This blog entry was written to accompany my podcast for the September 5, 2010 broadcast of the 365 Days of Astronomy. The podcast can be listened to here.

One of the high points of my stay in Germany recently was a visit I made to Nördlingen on the border between the provinces of Bavaria and Baden-Württemberg in southern Germany. It’s a beautiful place. It is enclosed by a defensive wall that dates back to the 14th Century – there are only three towns in Germany with this claim to fame. All the buildings are full of character. The town was the site of two battles during the Thirty Years War and were it not for the cars and the shops, you could easily imagine yourself in another time, another era.

But beautiful and all though the town is, this is not the reason I went there. It’s Nordlingen’s surroundings that interested me the most. The town is located in a region known as the Ries: a round, flat plain with an approximate diameter of around 23 km (15 miles).  This area is quite different to the surrounding countryside as the following scale model clearly indicates.

For many centuries, the prevailing idea about how this geological feature came to be was that it was an ancient volcanic caldera. The trouble was that much of the boulders and debris surrounding the  region were of non-volcanic origin. Many ideas were presented as to how this material got there, but it’s didn’t fully add up. The origins of the Ries remained controversial until fifty years ago.

Enter Eugene “Gene” Shoemaker. Gene was an astronomer and he had a few questions. When he looked at the Moon he saw a landscape quite different to the Earth. Everywhere on the Moon he saw craters. Big craters, small craters, enormous craters. Why then was the Earth practically devoid of them? Was it credible that the Moon could be subject to the slings and arrows of outrageous impacts while its larger sister, our planetary home, missed them all? He was convinced that the evidence for impact craters must exist on Earth, but where were they all? Gene had a good idea what kind of material would be created when a large object hit the Earth. It was just a matter of finding it.

Gene found the answer in Nordlingen. During a visit to the town in 1960 he became fascinated by the stones of St Georg’s Church in the centre of the town. He immediately realised that the church walls contained coesite, a material only created as a result of a massive meteorite impact. The rock had been mined locally from the Ries. This lead to a simple, stark conclusion. The Ries had been formed as a result of a gigantic meteor impact. “It was the first big impact crater on the Earth that we could prove was an impact crater, and that just changed the whole ballgame”, said Shoemaker.
Here is what we know. 15 million years ago, two large objects, one measuring up to 1km in diameter, crashed into southern Germany. The large object hit Nordlingen. Hitting the ground at a speed of 45,000 km per hour, it punched a hole 4km deep into Earth’s crust, vaporising on impact. The surrounding rocks were compressed to a quarter of their size by the impact and they responded with an explosion measuring 18,000 megatonnes of TNT, hundreds of times larger than the greatest nuclear bomb ever detonated on this planet. An enormous shock wave killed all living things for a hundred kilometers in every direction with devastating effects felt much further afield. A mushroom cloud 30km high was generated. Much of this cloud, composed of melted rock from deep within the crust, subsequently fell back to earth, covering the crater and the region around the Ries with a material known today as Suevite. The church of St Georg in Nordlingen is built from this material.
A massive amount of bedrock was ejected ballistically, forming rocks known as Bunte Breccia. The deepest rocks landed close to the impact zone while rocks close to the surface were hurled over great distances. Some limestone blocks have been found 70km from the crater while glassy rocks known as Moldavites have been discovered 400km away in the Czech Republic.
The 1km deep hole left by the impact became a lake and life returned to the Ries. Over time the lake itself became clogged with sediment and subsequent glaciations flattened out the region into the wide plain we see today.
A particularly good place to see the crater expanse is the Daniel, the steeple of the aforementioned St. Georg’s Church. From a height of 80 metres you can see in all directions the flat, fertile countryside with the hills forming the outer crater in the far distance.
A smaller meteorite simultaneously hit the region of Steinheim am Albuch, 40km away from Nordlingen. While the resulting crater was much smaller – just 3km in diameter – a distinct central uplift remains. Steinheim is a village well worth visiting. There is an excellent little museum in the hamlet of Sontheim im Stubenthal and plenty of well marked trails with wonderful views of the crater.
Rieskrater Museum
Hintere Gerbergasse 3
86720 Nördlingen, Deutschland
09081 273822-0
Hochfeldweg 5
89555 Steinheim, Deutschland