© Detlev van Ravenswaay, Astrofoto, Peter Arnold Images, PhotolibraryAn asteroid hurtles toward Earth in an artist's rendering.
Double impact may have caused tsunami, global cooling

Pieces of a giant asteroid or comet that broke apart over Earth may have crashed off Australia about 1,500 years ago, says a scientist who has found evidence of the possible impact craters.

Satellite measurements of the Gulf of Carpentaria (see map) revealed tiny changes in sea level that are signs of impact craters on the seabed below, according to new research by marine geophysicist Dallas Abbott.

Based on the satellite data, one crater should be about 11 miles (18 kilometers) wide, while the other should be 7.4 miles (12 kilometers) wide.

For years Abbott, of Columbia University's Lamont-Doherty Earth Observatory, has argued that V-shaped sand dunes along the gulf coast are evidence of a tsunami triggered by an impact.

"These dunes are like arrows that point toward their source," Abbott said. In this case, the dunes converge on a single point in the gulf - the same spot where Abbott found the two sea-surface depressions.

The new work is the latest among several clues linking a major impact event to an episode of global cooling that affected crop harvests from A.D. 536 to 545, Abbott contends.

According to the theory, material thrown high into the atmosphere by the Carpentaria strike probably triggered the cooling, which has been pinpointed in tree-ring data from Asia and Europe.

What's more, around the same time the Roman Empire was falling apart in Europe, Aborigines in Australia may have witnessed and recorded the double impact, she said.

Aborigine Eyewitnesses

Based on the new research, Abbott thinks the two craters were made by an object that split into pieces as it approached Earth.

To make a pair of craters this big in the seafloor's soft sediments, the original object must have been about 2,000 feet (600 meters) across before it broke up, she said.

Core samples from the region back up the case for such an impact, Abbott added. Previous research had found that the samples contain smooth, magnetic spherules, which were probably created when the object's explosive landing melted material and blasted it into the sky.

Furthermore, a 2004 paper in the journal Astronomy and Geophysics suggested that the circa-A.D. 500 global cooling event might have been caused by dust from an impact of approximately the size Abbott has now calculated for Carpentaria.

It's even possible the impact had eyewitnesses: Aboriginal rock art from the region seems to have recorded the event, although the researchers examining this art declined to discuss details until after their paper has been published.

Still, Duane Hamacher, a Ph.D. student at Macquarie University in Sydney not involved with the rock-art work, recently demonstrated that Aboriginal stories can be used to locate meteorite craters.

"Numerous examples of fiery stars falling from the sky and striking the earth, causing death and destruction, are found throughout Aboriginal Dreamings [spiritual folk stories] across Australia," Hamacher wrote on his blog.

"The descriptions seem to indicate that the events were witnessed, not simply 'made-up.'"

In findings yet to be published, Hamacher used one set of Aboriginal stories, along with images in Google Earth, to locate a 919-foot-wide (280-meter-wide) impact crater in Palm Valley, in Australia's Northern Territory.

Too Many Meteorite Strikes?

But some experts are skeptical of Abbott's conclusions, which were presented last December at a meeting of the American Geophysical Union in San Francisco.

One potential problem is the presence of two separate craters at the Gulf of Carpentaria site, said physicist Mark Boslough of Sandia National Laboratories in Albuquerque, New Mexico.

If a large impactor had broken up on its final approach to Earth, he said, the fragments would still have been very, very close together when they landed: "It essentially will behave as one piece," creating a single crater, Boslough said.

In addition, he said, Abbott and other members of an informal association called the Holocene Impact Working Group are finding evidence for more impact events than astronomers calculate should be possible.

Abbott and colleagues argue that several climate events during the Holocene epoch - 11,500 years ago to the present - were actually triggered by impacts, and therefore such large impacts are more common than currently believed.

Boslough and other experts, meanwhile, have been cataloging asteroids and other bodies that cross Earth's orbit and calculating how frequently space rocks should strike the planet.

"We have a pretty good idea about how many there are and what the frequency of impact should be, and the abundances based on [the working group's claimed crater count] are orders of magnitude greater than what astronomers observe," Boslough said.

"It's pretty hard to imagine where these things could be coming from so that astronomers wouldn't see them."

Instead, it's more likely that the craters found by the working group have volcanic origins, the impact skeptics conclude.

Abbott acknowledges that her case for Carpentaria isn't 100-percent proven. But in general, she said, "I think we're getting very close to being able to show there were a lot of impacts in the last 10,000 years."