GLOBAL SURFACE MODIFICATION OF VESTA FOLLOWING THE RHEASILVIA IMPACT

The geologically recent (~1 Gya) Rheasilvia impact basin on Vesta is nearly equal in size to the diameter of the asteroid. In addition to blanketing nearly half the body in ejecta, the impact induced profound changes to the surface morphology of the body. In order to understand the effects of such a large impact on the asteroid, we simulate the event using the iSALE shock physics code. Impact parameters are determined by fitting the observed topography of the Rheasilvia basin to model output, and target parameters are based on Dawn observations. By tracking massless tracer particles distributed throughout the target, we are able to calculate the strain field as a function of time. Strong extensional strains are induced in the surface of the asteroid by the passage of the impact shock wave. The mode and magnitude of these strains suggest that the prominent troughs observed orthogonal to the basin center are graben which opened immediately following the impact. We calculate the effects of crater erasure due to impact induced seismic shaking, and find that the Rheasilvia impact event was capable of erasing craters several hundred meters in diameter globally, and significantly degrading craters several kilometers in size. This helps to explain the small crater deficiency observed in the highly cratered terrains of Vesta’s northern hemisphere. The sound speed of Vesta’s iron core is lower than in the mantle, resulting in antipodal focusing of the impact shock wave, strong spallation at the north pole, and a broad topographic high several kilometers in altitude. This effect is strongly dependent on mantle porosity, which serves to both reduce the mantle sound speed and damp the shock wave. By comparing model north pole topography to observations it may be possible to characterize physical parameters of Vesta’s mantle. The impact velocity of the Rheasilvia event was not high enough to vaporize the projectile and the crater collapse process concentrates remnant impactor material into the center of the basin. This suggests that the central peak of Rheasilvia should be largely composed of broken and partially melted impactor material.