Atmospheric effects on ejecta emplacement

Abstract

Laboratory experiments allow the investigation of complex interactions between impacts and an atmosphere. Although small in scale, they can provide essential first‐order constraints on the processes affecting late‐stage ballistic ejecta and styles of ejecta emplacement around much larger craters on planetary surfaces. The laboratory experiments involved impacting different fine‐grained particulate targets under varying atmospheric pressure and density (different gas compositions). During crater formation, ballistic ejecta form the classic cone‐shaped profile observed under vacuum conditions. As atmospheric density increases (for a given pressure), however, the ejecta curtain bulges at the base and pinches above. This systematic change in the ejecta curtain reflects the combined effects of deceleration of ejecta smaller than a critical size and entrainment of these ejecta within atmospheric vortices created as the outward moving wall of ejecta displaces the atmosphere. Additionally, a systematic change in emplacement style occurs as a function of atmospheric pressure (largely independent of density): contiguous ejecta rampart superposing ballistically emplaced deposits (0.06 to 0.3 bar); ejecta flow lobes (0.3 to 0.7 bar); and radial patterns (>0.8 bar). Underlying processes controlling such systematic changes in emplacement style were revealed by observing the evolution of the ejecta curtain, by changing target materials (including layered targets and low‐density particulates), by varying atmospheric density, by changing impact angle, and by comparing the ejecta run‐out distances with first‐order models of turbidity flows. Three distinct ejecta emplacement processes can be characterized. Ejecta ramparts result from coarser clasts sorted and driven outward by vortical winds behind the outward moving ejecta curtain. This style of “wind‐modified” emplacement represents minimal ejecta entrainment and is enhanced by a bimodal size distribution in the ejecta. Such “eddy‐supported flows” are observed to increase in run‐out distance (scaled to crater size) with increasing atmospheric pressure. By analogy with turbidity flows, this scaled distance should increase as R1/2 for a given atmospheric pressure and degree of entrainment. Ejecta flows with much greater run‐out distances develop as the turbulent power in atmospheric response winds increase. Such flows overrun and scour the inner ejecta facies, thereby producing distinct inner and outer facies. The degree of ejecta entrainment depends on the dimensionless ratio of drag to gravity forces acting on individual ejecta and the intensity of the winds created by the outward moving curtain. Entrainment increases with increasing atmospheric density and ejection velocity (crater size) but decreases with ejecta density and size. The intensity of curtain‐generated winds increases with ejection velocity (crater size). The dimensionless drag ratio characterizing the laboratory experiments can be applied to Mars since the reduced atmospheric density is offset by the increased ejection velocities for kilometer‐scale events. For a given crater size (ejection velocity) and atmospheric conditions, a wide range of nonballistic ejecta emplacement styles could occur simply by varying ejecta sizes even without the presence of water. Alternatively, the onset crater diameter for nonballistic emplacement styles can reflect the range of ejecta sizes possible from the diverse martian geologic history (massive basalts to fine‐grained aeolian deposits). Scaling considerations further predict that ejecta run‐out distances scaled to crater size on Mars should increase as R1/2; hence long run‐out flows dependent on crater diameter need not reflect depth to a buried reservoir of water. On Venus, however, the dense atmosphere maximizes entrainment and results in ejecta flow densities approaching a constant fraction of the atmospheric density. Under such conditions, ejecta run‐out distances should decrease as R−1/2.