BUFFETED by the might of the hot Khamsin wind that sweeps across the Egyptian Sahara, the mountainous dunes of the Great Sand Sea are the stuff of legends. Here, ancient armies lie buried and the fabulous wealth of lost cities awaits discovery. But in the 1930s, these myths came under threat. Explorers arrived with camels, cars and flimsy biplanes and criss-crossed the blistering sands, searching for a legendary oasis called Zerzura. Though they never found Zerzura, Patrick Clayton, a surveyor with the Egyptian Geological Survey stumbled upon something almost as fantastic.
In December 1932, he was bumping across the dunes towards the high, wind-swept red rocks of the Saad plateau when he felt the tyres of his car crunch across chunks of glass. It was an incredibly clear, green-yellow glass that glittered like gems in the bright sun. Over the next few years he returned on expeditions to collect samples of this strange material, marking his last visit in 1934 with a simple, scribbled message that he tucked into an empty bottle and left amongst the glass.
Almost fifty years later, Italian explorer and archaeologist Giancarlo Negro stumbled across Clayton鈥檚 bottle as he picked his way across the site. 鈥淚 was amazed to see a whisky bottle full of sand, with a note sticking out of it,鈥� he recalls. Negro鈥檚 1985 expedition was the first in a series made by international teams of scientists aimed at unravelling the site鈥檚 secrets.
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The story that has begun to emerge since is both astonishing and mysterious. These glittering shards are the purest natural silica glass any one has ever found. And there may be more than 1400 tonnes of the stuff spread across a vast area of the desert. Some pieces contain tiny bubbles, wispy white deposits and swirling black patterns that hint at a tumultuous origin. Where on earth did this it from?
Tiny pieces of silica glass are fairly common in nature. When volcanic lava cools suddenly-as red-hot magma pours into the sea, for instance-molecules of silica in the lava freeze at random, creating an amorphous mass that resembles broken glass. But these materials are about 75 per cent amorphous silica at most. The rest is made up of crystals of quartz and oxides such as aluminium and iron. The desert glass is totally different: 鈥淚t鈥檚 the purest natural glass in the world,鈥� says Vincenzo de Michele, keeper of minerals at Milan鈥檚 Museum of Natural History, 鈥渨ith a silica content of 98 per cent.鈥�
This purity gives the desert glass some remarkable properties. Geochemist Peter Horn of the University of Munich has discovered that you can heat the material to 1700 掳C before it begins to melt-over 500 掳C higher than other natural glasses. You could turn the glass into excellent cooking dishes, says Horn. 鈥淚t can be dropped into cold water even when it is red hot and it doesn鈥檛 disintegrate,鈥� he says. 鈥淚t鈥檚 almost as good as the best high-tech glass.鈥�
Stroll across the desert site and you鈥檒l come across great big chunks of glass-some are larger than bowling balls and weigh as much as 26 kilograms. These massive pieces of dwarf lumps of natural glass found elsewhere. Also scattered about the site are clusters of sharp glass chips-the debris of prehistoric workshops-and ancient glass tools such as knives and hatchets, evidence of early interest in the silica glass.
Geologists have dreamt up some pretty bizarre theories to explain the origins of this remarkable material. Ulrich Jux, a geologist at the University of Cologne in Germany, for example, suggested that the silica glass may have formed at the bottom of a warm, volcanic lake. Over millions of years, water trickling through hot underground channels close to a volcano could have dissolved silica from the surrounding rocks. When this warm, silica-rich water collected into lakes and cooled, pure silica glass would begin to precipitate out.
Extraterrestrial clues
One person who isn鈥檛 convinced by Jux鈥檚 theory is Robert Rocchia from the environmental sciences lab at the French national agency for scientific research (CNRS) in Gif-sur-Yvette. In 1996, he and colleagues at half a dozen other French labs studied the molecular structure of the glass using infrared spectroscopy, looking for signs of hydroxide ions. These are usually found in amorphous silica formed at low temperature, but the researchers couldn鈥檛 find any in the samples.
Besides, geologists dated the glass at 28.5 million years old, but the dried-up remains of the ancient lakes that Jux had spotted near the site turned out to be far too young-just 9000 years old.
