In a land before time, a comet left its calling card (Illustration: Terry Bakker) Part of the meteorite found in the Egyptian desert (Image: Science Direct)

A charred black pebble found in the Egyptian desert may be a piece of a comet that shattered near Earth about 28 million years ago. If so, the stone would offer a first close-up glimpse of rock that formed at the very edges of the solar system.

The 30-gram stone was discovered by Egyptian geologist Aly Barakat in December 1996 in a part of the Sahara desert near the border with Libya. The 6000-square-kilometre region is famous for being strewn with fragments of pale green glass, thought to have formed millions of years ago during a meteorite strike. Ancient Egyptians used a piece of this Libyan desert glass to make a scarab for King Tutankhamen’s burial jewellery.


Barakat thought the darker stone might be a kind of black diamond called carbonado, so he sent a sample to Marco Andreoli at the University of the Witwatersrand in South Africa.

Years of analysis revealed that the rock may be something even rarer: the first large-scale piece of a comet. Until now, scientists had found only grains of cometary material in Earth’s atmosphere and in carbon-rich dust in Antarctic ice. NASA’s Stardust spacecraft also returned a few dust grains from comet Wild 2 in 2006.

“We can study material from the outermost solar system for the first time,” says Jan Kramers, a member of Andreoli’s team at the University of Johannesburg in South Africa. “This is actually quite exciting.”

Space diamonds

Kramers and his colleagues subjected a 1-gram sample of the stone to a barrage of tests to determine its composition. They found that the rock has a high ratio of oxygen to carbon – an unearthly mix similar to that seen in the Wild 2 samples and Antarctic dust fragments. They also found that the black rock contains chunks of diamond, which could have formed when its carbon was compressed by the high pressure of an impact.

“People think of comets as a dirty snowball made of ice that evaporates in the sunshine,” says Kramers. “But the heat-resistant part of comets is carbon, and if you compress it, heat it up and subject it to shock, you get some kind of glassy carbon. This appears consistent.”

The team then heated other fragments of the rock to 1800˚C to release gases trapped inside. They detected the noble gases neon, krypton, xenon and argon. The argon isotope ratios confirmed that the rock is not originally from Earth. The other isotope ratios look nothing like those found in known meteorites, all of which came from asteroids, rocky objects that formed inside the orbit of Jupiter.

The team thinks that a comet hit our atmosphere and exploded over the desert, akin to what happened over the Tunguska region of Siberia in 1908. The explosion heated and compressed the sand to create the Libyan desert glass. The black stone is a piece from the comet’s solid core, which absorbed noble gases from our atmosphere on its way down and turned its carbon into diamond on impact.

“You have Libyan desert glass, which is basically molten Earth surface material, and you have a piece which is basically a shocked fragment of cometary core in the same area, so you start putting 2 and 2 together,” says Kramers.

Rosetta comet

“It is definitely a strange object, different from any meteorite we have sampled so far,” says Ingo Leya at the University of Bern in Switzerland, who was not involved in the analysis. But while Leya agrees that the rock probably comes from space, he is not yet convinced it is a sample of a comet. “The problem is, we have little cometary material as a standard to compare with. Therefore it is difficult to firmly conclude or exclude that this meteorite comes from a comet.”

The European Space Agency’s Rosetta spacecraft is heading for a rendezvous with comet 67P/Churyumov-Gerasimenko (Chury) in 2014. It will deploy a small lander that will examine that comet’s core up close, perhaps helping to settle the matter. In the meantime, Kramers hopes to find more samples of the Egyptian stone for study.

“It’s a very inaccessible part of the Sahara, but there must be more material lying around,” he says. “To go there and try to find more, you have to spend a lot of money, but not as much as to go and capture a comet in outer space.”

Journal reference: Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2013.09.003