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Abstract
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Recent work by several researchers has drawn attention to the large amount of energy that is coupled directly to the Earth's atmosphere from hypervelocity collisions, and the importance of this interaction, both as a geologic process and as a human hazard. Even for crater-forming impact events, the atmosphere plays an important, if not dominant role. According to recent calculations by Melosh and Collins [1], the iron projectile that formed Meteor Crater (Arizona) deposited more than 2.5 times as much energy directly into the atmosphere than it carried to the surface (the projectile was estimated to have 9.0 megatons of kinetic energy at the top of the atmosphere, and only 2.5 megatons when it struck the surface).
We present CTH simulations of airbursts in the Earth's lower atmosphere from hypervelocity asteroid impacts. The intent of these simulations was to explore the phenomenology associated with low-altitude airbursts, with the particular goal of determining whether the altitude of maximum energy deposition can be used as a reasonable estimate of the equivalent height of a point source explosion. Our simulations suggest that this is not a good approximation. The center of mass of an exploding projectile is transported downward in the form of a high-temperature jet of expanding gas. The jet descends by a significant fraction of the burst altitude before its velocity becomes subsonic. The time scale of this descent is similar to the time scale of the explosion itself, so the jet simultaneously couples its kinetic energy and its internal energy to the atmosphere. Because of this downward flow, larger blast waves and stronger thermal radiation pulses are felt at the surface than would be predicted by a point source explosion at the height where the burst was initiated. For impacts with a kinetic energy above some threshold, the hot jet of vaporized projectile (the descending "fireball") makes contact with the Earth's surface, where it expands radially. During the time of radial expansion, the fireball can maintain temperatures well above the melting temperature of silicate minerals, and its radial velocity can exceed the sound speed in air. We suggest that the surface materials can ablate by radiative/convective melting under these conditions, and then quench rapidly to form glass after the fireball cools and recedes. Possible examples of such airburst glasses are the Muong-Nong Tektites of Southeast Asia and the Libyan Desert Glass of western Egypt. We suggest an enhancement of entry dynamics models to account for the downward advection of shocked and heated material, and the lowering of the apparent airburst altitude. The actual differences between the effects on the ground from a point source approximation versus a full flow field still need to be quantified by running more realistic high-resolution 3-D simulations with a variety of impact parameters. A re-evaluation of the impact hazard is necessary to properly include this enhanced damage potential of low-altitude airbursts. |