If not volcanic lakes, then how about the red-hot lava pouring out of prehistoric volcanoes? There are at least two ancient volcanic craters in the area, says Rocchia, but they are hundreds of kilometres away from the site of the glass-probably too far away to have been involved. And Horn and Christian Koeberl, a geochemist at the University of Vienna, identified whitish inclusions in the glass as minerals such as cristobalite and baddeleyite-which form at temperatures far higher than those found in volcanic lava.
The best clue to the origin of the glass lies locked up in the swirling black marks resembling drops of ink that pepper some fragments. Rocchia bombarded these samples with neutrons to trigger gamma-ray emissions from elements trapped inside. The energy of these emissions can help identify elusive trace elements locked up in the glass. They made an intriguing discovery: 鈥淭he dark samples are very rich in iridium,鈥� says Rocchia. High iridium levels are typical of extraterrestrial bodies such as meteorites and comets. The proportions of other elements such as ruthenium, cobalt and iron told the same story. The only explanation, says Rocchia, is that the glass formed when a meteorite crashed into the desert.
This suggestion seems to make good sense. The local Nubian sandstone is rich in silica and, should you want to melt thousands of tonnes of the stuff, there is no better way to do it than with a large meteorite travelling at several kilometres per second. Smash it into the ground and the explosive impact would vaporise a huge area of the desert, melting rocks and sand at temperatures easily high enough to form minerals like baddeleyite. And as the molten rock cooled, it would turn to clear, green-yellow coloured glass.
A neat explanation, but peppered with holes: photographs taken by the Landsat and Discovery satellites show no sign of an impact crater at the glass site. NASA鈥檚 X-SAR radar imaging camera and the European Space Agency鈥檚 ERS radar satellites have also swept the area, this time probing beneath the surface of the sand with microwaves. They drew a blank, says Farouk El Baz, head of the center for remote sensing at Boston University.
There鈥檚 another problem, too. The desert glass seems too clear and pure to have been created by a monumental collision. Glass discovered at impact sites such as Wabar in Saudi Arabia is blackened and shattered by the tremendous forces and temperatures involved. These fragments are often embedded in a matrix of fused and broken rock called brecchia. And impact sites are usually littered with tiny fragments of iron from the meteorite. The Great Sand Sea, however, is surprisingly clear of this kind of debris. 鈥淲e have plenty of impact craters on Earth,鈥� says Koeberl, 鈥渂ut this is the only known occurrence on the whole Earth of such glass. Why did it form here and nowhere else?鈥�
De Michele and Romano Serra, an astrophysicist at the University of Bologna, believe they know the answer. During their expedition in 1996, Serra and de Michele made a thorough search of the site and discovered that the glass is concentrated in two areas: one oval shaped, and the other a ring 21 kilometres across and about 6 kilometres wide. The area at the centre of the ring is empty, says de Michele. Since a geological upheaval couldn鈥檛 create a feature that small, de Michele and Serra have another theory.
Soft impact
Imagine that a chondritic meteorite-a brittle lump of stone and organic matter about the size of a house-is crashing into the atmosphere with the energy of ten thousand express trains. The friction and massive shock wave this creates compresses and heats the atmosphere, shattering the brittle meteorite in midair. The heat from this explosion would toast the rock and sand beneath. 快猫短视频s call this huge blast a 鈥渟oft鈥� impact and most believe something similar happened above Tunguska in Siberia in 1908, flattening thousands of kilometres of forest (快猫短视频, Science, 26 October 1996, p 16).
A soft impact might just explain why the centre of the ring in the desert is free of glass: 鈥淭he ground could have responded in an elastic way to the blast wave and rebounded, leaving a ring and a central peak which later eroded,鈥� says de Michele. Serra calculates that the meteorite must have been 10 to 12 kilometres above the desert when it exploded.
