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uni'wissen 02-2013_ENG

the blast fling them there? How did the stone change? The blast creates pressures millions of times higher than that of the Earth’s atmosphere, causing the atoms in minerals like quartz crystal to rearrange themselves. This leads to the forma- tion of minerals like stishovite and coesite, even in the crater lab. The researchers participating in the MEMIN project have analyzed 24 impact experiments since 2009. They have tested rocks of various thickness and composition. One of the most as- tounding findings they have made is that the rock’s porosity – the amount of spaces in it allow- ing air to pass through – has a particularly great influence on the forces set free during the forma- tion of a crater. The air works like an airbag, low- ering the pressures released upon impact. In a porous mineral like tuff, for instance, the crater will be smaller and the consequences for the en- vironment less severe than in granite or gneiss. Another major influencing factor is water. “When the rock contains a lot of water, the effects of a meteorite’s impact are four times as great. This was previously unknown,” says Kenkmann. In the second funding phase, which began in July 2013, the MEMIN project is going into more detail: “We aren’t just interested in looking at how the rocks have become more dense afterwards; we want to be there when it happens. In order to do this, we need strong x-rays.” At the German Electron Synchrotron in Hamburg (DESY), a re- search center operated by the Helmholtz Asso- ciation, the scientists can follow the crystals as they change form during impact. A further ambi- tious research focus in the second funding phase The properties of the rocks change the effect of the impact: Meteorites leave smaller craters in rocks with lower water content (left) than in those with higher water content (right). Photos: MEMIN The impact event unfolds much more slowly in the laboratory than in nature. The aim of the ex- periments is to predict what would happen if the Earth were bombarded from space. The results need to be transferable to real, large-scale im- pact events. With the help of computer simula- tions, the researchers first calculate what forces are at work during an impact event on the Earth’s surface and then use the accelerator to test how their calculations match up to reality. When a meteorite is shot out of the EMI’s light- gas accelerator in Freiburg, it crashes into the rock in the tightly sealed and secured metal chamber at the end of the five meter-long pipe of the accelerator. There is a bang as if someone had struck a table with a hammer. The little crater in the stone block shows cracks and fissures. The scientists capture the miniature impact event on high-speed film at 100,000 images per sec- ond. They measure the pressure, temperature, and speed at numerous points in the impact chamber and inside the stone block. Crammed with sensors and cables, the block resembles a little computer made of stone. With Floral Foam and Vaseline The most interesting thing for Kenkmann now is to examine the tiny shards, drips, and frag- ments the meteorite has blasted out of the stone block. “A contraption made of floral foam and Vaseline proved to be the best way to catch the fragments blasted out of the rock,” he says. The fragments and the crater are now subjected to a close examination under the microscope: Which fragments were thrown how far, and how fast did “When the rock contains a lot of water, the effects of a meteorite’s impact are four times as great” 6