Animpact crater is adepression in the surface of asolidastronomical body formed by thehypervelocityimpact of a smaller object. In contrast tovolcanic craters, which result from explosion or internal collapse,[2] impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain.[3] 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 theApollo Program[4] 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.[5]
Impact craters are the dominant geographic features on many solid Solar System objects including theMoon,Mercury,Callisto,Ganymede, and most small moons andasteroids. On other planets and moons that experience more active surface geological processes, such asEarth,Venus,Europa,Io,Titan, andTriton, visible impact craters are less common because they becomeeroded, buried, or transformed bytectonic andvolcanic processes over time. Where such processes have destroyed most of the original cratertopography, the termsimpact structure or astrobleme are more commonly used. In early literature, before the significance of impact cratering was widely recognised, the termscryptoexplosion or cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth.[6]
The cratering records of very old surfaces, such as Mercury, the Moon, and the southern highlands of Mars, record a period ofintense 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.[7][8] This indicates that there should be far more relatively young craters on the planet than have been discovered so far. The cratering rate in the inner solar system fluctuates as a consequence of collisions in theasteroid belt that create a family of fragments that are often sent cascading into the inner solar system.[9] Formed in a collision 80 million years ago, theBaptistina family of asteroids is thought to have caused a large spike in the impact rate. The rate of impact cratering in the outer Solar System could be different from the inner Solar System.[10]
Although Earth's active surface processes quickly destroy the impact record, about 190 terrestrial impact craters have been identified.[11] 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. theSikhote-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 thestable interior regions of continents.[12] 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 thesubduction of the ocean floor into Earth's interior by processes ofplate tectonics.
Daniel M. Barringer, a mining engineer, was convinced already in 1903 that the crater he owned,Meteor Crater, was of cosmic origin. Most geologists at the time assumed it formed as the result of a volcanic steam eruption.[13]: 41–42
Eugene Shoemaker, pioneer impact crater researcher, here at a crystallographic microscope used to examine meteorites
In the 1920s, the American geologistWalter 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 probablyvolcanic in origin. However, in 1936, the geologistsJohn D. Boon andClaude C. Albritton Jr. revisited Bucher's studies and concluded that the craters that he studied were probably formed by impacts.[14]
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 toDavid H. Levy, Shoemaker "saw the craters on the Moon as logical impact sites that were formed not gradually, ineons, but explosively, in seconds." For hisPhD degree atPrinceton University (1960), under the guidance ofHarry Hammond Hess, Shoemaker studied the impact dynamics of Meteor Crater. Shoemaker noted that Meteor Crater had the same form and structure as twoexplosion craters created fromatomic bomb tests at theNevada Test Site, notablyJangle U in 1951 andTeapot Ess in 1955. In 1960,Edward C. T. Chao and Shoemaker identifiedcoesite (a form ofsilicon 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 withinsuevite atNördlinger Ries, proving its impact origin.[13]
Armed with the knowledge of shock-metamorphic features,Carlyle S. Beals and colleagues at theDominion Astrophysical Observatory inVictoria, British Columbia, Canada andWolf von Engelhardt of theUniversity 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 AmericanApollo Moon landings, which were in progress at the time, provided supportive evidence by recognizing the rate of impact cratering on theMoon.[15] 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.
A laboratory simulation of an impact event and crater formation
Impact cratering involves high velocity collisions between solid objects, typically much greater than thespeed of sound in those objects. Such hyper-velocity impacts produce physical effects such asmelting andvaporization 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 gravitationalescape velocity of about 11 km/s. The fastest impacts occur at about 72 km/s[16] in the "worst case" scenario in which an object in a retrograde near-parabolic orbit hits Earth. Themedian impact velocity on Earth is about 20 km/s.[17]
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. Meteors of up to 7,000 kg lose all their cosmic velocity due to atmospheric drag at a certain altitude (retardation point), and start to accelerate again due to Earth's gravity until the body reaches itsterminal velocity of 0.09 to 0.16 km/s.[16] 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 the atmosphere at all, and impact with their initial cosmic velocity if no prior disintegration occurs.[16]
Impacts at these high speeds produceshock waves in solid materials, and both impactor and the material impacted are rapidlycompressed 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 the impact crater. Impact-crater formation is therefore more closely analogous to cratering byhigh explosives than by mechanical displacement. Indeed, theenergy 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 byexplosions, they are nearly always circular – only very low-angle impacts cause significantly elliptical craters.[18]
This describes impacts on solid surfaces. Impacts on porous surfaces, such as that ofHyperion, may produce internal compression without ejecta, punching a hole in the surface without filling in nearby craters. This may explain the 'sponge-like' appearance of that moon.[19]
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 is already underway in others.
