Lectures in Astrobiology Volume II
Based on material delivered at several summer schools, this book is the first comprehensive textbook at the graduate level encompassing all aspects associated with the emerging field of astrobiology. Volume II gathers another set of extensive lectures cov
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9.1 Introduction 9.1.1 Terrestrial Craters In July 1994, the twenty-one small fragments of comet Shoemaker Levy 9 struck Jupiter. Images of these collisions appeared almost live on the Internet and on television, (see http://www2.jpl.nasa.gov/sl9/ for the event’s image library). The solar system is indeed a violent place; the numerous craters of various sizes visible on the moon also attest to this as well. This satellite itself is most likely the result of the collision, about ∼ 4.5 Ga ago between the young Earth and a Marssized planet (Canup 2004). Collision is a major process in planetary evolution; catastrophic asteroid or comet impacts have occurred and will continue to occur in the solar system. Today approximately 170 impact craters are known on Earth (Fig. 9.1); and a couple of new ones are discovered every year. A regularly updated list of craters is found on the following website: http://www.unb.ca/passc/ ImpactDatabase/. There, the craters are listed by name, size, location and many are illustrated by excellent images. The traces of many more collisions have been erased by geological processes such as tectonic, erosion, sedimentation, volcanism or the produced craters are hidden, buried under kilometers of younger sediments. Many more impact events have taken place in the oceans, which cover more than two thirds of the Earth’s surface. The size of terrestrial craters varies between a few tens of meters to more than 200km. Geology also influences the age and size distribution of the known impact craters. The majority is younger than 200 million years (Fig. 9.2). Small impact structures (< 5 km) erode rapidly and are thus under-represented, except for the most recent ones. Many craters lie on the old continental shields, such as Scandinavia, Canada or Australia, where they are not much affected by erosion or tectonics for extended periods. Many craters remain to be discovered in Africa or South America and of course at the bottom of the oceans, where so far no or little systematic searches have been carried out. Some 25 craters (∼15% of the currently known population) have formed in the oceans, but always in less than 200m water-depth, on the continental shelf (Dypvik and Jansa 2003). Plate tectonic activity has brought many to the
Philippe Claeys, Impact Events and the Evolution of the Earth. In: Muriel Gargaud et al. (Eds.), Lectures in Astrobiology, Vol. II, Adv. Astrobiol. Biogeophys., pp. 239–280 (2007) DOI 10.1007/10913314 9 Springer-Verlag Berlin Heidelberg 2007
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Fig. 9.1. Geographic distribution of known terrestrial craters; Chicxulub, Vredefort and Sudbury are in blue, the size of the dot is proportional to that of the crater (modified after French 1998)
surface and today only six of them remain in the marine realm. Their morphology and characteristics resemble those of craters formed on the emerged part of the continental crust (Dypvik et al. 2003). Several factors are responsible for the lack of identified oceanic impact structures. The ocean floors are young (< 200 million y
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