In a controversial study, gravity maps of the Antarctic continent have revealed what could be multiple asteroid impact sites.
Gravity anomalies in Antarctica might mark spots where hundreds of thousands of years ago the frozen continent was struck by giant meteorites or fragments of a comet.
These, scientists suggest, would have punched deep into the crust, shattering rock and producing zones of slightly lower gravity waiting to be found by modern instruments.
Such instruments, carried aboard satellites, aeroplanes, and snow machines, can measure the Earth’s gravity field with remarkable precision.
“The gravity meter we used [is so sensitive] that if I should set it on a desk and read the gravity from it, then set it on the floor, I can detect the difference,” says John Weihaupt a geophysicist at the University of Colorado.
As far back as 1976, Weihaupt found a large region of low gravity in Antarctica’s Victoria Land that he believes to be a meteor impact crater, at least 243 km across – about three times as wide as America’s Chesapeake Bay, also believed to have been formed by an asteroid impact.
But in new gravity maps of the entire continent published in the journal Lithosphere, Weihaupt’s team found 14 additional such anomalies, which they think might mark the sites of additional impacts.
The anomalies could be caused by other under-ice features, such as buried mountains or tectonic trenches.
But there is no evidence for such features beneath this part of Antarctica, Weihaupt’s team said. Rather, they appear to be places where the impact not only created a depression, but also reduced the rock’s density (and therefore its gravity) by shattering it into rubble, known
as breccia.
Shotgun blast
Weihaupt thinks the number of anomalies means ancient Antarctica may have been peppered with a shotgun blast of space rocks up to 2 km across.
Furthermore, he said, the anomalies are arrayed in an ellipse, with the largest at one end and the smallest at the other.
“This is exactly the pattern one gets from the distribution of meteorite craters in a multiple impact event,” he said – i.e., a situation in which a large asteroid or comet breaks up before impact.
When did the impact occur? “That’s a tricky question,” Weihaupt says. A definitive answer would require drilling all the way through the ice to the underlying rocks.
But impacts produce clouds of tiny particles called tektites (and smaller ‘mirotectites’) that later fall back to Earth. These have been found in Australia and in parts of Antarctica, in two layers, one dated to about 700,000 years ago, and the other to 481,000 years ago.
Scientists have long speculated about the effect on civilisation if a large meteor struck a continent, filling the air with sunlight-blocking dust. But nobody has thought much about what would happen if hits ice.
“The ramification for the environment . . . and mankind . .. are enormous,” Weihaupt’s coauthor, Alan Rice, of the Department of Earth and Planetary Sciences at the American Museum of Natural History, New York, said.
“While NASA is combing the skies for the asteroid that will kill us all, no one has given thought to the response of the Antarctic Ice Cap if that got hit.”
One prospect, however, is that much of the ice would destabilise, rapidly flowing into the sea and increasing sea level by perhaps 5 metres.
Other scientists are skeptical about whether the anomalies actually are craters. One problem, says Dallas Abbott, a marine geophysicist at Columbia University’s Lamont-Doherty Earth Observatory at Columbia University’s Lamont Doherty Earth Observatory is the mathematical techniques often used to rule out density variations in rocks deeper in the Earth.
“This introduces artefacts in the final data that are circular,” she says – precisely the types of low-gravity blobs observed by Weihaupt’s team.
Nor, she adds, has she ever seen impact debris in Antarctic seabed cores matching impacts at the suggested sites.
Mark Boslough a physicist at Sandia National Laboratories in Albuquerque who has studied the risk of impacts from near-Earth asteroids, agrees.
“Because large impact structures are extremely rare, any claim requires a very high standard of evidence,” he said by email. “Lots of features can masquerade as impact structures, and most turn out not to be.”
Weihaupt admits his theory is controversial. Nevertheless, he said, “[It] has the potential to explain quite a number of unexplained features and phenomena related to the Antarctic – some of which we are not yet prepared to take to the scientific community.”
In other words, stay tuned. If a shotgun blast of big meteorites hitting the Antarctic sounds exciting, Weihaupt’s team believes it is only beginning to make its case.