Nested Craters on Mars, 40.104° N, 125.005° E. These nested craters are probably caused by changes in the strength of the target material. This usually happens when a weaker material overlies a stronger material.[20]
In the absence ofatmosphere, the impact process begins when the impactor first touches the target surface. This contactaccelerates 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 a result, the impactor is compressed, its density rises, and thepressure within it increases dramatically. Peak pressures in large impacts exceed 1TPa to reach values more usually found deep in the interiors of planets, or generated artificially innuclear explosions.
In physical terms, ashock wave originates from the point of contact. As this shock wave expands, it decelerates and compresses 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 formscoesite andstishovite. Many other shock-related changes take place within both impactor and target as 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.[18]
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 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.[18]
Contact, compression, decompression, and the passage of the shock wave all occur within a few tenths of a second for 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 aparaboloid (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.[18]
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 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, a significant crater volume may also be formed by the permanent compaction of thepore 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 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, 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 exceedescape velocity and leave the impacted planet or moon entirely. The majority of the fastest material 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 bespalled at very high relative velocities from 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 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.
Weathering may change the aspect of a crater drastically. This mound onMars' north pole may be the result of an impact crater that was buried bysediment and subsequently re-exposed byerosion.
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 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 collapsebreccia, ejecta and melt rock, and a portion of the central crater floor may sometimes be flat.
Multi-ringed impact basin Valhalla on Jupiter's moonCallisto
Above a certain threshold size, which varies with planetary gravity, the collapse and modification of the transient cavity is much more extensive, and the resulting structure is called acomplex 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 the collapse is a process in which a material with little or no strength attempts to return to a state ofgravitational equilibrium.
Complex craters have uplifted centers, and they have typically broad flat shallow crater floors, andterraced 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 a regular sequence with increasing size: small complex craters with a central topographic peak are called central peak craters, for exampleTycho; intermediate-sized craters, in which the central peak is replaced by a ring of peaks, are calledpeak-ring craters, for exampleSchrödinger; and the largest craters contain multiple concentric topographic rings, and are calledmulti-ringed basins, for exampleOrientale. On icy (as opposed to rocky) bodies, other morphological forms appear that may have central pits rather than central peaks, and at 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, orpalimpsests.[21] The impact events usually feature a period of enhanced hydrothermal activity and circulation. In a study at Finnish Lappajärvi, researchers could establish that microorganisms has colonized the impact melt rocks in late stages of such hydrothermal circulation, when the temperatures had cooled to habitable conditions.[22]
Non-explosive volcanic craters can usually be distinguished from impact craters by their irregular shape and the association of volcanic flows and other volcanic materials. Impact craters produce melted rocks as well, but usually in smaller volumes with different characteristics.[6]
The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such asshatter 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 the uplifted center of a complex crater, however.[23][24]
Impacts produce distinctiveshock-metamorphic effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include:
A layer of shattered or "brecciated" rock under the floor of the crater. This layer is called a "breccia lens".[25]
Shatter cones, which are chevron-shaped impressions in rocks.[26] Such cones are formed most easily in fine-grained rocks.
High-temperature rock types, including laminated and welded blocks of sand,spherulites andtektites, or glassy spatters of molten rock. The impact origin of tektites has been questioned by some researchers; they have observed some volcanic features in tektites not found in impactites. Tektites are also drier (contain less water) than typical impactites. While rocks melted by the impact resemble volcanic rocks, they incorporate unmelted fragments of bedrock, form unusually large and unbroken fields, and have a much more mixed chemical composition than volcanic materials spewed up from within the Earth. They also may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt. Note: scientific literature has reported that some "shock" features, such as small shatter cones, which are often associated only with impact events, have been found also in terrestrial volcanic ejecta.[27]
Microscopic pressure deformations of minerals.[28] These include fracture patterns in crystals of quartz and feldspar, and formation of high-pressure materials such as diamond, derived from graphite and other carbon compounds, orstishovite andcoesite, varieties ofshocked quartz.
Buried craters, such as theDecorah crater, can be identified through drill coring, aerial electromagnetic resistivity imaging, and airborne gravity gradiometry.[29]
On Earth, impact craters have resulted in useful minerals. Some of the ores produced from impact related effects on Earth include ores ofiron,uranium,gold,copper, andnickel. It is estimated that the value of materials mined from impact structures is five billion dollars/year just for North America.[30] 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 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. TheVredeford Dome in the center of theWitwatersrand Basin 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).[31][32][33][34] The asteroid that struck the region was 9.7 km (6 mi) wide. TheSudbury Basin was caused by an impacting body over 9.7 km (6 mi) in diameter.[35][36] This basin is famous for its deposits ofnickel,copper, andplatinum group elements. An impact was involved in making theCarswell structure inSaskatchewan, Canada; it containsuranium deposits.[37][38][39]Hydrocarbons are common around impact structures. Fifty percent of impact structures in North America in hydrocarbon-bearingsedimentary basins contain oil/gas fields.[40][30] Also the Siljan impact crater in Sweden has hydrocarbon occurrences.[41]
On Earth, the recognition of impact craters is a branch of geology, and is related toplanetary geology in the study of other worlds. Out of many proposed craters, relatively few are confirmed. The following twenty are a sample of articles of confirmed and well-documented impact sites.
