Impact ejecta dynamics in an atmosphere: Experimental results and extrapolations

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Abstract
Laboratory experiments of impacts in an atmosphere provide important clues for processes accompanying large-body impacts on the Earth. Impacts of 0.635 cm aluminum projectiles at 6 km/s into fine pumice dust at one atmosphere generate a ball of ionized gas behind an expanding curtain of upward-moving ejecta (within the first 0.2 ms). The gas ball forms a toroid that dissolves as it is driven along the interior of the ejecta curtain. This process contrasts with near-surface explosions where a fireball envelopes the early-time crater growth. Later evolution of the ejecta curtain reflects two phenomena: the effects of atmospheric drag on the ejecta and the effects of the ejecta on the atmosphere. Aerodynamic drag significantly modifies the ballistic paths of ejecta that are smaller than a critical size range. Such ejecta are decelerated and form a distorted ejecta curtain while larger ejecta form the classical inverted cone-shaped ejecta curtain characteristic of impacts in a vacuum. The effects of ejecta on the atmosphere are recorded both in early-time drag-induced vortices that lead the ejecta curtain and in the late-time circulatory motion of the decelerated ejecta cloud. High-frame-rate Schlieren photographs reveal that the atmosphere at the base of the ejecta curtain is initially turbulent but later forms a vortex. The experiments suggest that although small-size ejecta may be decelerated by air-drag, they are not simply lofted and suspended but become incorporated in an ejecta cloud controlled by air flow produced by the response of the atmosphere to the impact. Extrapolation to large-body impacts on the Earth suggests several significant differences from the laboratory experiments, including large quantities of impact-generated vapor, the supersonic advance of the ejecta curtain, the lessened effect of air-drag due to the tenuous upper atmosphere, and the role of secondary cratering. The dispersal of a meteoritic signature seems possible by several early-time processes, including ballistic shadowing, early-time atmospheric shocks, and entrainment in hot turbulent gases within the ejecta curtain. High-speed ejecta (target and meteoritic material) escaping the lower atmosphere with insignificant dynamic drag re-enter the atmosphere at large distances and deposit most of their kinetic energy as thermal energy to the upper atmosphere. Micron-size ablation products may be important contributors to significant reductions in the solar flux received at the surface. Laboratory results show that a low-angle impact ricochets projectile fragments at high speeds. At large scales, this process may provide a particularly effective mechanism for the widespread distribution of meteoritic material as a massive reentry shower. Because the ricocheted material carries with it more than 50% of the impact energy, the fraction of energy partitioned to the atmosphere increases and the cratering efficiency decreases. The resulting crater would be shallower and smaller than a crater formed by a vertical impact of equivalent energy and would be easily eroded over geologic time. Although late-time ejecta and secondary ejecta may contribute to global dispersal of suspended debris, laboratory experiments indicate that a near-rim ejecta flow develops in response to formation of the crater and is deposited near the crater rim.

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