First Evidence Found of a Comet Strike on Earth

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About 28 million years ago a comet exploded over Egypt, creating a 3600°F (2000°C) blast wave that spread out over the desert below. The fiery shockwave melted the sand, forming copious amounts of yellow silica glass scattered over 2,300 square miles (6,000 square kilometers) of the Sahara.

 

Polished into the shape of a scarab beetle, a large piece of this glass found its way into a brooch owned by the famed Egyptian boy king Tutankhamen.
 

"Because there is no sign of an impact crater, it has been a mystery as to what kind of celestial event actually could have caused this debris field, but a small, black stone found lying in the middle of the glass area caught our attention,"

said study co-author David Block, an astronomer at Wits University in Johannesburg, South Africa.

Saharan Surprise

A tiny slice of the black pebble was put through isotopic analysis, which definitely ruled out that it came from a meteor. Instead, the analysis showed that the pebble possessed the unique chemical signature of a comet, measured in terms of elements such as argon and carbon.

"It was then basically a matter of running the movie backwards in time and predicting what temperatures were needed to create the conditions we find that make up the fragment today,"

Block says.

"So when I saw the result of the analyses, I was completely ecstatic to realize that such a piece of cosmic history has been found for the first time right on our doorstep."

While meteors are known to enter the Earth's atmosphere frequently—one can be seen as a shooting star every 15 minutes or so on any random night—not so with comets.
The implosions of comets in planetary atmospheres are exceedingly rare events—the only other definitive case of a comet hitting a planet was back in 1994 when comet Shoemaker-Levy 9 impacted Jupiter's atmosphere.

 

Astronomical Odds Inspire Caution
And it's because of this rarity that Earth scientist Andrew Glikson of the Australian National University in Canberra, who was not involved in this study, questions if these yellow glass objects, called tektites, might instead have been created through much more common meteoric events, as seen at many impact sites around the world.

"Why can't the material represent a large tektite formed by heating and melting of sand at the Earth's surface by an asteroid impact, such as, for example, the Australite tektites?"

asks Glikson.
While this extraterrestrial glass is considered common around many impact sites,  geologist Gerald Johnson, who was not involved in the study, says that beyond the compelling evidence the team presents in their chemical workup of the black pebble, it's not surprising that the research community may be wary of these results.
 

"A comet, mostly water, vaporizes in the atmosphere and leaves little for the geologic record, while the 'dirt' incorporated in the comet also is disseminated widely and is unlikely to be found because of impact dispersal, surface weathering, and erosion,"

said Johnson, a meteor impact researcher at the College of William and Mary in Williamsburg, Virginia.
 

 

"Undoubtedly, the Earth has been hit numerous times by comets, but our knowledge of these is lacking because comets leave such a poor record ... so this discovery is amazing."

Solar System Origins
Microscopic dust particles from these icy interlopers have been collected from the upper atmosphere and from Antarctic ice, and have been scooped up by space probes.

But having a chance to study sizable comet material firsthand would be exceptional, and Block and his team believe it can offer a unique chance to study the birth of our solar system.
Cosmic particles called presolar grains formed in the stellar cloud of gas and dust that gave birth to our solar system, and are thought to have remained within comets and meteors.

"My bet is that this little rock will unlock some unique secrets in time to come, specifically because it appears packed with presolar grains,"

said Block.

The comet study will be published in an upcoming issue of Earth and Planetary Science Letters.