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More than one object my have collided with the Earth 65 million years ago suggests new research published in the journal Meteoritics and Planetary Science. Simon P. Kelley, from the Open University, UK, and Eugene Gurov, from the National Academy of Ukraine, have re-examined the age of a buried 24 km wide crater in the Ukraine known as Boltysh. In 1993 scientists had determined this impact to be 73 million years old. However, through measurements of radioactive trace elements and their decay products, Kelley and Gurov have refined the age of Boltysh to 65.2 � 0.6 million years. The 170 km diameter crater, Chicxulub, which is thought to be largely responsible for the mass extinction at the end of the Cretaceous period, has an age is 65.5 � 0.6 million years and thus both craters formed at the same time as far as scientists can tell. Kelly and Gurov, say that if they did this suggests the Earth may have been hit by a swarm of comets.
Craters the size of Boltysh are thought to form roughly every 2 to 3 million years. The similar ages of Chicxulub and Boltysh thus do not prove that two objects collided with the Earth simultaneously, however, they do suggest it is a distinct possibility. The presence of two craters would suggest that several comets rather than asteroids hit the Earth at this time. Comets are often seen to break-up into a trail of icy comet nuclei as the pass through the inner solar system whereas, apart from much smaller satellites, asteroids travel alone. The end Cretaceous mass extinction might, therefore, have been caused by an event similar to the collision of fragments of Comet Shoemaker-Levy with Jupiter in 1994.
More info: Sky and Telescope Article
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