ScienceWeek

Physics Today: October 2007

ScienceWeek http://scienceweek.com/2007/071028h.htm

Varying cosmic-ray flux may explain cycles of biodiversity

Bertram Schwarzschild

The solar system oscillates through the Milky Way's midplane with a period of about 60 million years. That cyclic excursion might be causing periodic fivefold increases in Earth's exposure to extragalactic cosmic rays.

Two years ago physicist Richard Muller (University of California, Berkeley) and his student Robert Rohde discovered a 62-million-year periodicity in the wordwide diversity of marine animals. They had undertaken a Fourier analysis of the time variation of the total number of genera listed in John Sepkoski's authoritative Compendium of Fossil Marine Animal Genera, which begins about 542 Myr ago-when animals first developed the hard exo- and endoskeletons that facilitate fossilization.

Sepkoski and coworkers at the University of Chicago had been compiling this valuable data set for decades until his death in 1999. Dating the first appearances and final disappearances of tens of thousands of genera depends crucially on age estimates of the stratigraphic layers in which their fossils are found. In 2004 the International Commission on Stratigraphy released a much improved chronology of very old strata based on the best available radioactive potassium–argon dating. "As soon as we applied the new ICS chronology to Sepkoski's compendium," recalls Muller, "the 62-million-year periodicity leapt out at us with unambiguous clarity [see figure 1]. It cried out for explanation, but we couldn't come up with one."

"We had actually been looking for a 26-Myr peak in support of my 20-year-old Nemesis conjecture," says Muller. Nemesis is the name he gave to a hypothetical faint stellar partner of the Sun that was supposed to pass through the solar system's Oort cloud once every 26 Myr, loosing a deadly cometary bombardment of Earth (see PHYSICS TODAY, February 1987, page 17).

BOOK SOURCES:

cosmic rays
Milky Way
biodiversity