  Using moving mesh hydrodynamic simulations, we determine the shock
propagation and resulting ground velocities for a planet hit by a high velocity
impactor. We use our results to determine the atmospheric mass loss caused by
the resulting ground motion due to the impact shock wave. We find that there
are two distinct shock propagation regimes: In the limit in which the impactor
is significantly smaller than the target ($R_i<< R_t$), the solutions are
self-similar and the shock velocity at a fixed point on the target scale as
$m_i^{2/3}$, where $m_i$ is the mass of the impactor. In addition, the ground
velocities follow a universal profile given by
$v_g/v_i=(14.2x^2-25.3x+11.3)/(x^2-2.5x+1.9) +2\ln{R_i/R_t}$, where
$x=\sin\left(\theta/2\right)$, $\theta$ is the latitude on the target measured
from the impact site, and $v_g$ and $v_i$ are the ground velocity and impact
velocity, respectively. In contrast, in the limit in which the impactor is
comparable to the size of the target ($R_i \sim R_t$), we find that shock
velocities decline with the mass of the impactor significantly more weakly than
$m_i^{2/3}$. We use the resulting surface velocity profiles to calculate the
atmospheric mass loss for a large range of impactor masses and impact
velocities and apply them to the Kepler-36 system and the Moon forming impact.
Finally, we present and generalise our results in terms of the $v_g/v_i$ and
the impactor to target size ratio ($R_i/R_t$) such that they can easily be
applied to other collision scenarios.
