Scientists believe that a planet about the size of Mars collided with Earth about 4.5 billion years ago, splashing out material that quickly formed our moon. These types of collisions may also explain Venus' retrograde spin, and Uranus' 90 degree tilt. A surface that has not seen any rejuvenation for over 4 billion years may have a million craters per million square kilometers, ranging in size from 0.25 kilometer to over 512 kilometers. In contrast, a surface that is less than a million years old may have less than 1 crater per million square kilometers and the maximum size would be less than 4 km. In comparison, the Earth has a surface area of about 500 million kilometers.
Shock waves propagate into the target rocks and the impacting body.
The rock becomes almost fluid. A decrompression wave follows the shock wave. Decompressed material sprays out of the cavity as an expanding conical sheet. The ejecta represents overturned layers. In large impact structures, the rock walls slump inward soon after excavation of the initial cavity.