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55-500: Gruemberger is an old impact crater in the southern part of the Moon . When viewed from the Earth this crater appears oval in shape due to foreshortening, but it is actually relatively circular. The crater lies about 25 kilometers to the north-northwest of the larger and more prominent crater Moretus . The smaller crater Cysatus intrudes into the eastern rim of Gruemberger. About one and

110-433: A paraboloid (bowl-shaped) crater in which the centre has been pushed down, a significant volume of material has been ejected, and a topographically elevated crater rim has been pushed up. When this cavity has reached its maximum size, it is called the transient cavity. The depth of the transient cavity is typically a quarter to a third of its diameter. Ejecta thrown out of the crater do not include material excavated from

165-468: A certain altitude (retardation point), and start to accelerate again due to Earth's gravity until the body reaches its terminal velocity of 0.09 to 0.16 km/s. The larger the meteoroid (i.e. asteroids and comets) the more of its initial cosmic velocity it preserves. While an object of 9,000 kg maintains about 6% of its original velocity, one of 900,000 kg already preserves about 70%. Extremely large bodies (about 100,000 tonnes) are not slowed by

220-401: A half crater diameters to the north is the large crater Clavius . This crater has undergone steady erosion from minor impacts, so that the features along the rim and inner wall have become softened and rounded. Significant impacts nearby have covered the floor and sides with ejecta, and there are a multitude of tiny craterlets along the inner wall. The satellite crater Gruemberger A lies within

275-455: A hole in the surface without filling in nearby craters. This may explain the 'sponge-like' appearance of that moon. It is convenient to divide the impact process conceptually into three distinct stages: (1) initial contact and compression, (2) excavation, (3) modification and collapse. In practice, there is overlap between the three processes with, for example, the excavation of the crater continuing in some regions while modification and collapse

330-452: A large impact. The subsequent excavation of the crater occurs more slowly, and during this stage the flow of material is largely subsonic. During excavation, the crater grows as the accelerated target material moves away from the point of impact. The target's motion is initially downwards and outwards, but it becomes outwards and upwards. The flow initially produces an approximately hemispherical cavity that continues to grow, eventually producing

385-558: A regular sequence with increasing size: small complex craters with a central topographic peak are called central peak craters, for example Tycho ; intermediate-sized craters, in which the central peak is replaced by a ring of peaks, are called peak-ring craters , for example Schrödinger ; and the largest craters contain multiple concentric topographic rings, and are called multi-ringed basins , for example Orientale . On icy (as opposed to rocky) bodies, other morphological forms appear that may have central pits rather than central peaks, and at

440-409: A result, the impactor is compressed, its density rises, and the pressure within it increases dramatically. Peak pressures in large impacts exceed 1 T Pa to reach values more usually found deep in the interiors of planets, or generated artificially in nuclear explosions . In physical terms, a shock wave originates from the point of contact. As this shock wave expands, it decelerates and compresses

495-677: A sample of articles of confirmed and well-documented impact sites. See the Earth Impact Database , a website concerned with 190 (as of July 2019 ) scientifically confirmed impact craters on Earth. There are approximately twelve more impact craters/basins larger than 300 km on the Moon, five on Mercury, and four on Mars. Large basins, some unnamed but mostly smaller than 300 km, can also be found on Saturn's moons Dione, Rhea and Iapetus. Sikhote-Alin meteorite At around 10:30 AM on 12 February 1947, eyewitnesses in

550-441: A significant crater volume may also be formed by the permanent compaction of the pore space . Such compaction craters may be important on many asteroids, comets and small moons. In large impacts, as well as material displaced and ejected to form the crater, significant volumes of target material may be melted and vaporized together with the original impactor. Some of this impact melt rock may be ejected, but most of it remains within

605-406: A small angle, and high-temperature highly shocked material is expelled from the convergence zone with velocities that may be several times larger than the impact velocity. In most circumstances, the transient cavity is not stable and collapses under gravity. In small craters, less than about 4 km diameter on Earth, there is some limited collapse of the crater rim coupled with debris sliding down

