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New mass-extinction mechanism. It is not only difficult to find consensus on exactly how many mass extinction events have occurred in Earth’s history, but even more difficult to determine their causes. Besides meteorites, the most widely accepted causes of mass extinction include glaciation, volcanism and the formation/glaciation of the mega-continents Gondwana and Pangea. New research was presented in January at the American Astronomical Society meeting supporting the idea that a lesser-known extinction – the Pliocene-Pleistocene boundary 2 million years ago – was caused by cosmic rays from a nearby supernova (a massive exploding star). Narcisco Benítez at John Hopkins University in Baltimore, Maryland, and Jesús Maíz Appellániz of the Space Telescope Science Institute in Virginia traced the motion of a cluster of young, short-lived stars formed from the debris of about 20 supernovae that have exploded over the last 20 million years. The cluster, called the Scorpius-Centaurus OB association, passed within 130 light-years of Earth about two million years ago.

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