In laboratory experiments, high speed videos are used to detect and track
mm-size surface particle motions caused by a low velocity normal impact into
sand. Outside the final crater radius and prior to the landing of the ejecta
curtain, particle displacements are measured via particle image velocimetry and
with a cross-correlation method. Surface particles rebound and are also
permanently displaced with both peak and permanent displacements rapidly
decaying as a function of distance from the crater center. The surface begins
to move before most of the ejecta curtain has landed, but continues to move
after the subsurface seismic pulse has decayed. Ray angles for surface and
subsurface velocities are similar to those described by a Maxwell's Z-model.
This implies that the flow field outside the crater excavation region is a
continuation of the crater excavation flow. The ratio of final particle
displacement to crater radius resembles that measured for other impact craters.
Subjects: Astrophysics - Earth and Planetary Astrophysics; Astrophysics - Instrumentation and Methods for Astrophysics