Ancient Mass Extinctions Caused by Cosmic Radiation, Scientists Say

Scott Norris
for National Geographic News
April 20, 2007
Cosmic rays produced at the edge of our galaxy have devastated life on Earth every 62 million years, researchers say.

The finding suggests that biodiversity has been strongly influenced by the motion of the solar system through the Milky Way and of the galaxy's movement through intergalactic space.

Mikhail Medvedev and Adrian Melott, both of the University of Kansas, presented their new theory at a meeting of the American Physical Society earlier this month.

The theory offers the first explanation for a mysterious pattern previously noted in the fossil record.

"There are 62-million-year ups and downs in the number of marine animals over the last 550 million years," Melott said.

Until now, however, even the scientists who first discovered the cyclical pattern had not been able to explain it. (Read related story: "Mystery Undersea Extinction Cycle Discovered" [March 9, 2005].)

A number of possible explanations had been considered—including volcanic activity, comet impacts, and changes in sea level—but none could account for the phenomenon's regularity.

The Kansas researchers discovered that high rates of extinction in the cycle coincide almost perfectly with periodic "excursions" of the solar system outside the central plane of the Milky Way galaxy.

"Excursions to galactic north coincide with drops in biodiversity," Melott said.

During these periods, which include some of the largest mass extinctions known from the fossil record, Earth is bombarded with high levels of cosmic radiation.

The radiation may harm biodiversity by causing mutations or by triggering climate change, the researchers said.

Richard Muller is the University of California, Berkeley, physicist who first discovered the 62-million-year cycle with his graduate student Robert Rohde.

"We spent a year searching for possible mechanisms," Muller said.

"I was stunned when I learned that Medvedev and Melott had succeeded where we had failed, and I congratulate them."

Cosmic Cause

Our solar system travels through the disk-shaped Milky Way on a complicated circuit that takes about 225 million years to complete. (See an interactive map of the solar system.)

At regular intervals, the system's wanderings take it up and down through the thin central portion of the disk. The sun reaches its farthest distance from the central plane every 62 million years.

The entire galactic disk, meanwhile, is hurtling through the hot gas that surrounds it at about 125 miles (200 kilometers) a second.

"The movement [of the Milky Way] is not edge-on like a Frisbee," Melott noted. Instead, he said, it is flat, "like a pie in the face."

The new theory suggests that cosmic rays are continually generated in a shock wave produced where the galaxy's "northern" or forward side collides with surrounding gases.

As the solar system rises above the central plane it sticks out like a cherry on top of the flying galactic pie—closer to the source of the cosmic radiation.

"We're exposed to the shock front more when we're emerging on the north side of the galactic disk," Melott said.

At the same time, he explained, the solar system receives less protection from powerful magnetic fields that form a shield from cosmic radiation in the dense, central portion of the galaxy.

Impacts on Earth

Melott said his group applied their model to the largest existing fossil database, which reconfirmed the finding of a 62-million-year fluctuation in diversity.

In a paper recently accepted by Astrophysical Journal, the Kansas researchers discuss various possible mechanisms by which cosmic-ray exposure could result in mass extinctions.

One possibility is that organisms receive harmful doses of radiation from high-energy particles known as muons, which are produced by cosmic rays colliding with Earth's atmosphere.

"Cosmic rays themselves are not really that dangerous," said Medvedev. "They create [charged particles] that propagate down through the atmosphere—especially muons that can go below the sea level."

Changes in the chemistry of the atmosphere, and accompanying depletion of the ozone layer, may also cause increased mutations, he added.

In addition, charged particles produced by cosmic-ray bombardment may cause greatly increased cloud cover, leading to climate change.

The researchers said their model does not explain all major mass extinctions.

For example the demise of the dinosaurs, which is thought to have been caused by an asteroid impact, does not fit the 62-million-year cycle. (Read related story: "'Dinosaur-Killer' Asteroid Crater Imaged for First Time" [March 7, 2003].)

As for what lies ahead, the news is mixed. The solar system has recently passed the galactic mid-plane and is on its way up, Melott said, which could mean greater exposure to radiation.

But, he added, "the next cosmic ray effect is about ten million years ahead of us."

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