May 16, 2019– Mary Greeley News –A unique and beautiful yellow-green glass occurs in ‘the Great Sand Sea’, a huge and inhospitable area of moving sand dunes in the Egyptian Sahara.
The glass was first documented in 1932 by Peter Clayton, a topographer for the Geological Department of the Survey of Egypt. However, it is clear that locals have known about this type of glass for thousands of years.
Recently, an intrepid group of scientists ventured into the Great Sand Sea to find out more.
A Curtin University researcher has solved a nearly 100-year-old riddle by discovering that glass found in the Egyptian desert was created by a meteorite impact, rather than atmospheric airburst, in findings that have implications for understanding the threat posed by asteroids.
Published in leading journal Geology, the research examined tiny grains of the mineral zircon in samples of Libyan desert glass, which formed 29 million years ago and is found over several thousand square kilometers in western Egypt. Nearly pure silica, the canary yellow glass was famously used to make a scarab that is part of King Tut’s Pectoral.
Libyan desert glass is a naturally occurring kind of glass that’s found in the eastern stretches of the Sahara Desert, in eastern Libya and western Egypt. Its rare yellow color has seen it used decoratively going back to the age of Tutankhamun, but desert glass has been around a lot longer than that.
Lead author Dr. Aaron Cavosie, from the Space Science and Technology Centre in Curtin’s School of Earth and Planetary Sciences, said zircons in the glass preserved evidence of the former presence of a high-pressure mineral named reidite, which only forms during a meteorite impact not from airbursts.
“It has been a topic of ongoing debate as to whether the glass formed during meteorite impact, or during an airburst, which happens when asteroids called Near Earth Objects explode and deposit energy in the Earth’s atmosphere,” Dr. Cavosie said.
“Both meteorite impacts and airbursts can cause melting, however, only meteorite impacts create shock waves that form high-pressure minerals, so finding evidence of former reidite confirms it was created as the result of a meteorite impact.”
Dr. Cavosie said the idea that the glass may have formed during a large atmospheric airburst gained popularity after a dramatic airburst over Russia in 2013, which caused extensive property damage and injury to humans but did not cause surface materials to melt.
“Previous models suggested that Libyan desert glass represented a large, 100-Mt class airburst, but our results show this is not the case,” Dr. Cavosie said.
“Meteorite impacts are catastrophic events, but they are not common. Airbursts happen more frequently, but we now know not to expect a Libyan desert glass-forming event in the near future, which is cause for some comfort.”
The research team also included Natural History Museum of Vienna Director, Professor Christian Koeberl.
Uranium testing has established that the glass was formed nearly 30 million years ago. Moreover, by measuring ratios of zircon it has been estimated that the temperature needed for formation was about 1,800 degrees Celsius – so ruling out both the low temperature formation and the volcanic theories.
Koeberl, Egyptian geologist Aly Barakat, and American impact physicist Mark Boslough, travelled by off roading vehicles over massive sand dunes for three days to the areas in which the glass occurs in the Great Sand Sea in search of the crater.
Unfortunately, they found no signs of one, and nor was any such crater apparent from satellite imagery. Despite this, Koeberl and Barakat still believe there was an impact, and that a crater will be discovered at some point. However, physicist Boslough is not so sure, and instead hypothesizes the likelihood that the meteorite would have exploded in the air and would thus have not left a crater.
Evidence for the Kebira Crater being an impact structure lacked direct field observations. Most recently, Longinelli and others studied the oxygen isotope and chemical composition of Libyan desert glass and samples of sands and sandstone from its proposed source areas. They found that the mean oxygen isotope values of the sandstone samples from the Kebira Crater differed greatly from their Libyan desert glass samples. Because of this, they concluded that the sandstone exposed in this feature can be ruled out as being the source of Libyan desert glass.
credit: In part with https://phys.org/news/2019-05-planetary-scientists-unravel-mystery-egyptian.html