There are approximately twelve more impact craters/basins larger than 300 km on the Moon, five on Mercury, and four on Mars.[43] Large basins, some unnamed but mostly smaller than 300 km, can also be found on Saturn's moons Dione, Rhea and Iapetus.
^Timmer, John (6 February 2014)."Spectacular new Martian impact crater spotted from orbit".Ars Technica. Archived fromthe original on 5 May 2022. Retrieved26 September 2022.The time window on the impact, between July 2010 and May 2012, simply represents the time between two different Context Camera photos of the same location
^Carr, M.H. (2006)The surface of Mars; Cambridge University Press: Cambridge, UK, p. 23.
^Grieve R.A.; Shoemaker, E.M. (1994). The Record of Past Impacts on Earth inHazards due to Comets and Asteroids, T. Gehrels, Ed.; University of Arizona Press, Tucson, AZ, pp. 417–464.
^Boon, John D.; Albritton, Claude C. Jr. (November 1936). "Meteorite craters and their possible relationship to "cryptovolcanic structures"".Field & Laboratory.5 (1):1–9.
^Grieve, R.A.F. (1990) Impact Cratering on the Earth.Scientific American, April 1990, p. 66.
^Barata, T.; Alves, E. I.; Machado, A.; Barberes, G. A. (November 2012). "Characterization of palimpsest craters on Mars".Planetary and Space Science.72 (1):62–69.Bibcode:2012P&SS...72...62B.doi:10.1016/j.pss.2012.09.015.
^Gustafsson, J., Osinski, G.R., Roberts, N.M.W. et al. Deep microbial colonization during impact-generated hydrothermal circulation at the Lappajärvi impact structure, Finland. Nat Commun 16, 8270 (2025).https://doi.org/10.1038/s41467-025-63603-y
^French, Bevan M (1998). "Chapter 5: Shock-Metamorphosed Rocks (Impactites) in Impact Structures".Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures.Lunar and Planetary Institute. pp. 61–78.OCLC40770730.
^abGrieve, R., V. Masaitis. 1994. The Economic Potential of Terrestrial Impact Craters. International Geology Review: 36, 105–151.
^Daly, R. 1947. The Vredefort ring structure of South Africa. Journal of Geology 55: 125145
^Hargraves, R. 1961. Shatter cones in the rocks of the Vredefort Ring. Transactions of the Geological Society of South Africa 64: 147–154
^Leroux H., Reimold W., Doukhan, J. 1994. A TEM investigation of shock metamorphism in quartz from the Vredefort Dome, South Africa. Tectonophysics 230: 223–230
^Martini, J. 1978. Coesite and stishovite in the Vredefort Dome, South Africa. Nature 272: 715–717
^Grieve, R., Stöffler D, A. Deutsch. 1991. The Sudbury Structure: controversial or misunderstood. Journal of Geophysical Research 96: 22 753–22 764
^French, B. 1970. Possible Relations Between Meteorite Impact and Igneous Petrogenesis As Indicated by the Sudbury Structure, Ontario, Canada. Bull. Volcan. 34, 466–517.
^Harper, C. 1983. The Geology and Uranium Deposits of the Central Part of the Carswell Structure, Northern Saskatchewan, Canada. Unpublished PhD Thesis, Colorado School of Mines, Golden, CO, USA, 337 pp
^Lainé, R., D. Alonso, M. Svab (eds). 1985. The Carswell Structure Uranium Deposits. Geological Association of Canada, Special Paper 29: 230 pp
^Grieve, R., V. Masaitis. 1994. The economic potential of terrestrial impact craters. International Geology Review 36: 105–151
^Drake, H., Roberts, N.M.W., Heim, C. et al. Timing and origin of natural gas accumulation in the Siljan impact structure, Sweden. Nat Commun 10, 4736 (2019).https://doi.org/10.1038/s41467-019-12728-y
Wood, Charles A.; Andersson, Leif (1978).New Morphometric Data for Fresh Lunar Craters. 9th Lunar and Planetary Science Conference. 13–17 March 1978. Houston, Texas.Bibcode:1978LPSC....9.3669W.
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