Dec 21, 2006
Strange specimens of natural glass found in the Egyptian desert are products of a meteorite slamming into Earth between 100,000 and 200,000 years ago, scientists have concluded.
The glass—known locally as Dakhla glass—represents the first clear evidence of a meteorite striking an area populated by humans.
At the time of the impact, the Dakhla Oasis, located in the western part of modern-day Egypt (http://www3.nationalgeographic.com/places/countries/country_egypt.html), resembled the African savanna and was inhabited by early humans, according to archaeological evidence (see Egypt map (http://www.nationalgeographic.com/xpeditions/atlas/index.html?Parent=africa&Rootmap=egypt).)
"This meteorite event would have been catastrophic for all living things," said Maxine Kleindienst, an anthropologist at the University of Toronto in Canada.
"Even a relatively small impact would have exterminated all life for [several] miles."
The origin of the glass had puzzled scientists since Kleindienst discovered it in 1987.
Some researchers had suggested the Stone Age glass may have been produced by burning vegetation or lightning strikes.
But a chemical analysis showed that the glass was created in temperatures so high that they could only have been the result of a meteorite impact.
Gordon Osinski, a geologist at the Canadian Space Agency in Saint-Hubert who conducted the analysis, found that the glass samples contain strands of molten quartz, a signature of meteorite impacts.
"We can now say for definite that they were caused by a meteorite impact," he said.
Osinski is the lead author of the paper detailing the findings, which was published online in ScienceDirect.
The glass deposits have been found in desert locations separated by tens of kilometers, suggesting a massive event.
But scientists have found no signs of an impact crater in the area.
"Usually from an impact like this, we should have a crater at least a kilometer [0.6 mile] across," Osinski said.
The absence of a crater, the scientists believe, suggests that the large space rock may have disintegrated upon entering Earth's atmosphere.
What happened may have been similar to the so-called Tunguska event, in which an asteroid exploded miles above the Earth's surface in a remote area of Siberia in 1908. That explosion felled an estimated 60 million trees over 830 square miles (2,150 square kilometers).
(See an interactive feature on asteroids (http://www.nationalgeographic.com/asteroids/).)
"There was no hole in the ground at Tunguska either," said Albert Haldemann, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, California, who has been using radar to scour the Egyptian desert for impact signs.
"In an air burst like that, contents of the explosion continue to travel downward … providing a gas pulse across the [Earth's] surface that could vitrify sediments," Haldemann explained.
Scientists know much more about what happens when meteorites hit hard rock than when they impact sand and sedimentary rock, as would have been the case in the Egyptian desert.
At the time, there was a large lake in the area, the researchers say.
"If there was an impact at the surface and it happened to hit the lake, it wouldn't be surprising if the [crater] was filled in," Haldemann said.
"Did the event boil the entire lake away, or did it just cause a really big wave to go across the lake? Maybe we can figure that out from the sediments."
Kleindienst, the anthropologist, has been excavating at the site for more than 20 years as part of the Dakhla Oasis project.
(Her research has been partially funded by the National Geographic Committee for Research and Exploration (http://www.nationalgeographic.com/research/index.html). The committee and National Geographic News are both divisions of the National Geographic Society.)
Kleindienst has obtained a large amount of evidence, including spears and scrapers, to show that humans continually inhabited this region of Egypt's Western Desert during the Middle Stone Age, from about 200,000 to 30,000 years ago.
She has even found glass in lake sediments with archaeological evidence of human habitation in the soil layers below and above it.
"There is no reason to suspect that humans were not there at the time that this catastrophe happened," she said.
The meteorite research has important implications for understanding the environmental and human history at the time, Kleindienst added.
"Calculations at Meteor Crater [in Arizona] give some idea of what the effect of a [relatively small] impact would be," she said.
"Life forms are killed or seriously injured for many tens of kilometers away from the impact.
"If this event happened during a humid period, the area might have been ecologically repopulated fairly quickly from surrounding areas," she added.
"But if it happened during a dry period, it might have taken a considerable period for life to be re-established in the oasis region."
Haldemann, who is also the deputy project scientist on NASA's Mars Exploration Rover project, says the meteorite strike underlines the interconnectedness between Earth and the rest of the solar system.
"We already know the environment of the whole Earth is tied together," he said.
"What we've been learning more and more in the last 20 years or so is that we're also tied to the solar system as a whole over longer time periods and that this interaction tends to be punctuated by these catastrophic events.
"Here we have evidence in the [Early Stone Age] records that this kind of thing can really happen to us."