The impact crater is typically the most common type of landform seen on the surface of most of the rocky and icy planets and satellites in our solar system. Impact craters form when a minor planetary body (meteoritic fragment, asteroid, or comet) strikes the surface of a larger body or major planet. A physical scar is excavated on the surface and much energy is dispersed in the process.
Most impact craters are generally circular, although elliptical impact craters are known from very low-angle or obliquely impacting projectiles. In addition, some impact craters have been tectonically deformed and thus are no longer circular. Impact craters may be exposed, buried, or partially buried. Geologists distinguish an impact crater, which is rather easily seen, from an impact structure, which is an impact crater that may be in a state of poor preservation. A meteorite crater is distinguished from other impact craters because there are fragments of the impacting body preserved near the crater. Typically, a meteorite crater is rather small feature under 1 km (0.62 mi) in diameter.
Volcanic activity can also produce circular depression (which are also sometimes called "craters," but these are not impact craters). Impact craters bear the evidence of hypervelocity impact (most cosmic objects are moving in a solar orbit at several dozen km/sec). These features include meteoritic fragments (as small craters) and shocked and shock-melted materials within and about the impact crater.
Impact craters are obliterated or covered over by younger materials where rates of volcanic activity are very high (e.g., Venus and Io) and where weathering, erosion, and sedimentation are highly active (e.g., Earth and parts of Mars). At present, there are about 1000 suspected impact craters on Venus and perhaps one or two on Io. On Earth, 150 to 200 impact craters and impact structures have been scrutinized sufficiently to prove their origin. There are several hundred other possible impact features that also have been identified. Given Earth's rather rapid weathering and tectonic cycling of crust, this is a relatively large preserved crater record. On Mars, there are several thousand impact craters in various stages of degradation. Even though preserved craters are rare on Earth, there is no reason to suspect that Earth has been bombarded any less intensively than the Moon (which has millions of impact craters), and thus the vast majority of Earth's impact features must have been erased.
Impact craters of Earth are subdivided into three distinctive groups based upon their shape, which are in turn related to crater size. The simple impact crater is a bowl-shaped feature (usually less than 1.2 mi [2 km] in diameter) with relatively high depth to diameter ratio. The complex impact crater has a low depth to diameter ratio and possesses a central uplift and a down-faulted and terraced rim structure. Complex impact craters on Earth range from the upper limit of simple impact craters to approximately 62.1 mi (100 k) in diameter. Multi-ring craters (also called multi-ring basins) are impact craters with depth to diameter ratios like complex impact craters, but they possess at least two outer, concentric rings (marked by normal faults with downward motion toward crater center). Earth has five known multi-ring impact basins, but many more are known on the Moon and other planets and satellites in the solar system, where they range from several hundred kilometers up to 4000 km (2485.5 mi) in diameter. The gravity of a planet or satellite and the strength of the surface material determine the transition diameter from simple to complex and complex to multi-ring impact crater morphology.
Impact craters go through three separate stages during formation. The contact and compression stage comes first. Contact occurs when the projectile first touches the planet or satellite's surface. Jetting of molten material from the planet's upper crust can occur at this stage and initial penetration of the crust begins (this is the origin of most tektites or impact glass objects). During compression, the projectile is compressed as it enters the target crustal material. Depending upon relative strength of the target and projectile, the projectile usually penetrates only a few times its diameter into the crust. Nearly all the vast kinetic energy of the projectile is imbued into the surrounding crust as shock-wave energy. This huge shock wave propagates outward radially into the crust from the point of projectile entry. At the end of compression, which lasts a tiny fraction of a second to two seconds at most (depends upon projectile size), the projectile is vaporized by a shock wave that bounces from the front of the projectile to the back and then forward. At this point, the projectile itself is no longer a factor in what happens. The subsequent excavation stage is driven by the shock wave propagating through the surrounding target crust. The expanding shock wave moves material along curved paths, thus ejecting debris from the continually opening crater cavity. This is the origin of the transient crater cavity. It may take several seconds to a few minutes to open this transient crater cavity. Material cast out of the opening crater during this phase forms an ejecta curtain that extends high above the impact area. This ejected material will fall back thus forming an ejecta blanket in and around the impact crater (usually extending out about 3 crater radii). During the final modification stage, gravity takes over and causes crater-rim collapse in simple impact craters. In complex and multi-ring impact craters, there is central peak or peak-ring uplift and coincident gravitational collapse in the rim area. Lingering effects of the modification stage may go on for many years after impact.
Impact crater densities are used in planetary geology to gauge the age of surfaces that have been exposed for long periods of geological time and have not been covered by volcanic flows or sediments. Impact crater sizes are also used to gauge age because the average size of impacting bodies has declined, on average, over time since the end of planetary accretion (early in our solar system's history). Sharpness or "freshness" of craters on some planetary surfaces is also a descriptive gauge of age of the crater itself. Crater studies on old planetary surfaces, like those of airless worlds like Mercury, the Moon, some icy satellites of the outer planets, and some asteroids allow age comparisons to be made between the body's surfaces. Also, crater studies allow estimates to be made of the change in density or "flux" of impacting bodies over time in the solar system.
On Earth (and perhaps early Mars), it is thought that impact events related to craters greater than 62.1 mi [100 km] in diameter likely had globally devastating effects. These effects, which may have led to global ecosystem instability or collapse, included: gas and dust discharge into the upper atmosphere (blocking sunlight and causing greenhouse effects); heating of the atmosphere due to re-entry of ballistic ejecta (causing extensive wildfires); seismic sea waves (causing tsunamis); and acid-rain production (causing damage to soils and oceans). There is much research currently underway to about the effect of cosmic impact events upon life on Earth during the geological past.
French, B.M. Traces of Catastrophe. Houston: Lunar and Planetary Institute, 1999.
Melosh, H.J. Impact Cratering. A Geologic Process. New York: Oxford University Press, 1989.
Montanari, A., and C. Koeberl. Impact Stratigrapy, the Italian Record. Berlin: Springer-Verlag, 2000.
David T. King Jr.