The earliest evidence of a comet entering Earth's atmosphere and exploding has been found. It rained down a shock wave of fire which obliterated every life form in its path, has been found. 

The comet entered Earth's atmosphere above Egypt about 28 million years ago. After it entered the atmosphere, it exploded, heating up the sand beneath it to a temperature of about 2 000 degrees Celsius, resulting in the formation of a huge amount of yellow silica glass which lies scattered over a 6 000 square kilometer area in the Sahara. A magnificent specimen of the glass, polished by ancient jewelers, is found in Tutankhamun's brooch with its striking yellow-brown scarab. 

The discovery is the first definitive proof of a comet striking Earth.

It resulted from a mysterious black pebble found years earlier by an Egyptian geologist in the area of the silica glass. After conducting highly sophisticated chemical analyses on this pebble, the authors came to the inescapable conclusion that it represented the very first known hand specimen of a comet nucleus, rather than simply an unusual type of meteorite. 

The team have named the diamond-bearing pebble "Hypatia" in honour of the first well known female mathematician, astronomer and philosopher, Hypatia of Alexandria.

Cartoon of the comet exploding in Earth's atmosphere above Egypt. Credit: Terry Bakker

The impact of the explosion also produced microscopic diamonds. "Diamonds are produced from carbon bearing material. Normally they form deep in the earth, where the pressure is high, but you can also generate very high pressure with shock. Part of the comet impacted and the shock of the impact produced the diamonds," says lead author Professor Jan Kramers of the University of Johannesburg. 

Comet material is very elusive. Comet fragments have not been found on Earth before except as microscopic sized dust particles in the upper atmosphere and some carbon-rich dust in the Antarctic ice. Space agencies have spent billions to secure the smallest amounts of pristine comet matter.

"NASA and ESA (European Space Agency) spend billions of dollars collecting a few micrograms of comet material and bringing it back to Earth, and now we've got a radical new approach of studying this material, without spending billions of dollars collecting it," says Kramers.

The study of Hypatia has grown into an international collaborative research program, coordinated by Andreoli, which involves a growing number of scientists drawn from a variety of disciplines. Dr Mario di Martino of Turin's Astrophysical Observatory has led several expeditions to the desert glass area.

"Comets contain the very secrets to unlocking the formation of our solar system and this discovery gives us an unprecedented opportunity to study comet material first hand," says co-author Professor David Block of Wits University.

Citation: Jan D. Kramers, Marco A.G. Andreoli, Maria Atanasova, Georgy A. Belyanin, David L. Block, Chris Franklyn, Chris Harris, Mpho Lekgoathi, Charles S. Montross, Tshepo Ntsoane, Vittoria Pischedda, Patience Segonyane, K.S. (Fanus) Viljoen, Johan E. Westraadt
'Unique chemistry of a diamond-bearing pebble from the Libyan Desert Glass strewnfield, SW Egypt: Evidence for a shocked comet fragment', Earth and Planetary Science Letters, Volume 382, 15 November 2013, Pages 21–31