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2007 Annual Report

2007 ANNUAL REPORT

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High-Performance Computing Provides Clues to Scientific Mystery

Enigmatic silica glass in the Sahara Desert has survived nearly 30 million years. How did it form?

desert glass
Libyan Desert Glass is found in an area spanning 6,500 km2, in the Great Sand Sea of the Western Desert of Egypt, near the border with Libya. In 1998, an Italian mineralogist showed that a carved scarab in King Tut‘s breastplate was made of this glass.
Most natural glasses are volcanic in origin and have chemical compositions consistent with rapid cooling of lava without opportunity for crystalline structure to grow. The rare exceptions are tektites, which are formed by shock melting associated with the very high velocity, or hypervelocity, impact of a comet or asteroid. But Libyan Desert Glass does not fall into either category, and has baffled scientists since its discovery by British explorers in 1932.

For physicist Mark Boslough and his Sandia colleagues, the 1994 collision of Comet Shoemaker-Levy 9 with Jupiter provided Sandia with a unique opportunity to model a hypervelocity atmospheric impact. Insights gained from those simulations and subsequent astronomical observations of the actual event led to a deeper understanding of the geologic process of impacts on Earth. The hypervelocity atmospheric impact thesis presented a likely scenario for the formation of Libyan Desert Glass.

High-resolution computer simulations of the impact process, requiring huge amounts of memory and processing power, support the hypothesis that the glass was formed by the radiative heating and ablation of sandstone and alluvium near ground zero, when a 100-megaton or larger atmospheric explosion resulted after the breakup of a comet or asteroid.

heat simulations
As these simulations show, the heat of an atmospheric explosion created a vapor plume more than 10 kilometers in diameter, which stayed in contact with the Earth for up to 20 seconds with very high temperatures and continuous wind blasts exceeding 100 meters per second. The conditions suggest one way the glass could have been created.


Using Sandia’s Red Storm supercomputer, Boslough ran high-resolution shock-physics simulations to show how a 120-meterdiameter asteroid entering the atmosphere at 20 kilometers per second (resulting in an explosion with a yield of about 110 megatons) breaks up just before hitting the ground. The fireball that is generated remains in contact with the Earth’s surface at temperatures exceeding the melting temperature of quartz for more than 20 seconds. At the same time, the air speed behind the blast wave exceeds several hundred meters per second. These conditions are consistent with rapid melting and quenching to form the Libyan Desert Glass.

These simulations require the massively parallel processing power provided with Sandia’s Red Storm computer.

The risk to humans from such impacts is small but not negligible. Because of the low frequency of these events, both their probability and consequences are difficult to determine. The most likely scenario that would cause damage and casualties would not be a crater-forming impact, but a large aerial burst similar to the one that created the unusual Libyan natural glass. This research is forcing risk assessments to recognize and account for the process of large aerial bursts.

For more information:
Mark Boslough, Ph.D., 505-845-8851, mbboslo@sandia.gov