As an ejecta curtain advances through an atmosphere, it creates a vortex ring. By analogy with smoke rings, the curtain-driven vortex ring develops instabilities that result in waves. The number of these waves depends upon the aspect ratio of the vortex ring (i.e., the ratio of the core vortex radius to the vortex radius) and the Reynolds number (or strength) of the flow in the vortex ring. In laboratory experiments the number of sinuous features at the edges of contiguous ejecta ramparts is consistent with the theoretical expectations for the origin of waves created in a curtain-driven vortex ring. Observing the formation of these sinuous features provides direct evidence that they indeed result from instabilities in the curtain-driven vortex 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 planets with atmospheres. Scaling relationships predict that the number of flow lobes observed for craters on both Venus and Mars should increase with increasing transient crater radius to the three-fourths power, a prediction that is consistent with observation. Consequently, the curtain-driven vortex may play an important role in controlling the morphology of ejecta on planets with atmospheres. Variations in the number of flow lobes for a given crater size probably reflect different impact conditions either in target properties (grain size, volatile content) and/or ambient atmospheric conditions. ¿ 1998 American Geophysical Union