Do pieces come from killer rock?

SAN FRANCISCO — Scientists have come up with new evidence that a giant space rock slammed into Earth and killed off most of its species not once in history, but twice.

Schoolchildren can relate the story of how the dinosaurs and many other creatures went extinct after an asteroid or comet hit the Earth 65 million years ago. But an even greater die-off occurred 250 million years ago in which as many as 90 percent of all species on Earth were snuffed out.

In 2001, a group of researchers made headlines when they argued that this extinction, too, took place when a comet or asteroid smashed into the Earth. Now, the scientists have reported new proof of a cosmic collision — tiny fragments, unearthed in Antarctica, of what they say is the killer rock itself.

If confirmed, the finding wouldn't be just the smoking gun linking the mass extinction to an extraterrestrial impact; it would be the bullet itself.

The new work, reported last month in Science and elaborated upon last week at an American Geophysical Union meeting, hasn't met with universal acceptance. Other scientists say it's hard to imagine how pieces of a space rock could survive for 250 million years, rather than weathering away as meteorites often do.

"It's mind-boggling to think that they could have survived 250 million years in those sediments," says Frank Kyte, a geochemist at UCLA.

In 1998, Kyte reported finding a piece of the meteorite that killed the dinosaurs in a core drilled from the bottom of the North Pacific Ocean. Over time, he has convinced most other specialists that the fragment is, indeed, a chunk of the doomsday rock.

Something similar may have just been found in 250 million-year-old rocks from Graphite Peak, Antarctica. Gregory Retallack, a geologist at the University of Oregon, collected the rocks in the early 1990s, looking at the geologic record preserving the boundary between the Permian and Triassic geologic periods.

Soon after, Retallack reported finding quartz grains in those rocks that had been marked by the shock of an extraterrestrial impact. He proposed that a collision had taken place 250 million years ago, but he never convinced many other researchers that he was right.

Things changed in 2001, when a team led by Luann Becker, now at the University of California, Santa Barbara, reported finding molecules called buckyballs in 250-million-year-old rocks from Meishan, China.

The soccer-ball-shaped buckyballs contained extraterrestrial gases, Becker's team argued, which suggested that an impact had occurred.