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660-502: A stamp for the 10th anniversary of the Sikhote-Alin meteorite shower. It reproduces a painting by P. I. Medvedev , a Soviet artist who witnessed the fall: he was sitting in his window starting a sketch when the fireball appeared and immediately began drawing what he saw. Because the meteor fell during the daytime, it was observed by many eyewitnesses. Evaluation of this observational data allowed V. G. Fesenkov , then chairman of

715-466: Is a massive fall, with the pre-atmospheric mass of the meteoroid estimated at approximately 90,000 kg (200,000 lb). A more recent estimate by Tsvetkov (and others) puts the mass at around 100,000 kg (220,000 lb). Krinov estimated the post-atmospheric mass of the meteoroid to be some 23,000 kg (51,000 lb). The strewn field for this meteorite covered an elliptical area of about 1.3 km (0.50 sq mi). Some of

770-402: Is already underway in others. In the absence of atmosphere , the impact process begins when the impactor first touches the target surface. This contact accelerates the target and decelerates the impactor. Because the impactor is moving so rapidly, the rear of the object moves a significant distance during the short-but-finite time taken for the deceleration to propagate across the impactor. As

825-512: Is ejected from close to the center of impact, and the slowest material is ejected close to the rim at low velocities to form an overturned coherent flap of ejecta immediately outside the rim. As ejecta escapes from the growing crater, it forms an expanding curtain in the shape of an inverted cone. The trajectory of individual particles within the curtain is thought to be largely ballistic. Small volumes of un-melted and relatively un-shocked material may be spalled at very high relative velocities from

880-459: Is estimated that the value of materials mined from impact structures is five billion dollars/year just for North America. The eventual usefulness of impact craters depends on several factors, especially the nature of the materials that were impacted and when the materials were affected. In some cases, the deposits were already in place and the impact brought them to the surface. These are called "progenetic economic deposits." Others were created during

935-404: Is sufficient to melt the impactor, and in larger impacts to vaporize most of it and to melt large volumes of the target. As well as being heated, the target near the impact is accelerated by the shock wave, and it continues moving away from the impact behind the decaying shock wave. Contact, compression, decompression, and the passage of the shock wave all occur within a few tenths of a second for

990-437: Is the largest goldfield in the world, which has supplied about 40% of all the gold ever mined in an impact structure (though the gold did not come from the bolide). The asteroid that struck the region was 9.7 km (6 mi) wide. The Sudbury Basin was caused by an impacting body over 9.7 km (6 mi) in diameter. This basin is famous for its deposits of nickel , copper , and platinum group elements . An impact

1045-497: The Nevada Test Site , notably Jangle U in 1951 and Teapot Ess in 1955. In 1960, Edward C. T. Chao and Shoemaker identified coesite (a form of silicon dioxide ) at Meteor Crater, proving the crater was formed from an impact generating extremely high temperatures and pressures. They followed this discovery with the identification of coesite within suevite at Nördlinger Ries , proving its impact origin. Armed with

1100-543: The Sikhote-Alin Mountains , Primorye , Soviet Union, observed a large bolide brighter than the sun that came out of the north and descended at an angle of about 41 degrees. The bright flash and the loud sound of the fall were observed for 300 kilometres (190 mi) around the point of impact not far from Luchegorsk and approximately 440 km (270 mi) northeast of Vladivostok . A smoke trail, estimated at 32 km (20 mi) long, remained in

1155-429: The speed of sound in those objects. Such hyper-velocity impacts produce physical effects such as melting and vaporization that do not occur in familiar sub-sonic collisions. On Earth, ignoring the slowing effects of travel through the atmosphere, the lowest impact velocity with an object from space is equal to the gravitational escape velocity of about 11 km/s. The fastest impacts occur at about 72 km/s in

