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17 January 2007

Ordovician Meteorite Shower Caused by One of the Biggest Asteroid Collisions in the Solar System

New study published by Isotope Dating Group (leader: Privatdozent Dr. Mario Trieloff) — "Even today, a quarter of the meteorites hitting the earth can be traced back to a major cosmic event 470 million years ago"

"470 million years ago, in the asteroid belt between the inner and outer solar system, one of the most colossal collision events in the history of the solar system took place when two asteroids probably measuring several hundred kilometres slammed into one another. This event released a swarm of relatively small fragments that were diverted to the earth-like planets of the inner solar system and rained down on the earth within a few million years, a brief period in geological terms. Remnants of small meteorites have recently been found in fossil deposits of the mid Ordovician, while kilometre-sized fragments have left huge impact craters measuring up to 30 km across."

Ekaterina Korochantseva is a doctoral student at the University of Heidelberg's Institute of Mineralogy and this is her summary of a new study from that quarter published in the January issue of the journal Meteoritics and Planetary Science (Vol. 42, pp, 113-129) under the title "L chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40Ar-39Ar dating" (authors Ekaterina V. Korochantseva, Mario Trieloff, Cyrill A. Lorenz, Alexei I. Buikin, Marina A. Ivanova, Winfried H. Schwarz, Jens Hopp, Elmar K. Jessberger).

"Even today, a quarter of the meteorites hitting the earth can be traced back to this major cosmic event 470 million years ago," explains Privatdozent Dr. Mario Trieloff, head of the Isotope Dating Group. About 10 years back a research group headed by Professor Birger Schmitz discovered fossil meteorites in a Swedish quarry, embedded in limestone from the mid Ordovician. As they were badly disintegrated it was impossible to allocate them with certainty to one of the known types of meteorite by means of chemical or mineralogical methods. With a sophisticated radiometric dating method the Heidelberg scientists have demonstrated that a group of meteoritic rocks (so-called L chondrites) still hitting the earth today stem from an asteroid ripped apart by a major collision exactly 470 (+/- 5) million years ago. This collision caused rock from the two asteroids involved to melt and also set off colossal shock waves with a maximum pressure of several hundred kilobars, comparable with the static pressure prevailing 2,000 kilometres down in the centre of the earth.

In addition, the Heidelberg scientists were able to show that the rock layers from the Ordovician in which the fossil meteorites were deposited are 467 (+/- 2) million years old. The connection is obvious. 470 million years ago, one of the biggest known collisions in the asteroid belt shattered a small planet a few hundred kilometres across. Fragments of various sizes were diverted earthwards and for a few million years about 100 times as many meteorites rained down on the earth as at any other period in the Neozoic era. The impact of kilometre-sized fragments was also considerably more frequent. The energy released by such impacts is vast and must have had global effects at the time. A crater 30 kilometres wide is caused by impact energy equivalent to about 10 million nuclear bombs of the kind dropped on Hiroshima. This is equivalent to a mountain higher than Ben Nevis hitting the earth at a speed of 50,000 kph. The amount of cosmic dust in the atmosphere will also have been above-normal for a period of several million years. Today about half of all the dust particles in the upper atmosphere are extraterrestrial in origin. At the time the proportion of this dust, which remains where it is for a very long time, was about 100 times higher than it is today. Prof. Birger Schmitz, who discovered the fossil meteorites, conjectures that the biodiversity explosion triggered in the mid to late Ordovician has something to do with the climate changes and the cosmic bombardment in that period. In other words, a profound transformation of our biosphere was caused by a process taking place in the outer reaches of the solar system.

"We are especially gratified by the fact that the precise dating of this major cosmic event was achieved using a Heidelberg dating method," says Rainer Altherr, director of the Institute of Mineralogy. "The so-called 40Ar-39Ar technique is an improved version of the K-Ar method developed in Heidelberg in the 1950s by Wolfgang Gentner. Isotope dating plays an increasingly significant role in the modern earth sciences, notably here in Heidelberg, and is supremely well suited to act as a bridge between various disciplines like astronomy, solar system research, geology/palaeontology, mineralogy and the environmental sciences."

The research was joint-funded by the German Research Foundation, the Geesthacht Research Centre and the Klaus Tschira Foundation gGmbH (Heidelberg).

Please address any inquiries to:
Dr. Mario Trieloff
Institute of Mineralogy
University of Heidelberg

General inquiries from journalists should be addressed to:
Dr. Michael Schwarz
Press Officer of the University of Heidelberg
phone: 06221/542310, fax: 54317

Irene Thewalt
phone: 06221/542310, fax: 542317

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