A LABORATORY MODEL OF SPLASH-FORM TEKTITES

Abstract

We assume that tektites are produced by terrestrial impacts, either directly from the splashing of shock melt, or possibly as condensates from the impact vapor cloud. In either case, the final product is a fluid drop falling through air, and this physical system is our subject of focus. We interpret and extend the dynamics and stability of spinning, translating fluid drops to make inferences concerning the dynamic history of tektites. Drop shape is uniquely prescribed by normal force balance at the tektite surface. The shapes of drops progress with change in the non-dimensional group Bo, called the Bond number, which is a ratio of density, angular speed, and drop radius to surface tension. As Bond number increases, the tektite shape progresses from a sphere to a dumbbell or an oblate ellipsoid, and then to a biconcave shape. A laboratory model for tektites is developed that consists of rolling or tumbling molten metallic drops either in a cylindrical drum or down a ramp into air or a quench bath. The model reproduces all of the known forms of splash-form tektites, including spheres, oblate ellipsoids, dumbbells, teardrops and tori. The laboratory also highlights important differences between rolling drops and drops tumbling while in flight; for example, toroidal drops are much more stable when rolling. We conclude that molten tektites can exist as equilibrium bodies of revolution only up to 3 mm, based on an analysis of capillary length. Smaller drops are the product of break-up at greater than terminal velocity. Larger tektites are necessarily non-equilibrium forms. This underscores the importance of cooling and quenching in flight, since many tektites greatly exceed the maximum sizes anticipated based on any reasonable relative flight speed estimate, suggesting that their break-up time greatly exceeded their cooling time. This is supported by the large fraction of tektites that show a high-viscosity crust that evidently cracked while in flight.