We know that the earth was attacked by asteroids and comets in its history. We just have to remember the sad ending of the dinosaurs. However, even if there were many asteroids and comets falling on earth, it is hard to study them. Erosion and geological changes like volcanos and water makes it hard to identify the craters and to study its age. Much better are the circumstances on the moon. There is no water and no volcanos. Therefore, the craters stay “untouched”. And when there would be a time in which many asteroids and comets attacked the moon, it just seems logical to assume that in the same time there were also asteroids and comets falling on earth as they are close neighbours. Therefore, knowing the history of the crater impacts of the moon also reveals some informations about the history of our earth. Interestingly, lunar probes collected by Apollo astronauts showed that around 3.9 billion years ago the crust of the moon was so hot that all rocks in it metamorphosed. This was the birth of the lunar cataclysm hypothesis: the heating up of the crust may be created by a large number of asteroid and/or cometary collisions in a brief pulse of time. However, did the lunar cataclysm really happen? The Apollo astronauts just collected probes in a small area so just because these probes are from the same age, it has not to be valid for the whole moon surface. However, in December 2000, Cohen et al. published a study which results support the hypothesis. They studied the ages of 4 lunar meteorites which were found on earth. The lunar meteorites are like random samples from the moon surface and indeed the scientists found in them nothing which was older than 3.9 billion years. So that supports the hypothesis that at this time point many collision events happened on the moon. And maybe this also affected our earth. Remembering that the earliest isotopic evidence Scientists are now wondering if this cataclysmic bombardment may have affected life on Earth or been involved in life's origins. The earliest isotopic evidence of life is ~3.8 billion years old. Can this be an accident? (please read here more about it: http://www.lpi.usra.edu/exploration/science/lunarCat/ ) "Support for the lunar cataclysm hypothesis from lunar meteorite impact melt ages."
B. A. Cohen, T. D. Swindle, and D. A. Kring. Science 290.5497 (2000): 1754-1756.
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