NUMERICAL MODELING OF AERIAL BURSTS AND ABLATION MELTING OF LIBYAN DESERT GLASS

Libyan Desert Glass (LDG) fragments are scattered over an area spanning 6500 km² in western Egypt, but the original source area may be much smaller.  This silica-rich glass has a reported fission track age of 28.5 Ma, and is found primarily on and within Quaternary colluvium and alluvium in the narrow corridors between linear dunes of the Great Sand Sea.  Multiple lines of geochemical evidence strongly suggest that it is the product of an impact event, but there is no associated impact structure.  We have performed numerical modeling to assess the hypothesis that the source material was melted by radiative/convective heating from the low altitude explosion of a small asteroid or comet. High-resolution simulations were performed using CTH with adaptive mesh refinement of the atmospheric entry of a 120-meter-diameter sphere of dunite.  With an initial velocity of 20 km/s, the kinetic energy of about 4.5·10¹⁷ Joules converts to an explosive yield of 108 Megatons.  Most of this energy is coupled directly into the atmosphere as the asteroid ablates and explodes before it hits the ground.  The resulting fireball contains air and ablated meteoritic material at temperatures exceeding the melting temperature of quartz in direct contact with the surface over a 10 km diameter area for more than 10 s after the explosion.  Where the fireball is in contact with the ground, wind velocities exceed the sound speed over this time interval.  This model is qualitatively different from a conventional impact, in which the fireball is embedded within the ejecta curtain and does not come into contact with the surface.  The combination of high temperatures and high-velocity shear flow leads to strong coupling of thermal radiation, with melting and stripping of ablated silica melt that subsequently quenches to form the LDG. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.