Northern Exposure: The inhospitable side of the galaxy?

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.

COSMIC PERIL. As the Milky Way (represented here by a similar galaxy) whirls around, the solar system’s wobbly trajectory (green curve) periodically exposes Earth to harsh cosmic rays. The white dashed line shows the sun’s motion as it would appear in the plane of the galaxy. Medvedev/ Univ. of Kansas

Biodiversity has had well-known ups and downs over the eons, 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.

A particle flow similar to the solar wind emanates from the Milky Way as a whole, and as the galaxy moves, that wind runs into the tenuous medium that pervades intergalactic space. The collision creates a shock wave. The Kansas team calculates that when electrically charged particles rebound within the shock wave, they gain enough energy to turn into cosmic rays.

When a cosmic ray hits the upper layers of the atmosphere, it triggers a shower of millions of energetic electrons and other particles, some of which can penetrate to land and into the oceans. The particles have a variety of effects. For example, they may alter cloud coverage or damage DNA, with potentially fatal consequences for entire species.

“Drops in biodiversity correspond to peaks in cosmic rays,” Medvedev says. However, he and his colleagues stress that they haven’t identified the mechanism linking cosmic rays and extinctions.

“I was stunned when I learned that [Medvedev’s team] had succeeded where we had failed” at explaining the 62-million-year cycle, Muller says.

Charles Dermer, an astrophysicist at the Naval Research Laboratory in Washington, D.C., says that the new explanation is “very tantalizing” but that it rests on Rohde and Muller’s biodiversity cycles, which are not firmly established.

Medvedev and his colleagues say that the cosmic ray bombardments would also increase gamma rays from the north side of the galaxy, a prediction that new gamma-ray observatories may test in the next few years.

The researchers presented their work this week, in Jacksonville, Fla., at a meeting of the American Physical Society. The report is also due to appear in Astrophysical Journal.

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