Lobateness of impact ejecta deposits from atmospheric interactions

As an ejecta curtain advances through an atmosphere, it creates a vortex ri ng. By analogy with smoke rings, the curtain-driven vortex ring develops in stabilities that result in waves. The number of these waves depends upon th e aspect ratio of the vortex ring (i.e., the ratio of the core Vortex radiu s to the vortex radius) and the Reynolds number (or strength) of the flow i n the vertex ring. In laboratory experiments the number of sinuous features at the edges of contiguous ejecta ramparts is consistent with the theoreti cal expectations for the origin of waves created in a curtain-driven vortex ring. Observing the formation of these sinuous features provides direct ev idence that they indeed result from instabilities in the curtain-driven vor tex ring. Scaling relations for curtain velocity, curtain size, and time of crater formation permit testing whether or not such instabilities explain the lobateness or sinuosity of distal ejecta facies at broad scales on plan ets with atmospheres. Scaling relationships predict that the number of flow lobes observed for craters on both Venus and Mars should increase with inc reasing transient crater radius to the three-fourths power, a prediction th at is consistent with observation. Consequently, the curtain-driven vortex may play an important role in controlling the morphology of ejecta on plane ts with atmospheres. Variations in the number of flow lobes for a given cra ter size probably reflect different impact conditions either in target prop erties (grain size, volatile content) and/or ambient atmospheric conditions .