PLANAR SHOCK INTERACTIONS IN THE NEAR-SURFACE OF IMPACT TARGETS

Spallation accelerates lightly shocked ejecta fragments to speeds that can exceed the escape velocity of the parent body. We present high-resolution simulations of nonlinear shock interactions in the near surface. Initial results show the acceleration of near-surface material to velocities up to 1.8 times greater than the peak particle velocity in the detached shock, while experiencing little to no shock pressure. These simulations suggest a possible nonlinear spallation mechanism to produce the high-velocity, low show pressure meteorites from other planets. Here we present the numerical simulations that test the production of spall through nonlinear shock interactions in the near surface.

We simulate near-surface shock interactions using the SALES_2 hydrocode and the Murnaghan equation of state. We model a planar shock approaching the surface at a constant angle phi, which ranges from 15 to 75 degrees. We model the target as a solid with a nonlinear Murnaghan equation of state. This idealized equation of state supports nonlinear shocks but is temperature independent. We track the maximum pressure and maximum velocity attained in every cell in our simulations and compare them to the Hugoniot equations that describe the material conditions in front of and behind the shock.

Our simulations demonstrate that nonlinear shock interactions in the near surface produce lightly shocked high-velocity material for planar shocks. The spall is the result of the free surface boundary condition, which forces a pressure gradient from the peak shock pressure to the zero pressure boundary. The nonlinear shock interactions occur where the pressure contours curve to accommodate the free surface. The material within this spall zone is ejected at speeds up to 1.8 km s^-1 for an imposed pulse of 1 km s^-1. Where the ejection velocities are highest, the maximum pressure attained in each cell is effectively zero.