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1210-399: The stable interior regions of continents . Few undersea craters have been discovered because of the difficulty of surveying the sea floor, the rapid rate of change of the ocean bottom, and the subduction of the ocean floor into Earth's interior by processes of plate tectonics . Daniel M. Barringer, a mining engineer, was convinced already in 1903 that the crater he owned, Meteor Crater ,

1265-493: The "worst case" scenario in which an object in a retrograde near-parabolic orbit hits Earth. The median impact velocity on Earth is about 20 km/s. However, the slowing effects of travel through the atmosphere rapidly decelerate any potential impactor, especially in the lowest 12 kilometres where 90% of the Earth's atmospheric mass lies. Meteorites of up to 7,000 kg lose all their cosmic velocity due to atmospheric drag at

1320-480: The Sikhote-Alin Meteorite are basically of two types: The first type probably broke off the main object early in the descent. These pieces are characterized by regmaglypts (cavities resembling thumbprints) on the surface of each specimen. The second type are fragments which were either torn apart during the atmospheric explosions or blasted apart upon impact on the frozen ground. Most resulted from

1375-478: The actual impact. The great energy involved caused melting. Useful minerals formed as a result of this energy are classified as "syngenetic deposits." The third type, called "epigenetic deposits," is caused by the creation of a basin from the impact. Many of the minerals that our modern lives depend on are associated with impacts in the past. The Vredeford Dome in the center of the Witwatersrand Basin

1430-469: The association of volcanic flows and other volcanic materials. Impact craters produce melted rocks as well, but usually in smaller volumes with different characteristics. The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as shatter cones , melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in

1485-432: The atmosphere at all, and impact with their initial cosmic velocity if no prior disintegration occurs. Impacts at these high speeds produce shock waves in solid materials, and both impactor and the material impacted are rapidly compressed to high density. Following initial compression, the high-density, over-compressed region rapidly depressurizes, exploding violently, to set in train the sequence of events that produces

1540-451: The collapse and modification of the transient cavity is much more extensive, and the resulting structure is called a complex crater . The collapse of the transient cavity is driven by gravity, and involves both the uplift of the central region and the inward collapse of the rim. The central uplift is not the result of elastic rebound, which is a process in which a material with elastic strength attempts to return to its original geometry; rather

1595-468: The collapse is a process in which a material with little or no strength attempts to return to a state of gravitational equilibrium . Complex craters have uplifted centers, and they have typically broad flat shallow crater floors, and terraced walls . At the largest sizes, one or more exterior or interior rings may appear, and the structure may be labeled an impact basin rather than an impact crater. Complex-crater morphology on rocky planets appears to follow

1650-451: The crater walls and drainage of impact melts into the deeper cavity. The resultant structure is called a simple crater, and it remains bowl-shaped and superficially similar to the transient crater. In simple craters, the original excavation cavity is overlain by a lens of collapse breccia , ejecta and melt rock, and a portion of the central crater floor may sometimes be flat. Above a certain threshold size, which varies with planetary gravity,

1705-405: The craters on the Moon as logical impact sites that were formed not gradually, in eons , but explosively, in seconds." For his PhD degree at Princeton University (1960), under the guidance of Harry Hammond Hess , Shoemaker studied the impact dynamics of Meteor Crater. Shoemaker noted that Meteor Crater had the same form and structure as two explosion craters created from atomic bomb tests at

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1760-498: The dominant geographic features on many solid Solar System objects including the Moon , Mercury , Callisto , Ganymede , and most small moons and asteroids . On other planets and moons that experience more active surface geological processes, such as Earth , Venus , Europa , Io , Titan , and Triton , visible impact craters are less common because they become eroded , buried, or transformed by tectonic and volcanic processes over time. Where such processes have destroyed most of

1815-407: The expanding vapor cloud may rise to many times the scale height of the atmosphere, effectively expanding into free space. Most material ejected from the crater is deposited within a few crater radii, but a small fraction may travel large distances at high velocity, and in large impacts it may exceed escape velocity and leave the impacted planet or moon entirely. The majority of the fastest material

