Toward the end of the Eocene epoch 35 million years ago, temperatures plummeted and the Earth's primitive inhabitants endured a cold spell that lasted 100,000 years. The length of that ordeal and what brought it on have long puzzled experts. Various theories to explain it have been put forward--from variations in solar output to changes in the tilt of the Earth's axis--and generally dismissed. Now two scientists, writing in the Journal of Geophysical Research, have proposed a novel idea: the possibility that our planet was once encircled by a huge, Saturn-like ring created by a glancing blow from an asteroid.
It wouldn't have been the first time Earth's climate was altered by a large incoming object. Thirty million years earlier, the impact of the asteroid believed to have wiped out the dinosaurs enshrouded the Earth in dust that blocked sunlight and caused temperatures to fall. But that shroud dissipated, possibly in a matter of months, and the climate quickly warmed up again. Whatever caused the Eocene cooling must have lingered much longer.
The most likely explanation, say Peter Fawcett of the University of New Mexico and Mark Boslough of the Sandia National Laboratory, is planetary rings. They speculate that a large asteroid hit the Earth at an oblique angle and plowed across the surface for some distance before ricocheting away. In the process, the theory goes, it sprayed molten and vaporized Earth and asteroid chunks into space, where some pieces went into orbit and eventually formed an opaque ring.
Using a computerized climate model, the scientists calculated the consequences. "Like the rings of Saturn," suggests Boslough, "this ring cast a shadow over the tropics, drastically affecting the global climate." And unlike the dinosaurs' dust shroud, he says, the Eocene ring and its shadow persisted until the orbiting chunks were slowed by the upper fringes of the atmosphere, then finally dropped back to Earth. --By Leon Jaroff