De Michele is particularly impressed by the size of the glass chunks: 鈥淭his points to a thick mantle of glass and to an enormous amount of heat,鈥� he says. 鈥淸Molten] silica is highly viscous, yet the streaks in some samples show that it was flowing like a river.鈥�
According to Mark Boslough of Sandia National Labs at Albuquerque in New Mexico, a meteorite 30 metres across could create an explosion equivalent to a 3-megaton nuclear bomb-easily hot enough to melt thousands of tonnes of sand. And when this meteorite hit the atmosphere, a plume of air would rocket outwards into space like the splashes thrown up by a rock as it drops into water. As the plume came down, its kinetic energy would have heated the atmosphere to more than 2000 掳C, says Boslough. At this temperature, the hot air would have sprayed infrared radiation onto the desert, melting the sand like sugar beneath a blowtorch.
Serra believes that this thermal blanket might have kept the glass sizzling at thousands of degrees for over a week.
Not everyone agrees with this view. 鈥淎irblast melting alone would not work,鈥� objects Koeberl. 鈥淭o melt hundreds of kilometres of desert, you need a big body. But big bodies don鈥檛 make airblasts, unless you make unrealistic assumptions about their density and composition. They make impacts on the ground.鈥� And because the glass is contaminated, he says, it suggests the meteorite made contact with the glass.
Koeberl believes that a large meteorite raced through the Earth鈥檚 atmosphere at a very shallow angle and skimmed across the surface of the Sahara like a stone skipping across a pond. In the moments the meteorite spent in contact with the desert, friction would have created enough heat to melt the sand and rock. This could create far more melted silica than a meteorite smashing straight into the ground. And it wouldn鈥檛 leave a deep crater: 鈥淚n 28 million years a lot of sedimentation and infill can happen,鈥� says Koeberl. 鈥淭he crater might still be there, covered by hundred of metres of sand.鈥�
But there may be a simple way to overcome Koeberl鈥檚 objections to a soft impact yet still account for the huge amount of melted glass at the site. According to calculations made by Boslough, massive amounts of heat could have come from an impact involving multiple soft impacts-when more than one piece of meteorite dropped into the atmosphere and exploded. Much the same thing occurred when pieces of Comet Shoemaker-Levy smashed into Jupiter in 1996. 鈥淐lose-packed arrays of soft impacts lead to dense plumes, generating higher temperatures,鈥� he says.
Even without a crater, Rocchia prefers to stick with the hard impact theory. Researchers have found shocked quartz grains inside silica glass, he says. 鈥淭hey are unlikely to result from an atmospheric explosion.鈥� But the arguments look set to continue: Rocchia plans to return to the Great Sand Sea to hunt for signs of his crater. Meanwhile, Serra is devoting much of his efforts to studying Tunguska in the hope of strengthening the air blast theory.
Even if we never learn exactly what created the beautiful desert glass, it is helping us to appreciate how vulnerable our planet is to meteorite impact. 鈥淓vents like [this one] or Tunguska are much more frequent than previously thought,鈥� says Serra. In fact, estimates suggest that impacts caused by objects 30 to 40 metres across happen once every one or two centuries. Smaller events caused by 10 to 20-metre objects may occur once a month, says Serra. But soft impacts leave no trace in the geological record and easily pass unnoticed. This may change as increasing numbers of sensors, orbiting the Earth on satellites, keep watch for their fiery signatures.
But this remarkable glass has another tale to tell. In 1996 Negro and de Michele were wondering through the Egyptian Museum in Cairo when they spotted a tiny carved scarab, part of a piece of jewellery found by Howard Carter in Tutankhamen鈥檚 tomb. According to Carter鈥檚 records, it was carved from a kind of quartz called chalcedony. But to Negro and de Michele, the yellow-green mineral bore a striking resemblance to their mysterious desert glass.
By October 1998 they had the necessary permits and returned to study the jewel. Escorted by a crowd of officials and soldiers, they nervously opened the case and measured the optical properties of the scarab. They almost exactly matched that of samples of the silica glass.
Prehistoric glass tools made almost 100 000 years ago lie scattered around the desert site. But until the discovery of the scarab, says de Michele, no one had any idea that the ancient Egyptians knew about the glass or had travelled so far-almost 700 kilometres-into the desert. The scarab remains the only silica glass jewel discovered among the treasures of ancient Egypt. 鈥淧erhaps they used it as a rarity,鈥� says Negro, 鈥減robably thinking, as Clayton did, it was a cosmic gem.鈥�
