Ancient Mass Extinctions, Cyclic and Caused by Cosmic Radiation?

At the end of the Cretaceous Period, approximately 65 million years ago, a catastrophic extinction event ended dinosaurs' dominance on land. One group of dinosaurs is known to have survived to the present day. Taxonomists consider modern birds to be direct descendants of theropod dinosaurs.

The rise and fall of species on Earth might be driven in part by the undulating motions of our solar system as it travels through the disk of the Milky Way, scientists say.

According to Mikhail Medvedev and Adrian Melott, researches at University of Kansas, cosmic rays produced at the edge of our galaxy have devastated life on Earth every 62 million years.

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

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.

Their idea hinges upon the fact that, appearances aside, stars are not fixed in space. They move around, sometimes rushing headlong through galaxies, or approaching close enough to one another for brief cosmic trysts.

In particular, our sun moves toward and away from the Milky Way's center, and also up and down through the galactic plane. One complete up-and-down cycle takes 64 million years, suspiciously similar to Earth's biodiversity cycle.

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.

However, the researchers said their model does not explain all major mass extinctions.

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.

"The next cosmic ray effect is about ten million years ahead of us", he said.