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Abstract— A simple analytical solution for subsurface particle motions during impact cratering is useful for tracking the evolution of the transient crater shape at late times. A specific example of such an analytical solution is Maxwell's Z-Model, which is based on a point-source assumption. Here, the parameters for this model are constrained using measured ejection angles from both vertical and oblique experimental impacts at the NASA Ames Vertical Gun Range. Data from experiments reveal that impacts at angles as high as 45° to the target's surface generate subsurface flow-fields that are significantly different from those created by vertical impacts. The initial momentum of the projectile induces a subsurface momentum-driven flow-field that evolves in three dimensions of space and in time to an excavation flow-field during both vertical and oblique impacts. A single, stationary point-source model (specifically Maxwell's Z-Model), however, is found inadequate to explain this detailed evolution of the subsurface flow-field during oblique impacts. Because 45° is the most likely impact angle on planetary surfaces, a new analytical model based on a migrating point-source could prove quite useful. Such a model must address the effects of the subsurface flow-field evolution on crater excavation, ejecta deposition, and transient crater morphometry.