An impact crater is a physical scar on a planetary body's surface (topographic depression or geological structure) that is the result of hypervelocity impact by a minor planet, such as an asteroid, comet, or meteorite. 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 a rather small feature under 0.6 mi (1 km) in diameter.
The impact crater is the most common landform on the surface of most of the rocky and icy planets and satellites in our solar system. Impact craters are obliterated or covered over by younger materials where re-surfacing rates are high (e.g., Venus and Io) and where weathering and erosion are intensive (e.g., Earth and parts of Mars). At present, there are about 150 to 200 impact craters and impact structures on Earth that 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. 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, and thus, the vast majority of Earth's impact features must have been erased.
Impact craters are subdivided into three distinctive morphologic classes, which are related to crater size. The simple impact crater is a bowl-shaped feature with relatively high depth to diameter ratio. Most simple impact craters on Earth are less than 1.2 mi (2 km) in diameter. The complex impact crater has a low depth to diameter ratio and possesses a central uplift and a down-faulted and terraced rim structure. Some large complex impact craters possess an uplifted inner ring structure rather than a simple central uplift and they have a down-faulted and terraced rim as well. Complex impact craters on Earth range from the upper limit of simple impact craters to approximately 62 mi (100 km) in diameter. Multiring 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). The five multi-ring impact craters known on Earth range from 6224 mi (10000 km) in diameter. On the Moon and other planets and satellites in the solar system, the range of multiring crater diameters is from several hundred miles up to 2,485 mi (4,000 km) in diameter. A planet or satellite's gravity 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 stages during formation. Contact and compression is the initial stage. Contact occurs when the projectile first touches the planet's 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. 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. The average velocity of a cosmic projectile is approximately 12.4 mi/sec (20 km/sec) and nearly all the vast kinetic energy of this projectile is imparted to 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 subsequently. 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 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, depending upon the kinetic energy imparted by the projectile. 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. 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.
There is a general relationship between impact-crater diameter, approximate projectile diameter, energy released (in joules (J) and megatons of TNT (MT)). Generally, the ratio 20:1 relates crater diameter to projectile diameter. Kinetic energy imparted to the target may be computed using the formula KE = ½ mv2, where m is projectile mass and v is its velocity.
Further, observational data for asteroids and comets give us a general idea of impact frequency (n/106 years), and mean interval between such impacts for projectiles of given sizes. All this can be combined to give scientists an idea of the magnitude of impact energy release and how often it occurs. For example, a.62 mi (1 km) diameter impact crater would be made by a projectile 165 ft (50 m) in diameter, which would release approximately 4.6 1016 J (= 11 MT) of energy upon impact. Such an impact would occur approximately 640 times per million (106) years, or on average about once per 1,600 years. For a 3.1 mi (5 km) diameter impact crater, an 820 ft (250 m) diameter projectile is required. Approximately 5.7 1018 J (= 1,400 MT) energy is released in such an event, which would occur approximately 35 times per million years (or once per 28,500 years on average). For a 6.2 mi (10 km) diameter impact crater, a 1,640 ft (500 m) diameter projectile is needed. Approximately 4.6 1019 J (= 11,000 MT) of energy would be released. Scientists expect that such events happen approximately 10 times per million years (or on average once per 100,000 years). For a 31 mi (50 km) diameter crater (made by a 1.6 mi (2.5 km) diameter projectile), we can expect a 5.8 1021 J (= 1.3 106 MT) energy release. This would happen approximately 0.22 times per million years or on average once per 4.5 million years. For a 62 mi (100 km) diameter crater (made by a 3.1 mi (5 km) diameter projectile), we can expect a 4.6 1022 J (= 1.1 107 MT) energy release. This would happen approximately 0.04 times per million years, or on average once per 26 million years. To put the energy release in perspective, the largest nuclear weapon ever tested on Earth yielded 58 MT. If all nuclear weapons that existed at the height of the Cold War were exploded at once, the yield would be approximately 105 MT. The impact event linked to the dinosaur extinction (Chicxulub impact structure, Mexico) has a diameter of nearly 124 mi (200 km).
It is thought that impact events related to craters greater than 62 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 find the effect of cosmic impact events upon life on Earth during the geological past.
See also Asteroids; Barringer meteor crater; Comets; K-T event; Meteoroids and meteorites; Shock metamorphism
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