1870-658: The fragments made impact craters , the largest of which was about 26 m (85 ft) across and 6 m (20 ft) deep. Fragments of the meteorite were also driven into the surrounding trees, embedding themselves. The Sikhote-Alin meteorite is classified as an iron meteorite belonging to the meteorite group IIAB and with a coarse octahedrite structure. It is composed of approximately 93% iron , 5.9% nickel , 0.42% cobalt , 0.46% phosphorus , and 0.28% sulfur , with trace amounts of germanium and iridium . Minerals present include taenite , plessite , troilite , chromite , kamacite , and schreibersite . Specimens of

1925-401: The full depth of the transient cavity; typically the depth of maximum excavation is only about a third of the total depth. As a result, about one third of the volume of the transient crater is formed by the ejection of material, and the remaining two thirds is formed by the displacement of material downwards, outwards and upwards, to form the elevated rim. For impacts into highly porous materials,

1980-448: The geologists John D. Boon and Claude C. Albritton Jr. revisited Bucher's studies and concluded that the craters that he studied were probably formed by impacts. Grove Karl Gilbert suggested in 1893 that the Moon's craters were formed by large asteroid impacts. Ralph Baldwin in 1949 wrote that the Moon's craters were mostly of impact origin. Around 1960, Gene Shoemaker revived the idea. According to David H. Levy , Shoemaker "saw

2035-599: The impact crater. Impact-crater formation is therefore more closely analogous to cratering by high explosives than by mechanical displacement. Indeed, the energy density of some material involved in the formation of impact craters is many times higher than that generated by high explosives. Since craters are caused by explosions , they are nearly always circular – only very low-angle impacts cause significantly elliptical craters. This describes impacts on solid surfaces. Impacts on porous surfaces, such as that of Hyperion , may produce internal compression without ejecta, punching

2090-521: The impactor, and it accelerates and compresses the target. Stress levels within the shock wave far exceed the strength of solid materials; consequently, both the impactor and the target close to the impact site are irreversibly damaged. Many crystalline minerals can be transformed into higher-density phases by shock waves; for example, the common mineral quartz can be transformed into the higher-pressure forms coesite and stishovite . Many other shock-related changes take place within both impactor and target as

2145-549: The interior, along the inner wall to the west-southwest. By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Gruemberger. Impact crater An impact crater is a depression in the surface of a solid astronomical body formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters , which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than

2200-764: The knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Astrophysical Observatory in Victoria, British Columbia , Canada and Wolf von Engelhardt of the University of Tübingen in Germany began a methodical search for impact craters. By 1970, they had tentatively identified more than 50. Although their work was controversial, the American Apollo Moon landings, which were in progress at

2255-649: The largest sizes may contain many concentric rings. Valhalla on Callisto is an example of this type. Long after an impact event, a crater may be further modified by erosion, mass wasting processes, viscous relaxation, or erased entirely. These effects are most prominent on geologically and meteorologically active bodies such as Earth, Titan, Triton, and Io. However, heavily modified craters may be found on more primordial bodies such as Callisto, where many ancient craters flatten into bright ghost craters, or palimpsests . Non-explosive volcanic craters can usually be distinguished from impact craters by their irregular shape and

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2310-468: The meteorite committee of the USSR Academy of Science , to estimate the meteoroid's orbit before it encountered the Earth. This orbit was ellipse -shaped, with its point of greatest distance from the sun situated within the asteroid belt , similar to many other small bodies crossing the orbit of the Earth. Such an orbit was probably created by collisions within the asteroid belt. Sikhote-Alin

2365-411: The original crater topography , the terms impact structure or astrobleme are more commonly used. In early literature, before the significance of impact cratering was widely recognised, the terms cryptoexplosion or cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth. The cratering records of very old surfaces, such as Mercury, the Moon, and

