big enough, they cast most of the snow surface into shadow. This slows down subsequent evaporation of the ice, they will report in Physical Review Letters1.

Dirty caps

“It’s fascinatingly charming, and a really nice piece of physics,” says glaciologist Elizabeth Morris at the Scott Polar Research Institute in Cambridge, UK.

The process is accelerated by a fine coating of dirt on the snow surface. As the troughs deepen they expose clean snow that is prone to further evaporation, whereas dirt in the old snow at the peaks covers the ice crystals like a cap and insulates them.

This seems to contradict the observation that snow or ice melts faster if it’s dirty than if it’s clean, because it absorbs more sunlight. But Bergeron explains that whether a layer of dirt acts primarily as an insulator or an absorber depends on how thick it is.

Bergeron and colleagues think that glaciers and ice fields at risk of melting could be protected by scattering a little dirt or dust on their surface to promote the formation of penitentes. “That’s one of the things we now want to investigate,” Bergeron says.

Melt warning

The research comes as scientists advising the British government publish a report warning that major world deposits of ice, including the Greenland ice cap and parts of the Antarctic, are in danger of melting.

Their 30 January report, launched about a year after their meeting in February 2005 (see ‘ UK climate meeting calls for action’), warns that the risks associated with climate change seem to be more serious than thought just a few years ago.

Creating penitentes could potentially form a small part of efforts to help reduce the rate of ice loss, should the idea prove valid. “It makes sense to me,” Morris says. If this idea doesn’t pan out, Bergeron is also interested in applying the physics of penitentes’ to microelectronics. He is keen to make tiny pillars just a few thousandths of a millimetre across, by using laser beams to blast material off the surface of silicon wafers. Such fields of microscopic spikes could be used to create finely focused electron beams, he says.

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  1. Bergeron V., et al. Phys. Rev. Lett. in press.


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