Structures in ice and snow
A snowball may slumber through the summer in a deep-freeze without damage, but it is quite a different matter to get snow samples into a laboratory without changing them on the way. “It’s pretty futile to transport snow,” says Johannes Freitag of the Alfred Wegener Institute for Polar und Marine Research (AWI) in Bremerhaven. “That is, of course, if you’re interested in the fine details of its structure like we are.” Quite apart from the fact that snow reacts to even the slightest temperature variations, the transport would simply cause too much mechanical stress. So Freitag has taken his laboratory to the snow instead – to the Kohnen Antarctic Station, situated more than 500 kilometers inland from the coastal Neumayer Station. There, he and 27 colleagues are working on the European Project for Ice Coring in Ant- arctica EPICA, which aims to obtain older and more accurate climatic records by drilling several kilometers deep into the ice sheet covering the continent. The ice cores contain air whose composition is identical to that of the atmosphere at that time. The gases get trapped into snow, firn and finally into ice at a depth of 80 to 100 meters. Not until then, the tiny bubbles are so tightly sealed that no further exchange with the atmosphere can take place. This means that the icy prison can be as much as a thousand years older than the air it holds. Until now, researchers had examined the structures of their snow and ice samples in situ by looking at thin sections under the microscope. But on this expedition they are for the first time carrying two micro CAT scanners that deliver far more accurate data. MAVI, a modular software system from the Fraunhofer Institute for Industrial Mathematics ITWM, helps to analyze the 3D X-ray images. It normally detects and quantifies geometrical parameters of microstructures – such as foams, fiber composites, textiles, or concrete and other construction materials. “Defined in terms of its structure, snow is an open-pored foam,” says Katja Schladitz of the department for models and algorithms in image processing. “We have programmed a special MAVI module to analyze it.” In addition to structural parameters, the researchers in the Antarctic are using MAVI primarily to determine characteristics that describe the cohesion of the pores and their transition from an open to a closed condition. These are computed with data for the mass transport of the different gases occurring in the air. However, several more missions will have to be carried out before a mathematical model will predict the metamorphosis of snow suffiently accurate.
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