2420-711: The outer Solar System could be different from the inner Solar System. Although Earth's active surface processes quickly destroy the impact record, about 190 terrestrial impact craters have been identified. These range in diameter from a few tens of meters up to about 300 km (190 mi), and they range in age from recent times (e.g. the Sikhote-Alin craters in Russia whose creation was witnessed in 1947) to more than two billion years, though most are less than 500 million years old because geological processes tend to obliterate older craters. They are also selectively found in

2475-479: The planet than have been discovered so far. The cratering rate in the inner solar system fluctuates as a consequence of collisions in the asteroid belt that create a family of fragments that are often sent cascading into the inner solar system. Formed in a collision 80 million years ago, the Baptistina family of asteroids is thought to have caused a large spike in the impact rate. The rate of impact cratering in

2530-419: The shock wave passes through, and some of these changes can be used as diagnostic tools to determine whether particular geological features were produced by impact cratering. As the shock wave decays, the shocked region decompresses towards more usual pressures and densities. The damage produced by the shock wave raises the temperature of the material. In all but the smallest impacts this increase in temperature

2585-475: The sky for several hours. As the meteor, traveling at a speed of about 14 km/s (8.7 mi/s), entered the atmosphere, it began to break apart, and the fragments fell together, some burying themselves 6 metres (20 ft) deep. At an altitude of about 5.6 km (3.5 mi), the largest mass apparently broke up in an explosion called an air burst . On November 20, 1957 the Soviet Union issued

2640-464: The southern highlands of Mars, record a period of intense early bombardment in the inner Solar System around 3.9 billion years ago. The rate of crater production on Earth has since been considerably lower, but it is appreciable nonetheless. Earth experiences, on average, from one to three impacts large enough to produce a 20-kilometre-diameter (12 mi) crater every million years. This indicates that there should be far more relatively young craters on

2695-425: The surface of the target and from the rear of the impactor. Spalling provides a potential mechanism whereby material may be ejected into inter-planetary space largely undamaged, and whereby small volumes of the impactor may be preserved undamaged even in large impacts. Small volumes of high-speed material may also be generated early in the impact by jetting. This occurs when two surfaces converge rapidly and obliquely at

2750-507: The surrounding terrain. Impact craters are typically circular, though they can be elliptical in shape or even irregular due to events such as landslides. Impact craters range in size from microscopic craters seen on lunar rocks returned by the Apollo Program to simple bowl-shaped depressions and vast, complex, multi-ringed impact basins . Meteor Crater is a well-known example of a small impact crater on Earth. Impact craters are

2805-473: The time, provided supportive evidence by recognizing the rate of impact cratering on the Moon . Because the processes of erosion on the Moon are minimal, craters persist. Since the Earth could be expected to have roughly the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters. Impact cratering involves high velocity collisions between solid objects, typically much greater than

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2860-411: The transient crater, initially forming a layer of impact melt coating the interior of the transient cavity. In contrast, the hot dense vaporized material expands rapidly out of the growing cavity, carrying some solid and molten material within it as it does so. As this hot vapor cloud expands, it rises and cools much like the archetypal mushroom cloud generated by large nuclear explosions. In large impacts,

2915-403: The uplifted center of a complex crater, however. Impacts produce distinctive shock-metamorphic effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include: On Earth, impact craters have resulted in useful minerals. Some of the ores produced from impact related effects on Earth include ores of iron , uranium , gold , copper , and nickel . It

2970-684: Was involved in making the Carswell structure in Saskatchewan , Canada; it contains uranium deposits. Hydrocarbons are common around impact structures. Fifty percent of impact structures in North America in hydrocarbon-bearing sedimentary basins contain oil/gas fields. On Earth, the recognition of impact craters is a branch of geology, and is related to planetary geology in the study of other worlds. Out of many proposed craters, relatively few are confirmed. The following twenty are

3025-519: Was of cosmic origin. Most geologists at the time assumed it formed as the result of a volcanic steam eruption. In the 1920s, the American geologist Walter H. Bucher studied a number of sites now recognized as impact craters in the United States. He concluded they had been created by some great explosive event, but believed that this force was probably volcanic in origin. However, in 1936,

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