Magazine article Science News

Northern Exposure: The Inhospitable Side of the Galaxy?

Magazine article Science News

Northern Exposure: The Inhospitable Side of the Galaxy?

Article excerpt

The solar system's periodic visits to the northern side of the Milky Way expose life on Earth to extra cosmic rays that have caused catastrophic mass extinctions, two astrophysicists propose.

Biodiversity has had well-known ups and downs over the cons, with major extinctions followed by rebounds. In a 2005 study, Robert Rohde and Richard Muller of Lawrence Berkeley (Calif.) National Laboratory found that these swings were surprisingly regular, most of them taking place at intervals of about 62 million years. The researchers reached their conclusion after examining one of the most comprehensive long-term biodiversity surveys, a compilation of fossil data that charted the number of marine-life genera over the past 500 million years.

The extraordinary dinosaur kill 65 million years ago doesn't fit in the cyclic pattern, and experts widely blame it on the impact of a large asteroid.

To explain the cyclic pattern of mass extinctions, Rohde and Muller considered a phenomenon that has just about the right periodicity. As the solar system orbits around the galaxy, it swings from one side to the other of the galactic plane every 63 million years. Gravity from the rest of the galaxy's mass pulls the solar system back each time.

Perhaps when the sun is at the maximum distance from the galactic plane, Earth's biodiversity is at greatest risk, Rohde and Muller speculated. But that would put mass extinctions every 31.5 million years, not every 63 million. It wasn't clear why one side of the galaxy's plane would be more dangerous to life than the other.

Mikhail Medvedev and his colleagues of the University of Kansas in Lawrence now propose an explanation that rests on variations in the number of high-energy particles, known as cosmic rays, that strike Earth from space. They argue that because the galaxy is moving toward a large cluster of galaxies in the direction of the Virgo constellation, cosmic rays would be more abundant on the galaxy's north side--according to the view from Earth. …

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