Modeling airbursts by comets, asteroids, and nuclear detonations: shock metamorphism, meltglass, and microspherules

Abstract

Asteroid and comet impacts can produce a wide range of effects, varying from large crater-forming events to high-altitude, non-destructive airbursts. Numerous studies have used computer hydrocode to model airbursts, primarily focusing on high-altitude events with limited surface effects. Few have modeled so-called “touch-down” events when an airburst occurs at an altitude of less than ∼1000 m, and no known studies have simultaneously modeled changes in airburst pressures, temperatures, shockwave speeds, visible materials, and bulk material failure for such events. This study used the hydrocode software Autodyn-2D to investigate these interrelated variables. Four airburst scenarios are modeled: the Trinity nuclear airburst in New Mexico (1945), an 80-m asteroid, a 100-m comet, and a 140-m comet. Our investigation reveals that touch-down airbursts can demolish buildings and cause extensive ground-surface damage. The modeling also indicates that contrary to prevailing views, low-altitude touch-down airbursts can produce shock metamorphism when the airburst shockwave or fragments strike Earth’s surface at sufficiently high velocities, pressures, and temperatures. These conditions can also produce microspherules, meltglass, and shallow impact craters. Regardless of modeling uncertainties, it is known that bolides can burst just above the Earth’s surface, causing significant damage that is detectable in the geologic record. These results have important implications for using shocked quartz and melted materials to identify past touch-down airbursts in the absence of a typical impact crater. Although relatively rare, touch-down events are more common than large crater-forming events and are potentially more dangerous.