Impacts of small celestial bodies, in terms of energy density, occupy the range between ordinary chemical high explosives and nuclear explosions. The high initial energy density of impact gives them some features of an explosion (shock waves, melting and vaporization, mechanical disruption of target rocks). A near-surface burst creates an explosion crater, and an impact often results in the creation of an impact crater. The chain of processes connected to an impact crater’s formation is named “impact cratering” or simply “cratering.” The initial kinetic energy and momenta of the impacting body (“projectile”) generates shock waves (decaying with propagation to seismic waves), heats the material (at high impact velocities, to melt or to boil target rocks). A part of the kinetic energy is converted to target material motion, creating the crater cavity. The final crater geometry depends on the scale of event—while small craters are simple bowl-shaped cavities, large enough crater transient cavities collapse in the gravity field. If collapse takes place, the final crater has a complex geometry with central peaks and concentric inner rings. The boundary crater diameter, dividing simple and complex craters, varies with target body gravity and rock strength. Comparison of a crater’s morphology on remote planets and asteroids allows us to make some estimates about their mechanical parameters (e.g., strength and friction) even before future sample return missions. On many planets large impact craters can be seen, preserved much better than on the geologically active Earth. These observations help researchers to interpret the geological and geophysical data obtained for the relatively few and heavily modified large impact craters found on continents and (rarely) at the sea bottom.
A large impact crater beneath Hiawatha Glacier in northwest Greenland