The Great Pummeling

Deep in the layers of the Earth, ancient rock shows signs of a planet, 250 million years ago, teeming with plants, fish, reptiles and proto-mammals . Then, evidence of life around the globe all but vanishes.

In a mere 100,000 years, a blink of the eye in geological terms, 90 percent of the earth's life disappeared. Now, an analysis of meteorite fragments described in this week's Science magazine helps confirm one scenario--that space rock crashed into the earth at the end of the Permian period, devastating the environment in a catastrophe much like the one that wiped out the dinosaurs 185 million years later. This means that the earth was pounded from space not once, but twice. "It appears to us that the two largest mass extinctions in earth's history were both caused by collisions with ... meteoroids," says the study's author, Asish Basu, a geochemist at the University of Rochester.

Exactly what came to earth 250 million years ago and where it crashed remains mysterious. But scientists have a vivid picture of the resulting devastation. Imagine an asteroid or comet, perhaps the size of Mount Everest, slamming into a coastline, transmitting temperatures of several thousand degrees and pressures 1 million times greater than the weight of the earth's atmosphere. The impact sets off scorching flows of lava. A wave of hot vapor kills everything in its path. Tidal waves overwhelm the land. Clouds of debris block the sun. Lush vegetation and a world of creatures perish, and fungi inherit the earth. Paleontologists poetically call this event the "Great Dying." But some also argue that mass extinctions were necessary to advance evolution. After another passage of millions of years, dinosaurs thrived in the niches once occupied by other animals. The demise of the dinosaurs created room for mammals and, eventually, us.

If an extraterrestrial impact didn't cause the earlier die-out, then what did? Back then, the earth's landmass consisted of one supercontinent called Pangaea and a superocean, Panthalassa. Many scientists believe that life was already struggling when the meteoroid arrived, assuming it did. Earth suffered from volcanic eruptions in what is now Siberia, where lava flowed from huge fissures in the crust. Pangaea may have blocked the movement of water, causing disastrous changes in sea levels and climate.

Of course, the most dramatic evidence of a collision would be a hole in the ground. In the early 1980s, geophysicists searching for oil and gas buried in the Gulf of Mexico found an immense circle 110 miles across, now known as the Chicxulub crater, dating precisely to the demise of the dinosaurs. The crater is also rich in iridium, a chemical element most often found in objects in outer space. In fact, the discovery of a high iridium concentration in clay from that period first led to the landmark hypothesis that a giant impact killed the dinosaurs.

The 250 million year geological line, known as the Permian-Triassic boundary, lacks iridium, and scientists are still searching for a tell-tale crater. But Basu and others argue that neither are necessary to confirm a meteoroid collision. Other clues, they say, support the theory. The fragments he analyzed were buried in claystone in Graphite Peak, Antarctica, just above a bed containing the last occurrence of glossopteris--a tropical ginkgolike plant that went extinct. Diggers at that site had already turned up quartz crystals with multiple fractures suggesting that they were "shocked"--as if by a collision. Few earthly conditions can disfigure quartz, which is stable at the highest temperatures and pressures deep inside the earth's crust. The site also yielded exotic carbon molecules known as fullerenes or "buckyballs" that contain extraterrestrial gasses. Named after R. Buckminster Fuller, the inventor of the geodesic dome they resemble, buckyballs trap the noble gases helium and carbon inside their cagelike structures. Astrobiologist Luann Becker of the University of California, Santa Barbara, has examined the molecules from several Permian sites and concluded that they were formed outside the solar system and delivered to earth by a comet or asteroid.

Although shocked quartz and buckyballs provide intriguing evidence of an impact, so far their presence is considered inconclusive. Basu's pitted iron-rich meteorite fragments may be more persuasive because they are well-preserved, evidently unique to the Permian-Triassic boundary, and concentrated in one layer at Graphite Peak, suggesting a single event. Matching metallic grains have been found in Permian sites in southern China and Japan. Basu's research assistant, Rama Chakraborti, will be down in Antarctica with Luann Becker and a research team until Christmas, searching for more evidence. Meanwhile, the rest of us are left to wonder, when do we get hit next?