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Impact cratering occurs when meteorites strike planetary surfaces at hypervelocity (km/s), converting kinetic energy into shock waves, melting, and vaporization. Crater size, depth, and morphology depend on impactor size, velocity, impact angle, and target material properties and gravity.
From planetary formation, you know that the solar system assembled from collisions — dust grains accreted into planetesimals, planetesimals into protoplanets, and the leftover debris continued to bombard planetary surfaces for billions of years. From Newton's second law and conservation of energy, you know that a moving object carries kinetic energy (½mv²) and that forces cause acceleration. Impact cratering is what happens when these principles play out at extreme velocities — and the physics is unlike anything in everyday experience.
The key fact that makes impact cratering distinct from, say, dropping a rock on the ground is hypervelocity. Meteorites hit planetary surfaces at speeds of 10–70 km/s. At these velocities, the kinetic energy per unit mass far exceeds the strength of any rock or metal. When the impactor contacts the surface, the material cannot move out of the way fast enough, so it compresses. A shock wave propagates outward from the contact point into both the target rock and the impactor itself, subjecting the material to pressures of hundreds of gigapascals — millions of times atmospheric pressure. At these pressures, rock behaves like a fluid. The impactor and a comparable volume of target rock are melted or vaporized almost instantaneously, and the shock wave continues expanding hemispherically into the surrounding target, compressing, fracturing, and accelerating rock outward.
The crater forms in three stages. During the contact and compression stage (lasting only fractions of a second for a large impact), the shock wave transfers the impactor's kinetic energy to the target. In the excavation stage, the expanding shock wave and its trailing release wave accelerate target material outward and upward, excavating a bowl-shaped transient crater that is much larger than the impactor — typically 20–30 times the impactor's diameter. Material near the surface is ejected ballistically, forming an ejecta blanket around the crater, while deeper material flows outward along the crater walls. Finally, during the modification stage, the transient crater is unstable and collapses under gravity. For small craters (below ~4 km on Earth), the result is a simple bowl shape. For larger craters, the floor rebounds upward to form a central peak (like the splash-back when you drop a stone in water, frozen in rock), and the steep walls slump inward to create terraced rims — these are complex craters. The very largest impacts produce multi-ring basins where concentric rings of mountains surround the impact site.
A crucial and counterintuitive point: the crater is almost always circular regardless of impact angle. Because the shock wave propagates radially from the point of energy release and the excavation is driven by this symmetric expansion, even a 30° oblique impact produces a round crater. Only very shallow angles (below ~10–15°) create noticeably elliptical craters. This is why nearly every crater on the Moon, Mars, and Mercury is circular — it reflects the physics of shock wave expansion, not the trajectory of the impactor. Crater counting and morphology analysis remain the primary tools for dating planetary surfaces and understanding the bombardment history of the solar system.