SUBSURFACE FAILURE IN SPHERICAL BODIES: IMPLICATIONS FOR LINEAR FEATURES ON VESTA

This study presents a new, time-resolved study of damage evolution in spherical targets using a combination of laboratory and numerical experiments. The comparison reveals details about subsurface failure within spherical PMMA targets: incipient spallation occurs near the surface at the farside of the target, rarefaction waves coalesce to form a central column of damage centered on the impact point antipode, and deep failure planes result from shear deformation following passage of the shock wave. The orientation of the planar damage features, the number of sub-parallel failure planes, and the location and extent of shallow hazy failure depend on impact angle. Laboratory experiments performed at the NASA Ames Vertical Gun Range (AVGR) provide a clear view of damage growth and insight about particular damage structures. Three main regions characterize the damage in experiments: the intensely fractured and spalled region surrounding the impact point; central and antipodal failure planes; and shallow, near-surface incipient spallation and shear downrange from the impact point. Numerical simulations reveal that deep failure planes result from shear deformation following passage of the shock wave. Intuition gained from the laboratory combined with small-scale numerical experiments then prompts a new interpretation of surface features observed on asteroids. Large-scale comparisons focus on the grooved terrain observed by the Dawn spacecraft at 4 Vesta. Three-dimensional CTH calculations of large, oblique impacts onto a differentiated Vesta induce damage over large regions of the subsurface. Such simulations indicate that the linear features are likely surface expressions of subsurface failure planes and faulting. A combination of tensile damage and high values of shear stress reveal that these features are consistent with shear localization in subsurface failure planes (similar to what is seen in laboratory experiments). Comparison between the orientation of damage structures in the laboratory and failure regions within Vesta can be used to constrain impact parameters for the large south-pole basins on Vesta (e.g., the impact point and impact angle).