Impact crater

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Crater Engelier on Saturn's moon Iapetus Fresh crater on Mars showing a ray system of ejecta
Impact crater Tycho on the Moon
The Barringer Crater (Meteor Crater) east of Flagstaff, Arizona
Impact craters in the Solar System:

An impact crater is an approximately circular depression in the surface of a planet, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body. In contrast to volcanic 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] Although Meteor Crater is perhaps the best-known example of a small impact crater on Earth, impact craters range from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins.


The cratering records of very old surfaces, such as Mercury, the Moon, and 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 from one to three impacts large enough to produce a 20 km diameter crater about once every million years on average.[5][6] 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 the asteroid belt that create a family of fragments that are often sent cascading into the inner solar system.[7] Formed in a collision 160 million years ago, the Baptistina family of asteroids is thought to have caused a large spike in the impact rate, perhaps causing the Chicxulub impact that may have triggered the extinction of the non-avian dinosaurs 66 million years ago.[7] Note that the rate of impact cratering in the outer Solar System could be different from the inner Solar System.[8]

Although Earth's active surface processes quickly destroy the impact record, about 190 terrestrial impact craters have been identified.[9] These range in diameter from a few tens of meters up to about 300 km, 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 the stable interior regions of continents.[10] 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.

Impact craters are not to be confused with landforms that may appear similar, including calderas, sinkholes, glacial cirques, ring dikes, salt domes, and others.



Eugene Shoemaker, pioneer impact crater researcher, here at a crystallographic microscope used to examine meteorites


The concept of impact cratering remained more or less speculative until the 1960s. At that time a number of researchers, most notably Eugene M. Shoemaker, (co-discoverer of the comet Shoemaker-Levy 9), conducted detailed studies of a number of craters and recognized clear evidence that they had been created by impacts, specifically identifying the shock-metamorphic effects uniquely associated with impact events, of which the most familiar is shocked quartz.

Armed with 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 the time, provided supportive evidence by recognizing the rate of impact cratering on the Moon.[11] Because the processes of erosion on the Moon are minimal, craters persist almost indefinitely. 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.

Crater formation[edit]

A laboratory simulation of an impact event and crater formation

Impact cratering involves high velocity collisions between solid objects, typically much greater than the velocity 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[12] in the "worst case" scenario in which an object in a retrograde near-parabolic orbit hits Earth. (Because kinetic energy scales as velocity squared, Earth's gravity only contributes 1 km/s to this figure, not 11 km/s). The median impact velocity on Earth is about 20 km/s.[13]

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 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.[12] 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.[12]


This describes impacts on solid surfaces. Impacts on porous surfaces, such as that of Hyperion, 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.[15]


Contact and compression[edit]

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.[16]


In physical terms, a shock 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 forms coesite and stishovite. 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.[14]

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.[14]


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 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.[14]

Herschel Crater on Saturn's moon Mimas

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 the pore space. Such compaction craters may be important on many asteroids, comets and small moons.


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 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.


Modification and collapse[edit]

Weathering may change the aspect of a crater drastically. This mound on Mars' north pole may be the result of an impact crater that was buried by sediment and subsequently re-exposed by erosion.


Multi-ringed impact basin Valhalla on Jupiter's moon Callisto


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 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 the largest sizes may contain many concentric rings. Valhalla on Callisto is an example of this type.

Identifying impact craters[edit]

Impact structure of craters: simple and complex craters
Wells Creek crater in Tennessee, United States: a close-up of shatter cones developed in fine grained dolomite
Decorah crater: aerial electromagnetic resistivity map (USGS)
Meteor Crater in the U.S. state of Arizona, was the world's first confirmed impact crater.
Shoemaker Crater in Western Australia was renamed in memory of Gene Shoemaker.

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.


Impacts produce distinctive shock-metamorphic effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include:

  • Shatter cones, which are chevron-shaped impressions in rocks.[18] Such cones are formed most easily in fine-grained rocks.
  • High-temperature rock types, including laminated and welded blocks of sand, spherulites and tektites, 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.[19]
  • Microscopic pressure deformations of minerals.[20] 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, or stishovite and coesite, varieties of shocked quartz.
  • Buried craters, such as the Decorah crater, can be identified through drill coring, aerial electromagnetic resistivity imaging, and airborne gravity gradiometry.[21]

Lists of craters[edit]

Impact craters on Earth[edit]

On Earth, the recognition of impact craters is a branch of geology, as opposed to astronomy on 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.

See the Earth Impact Database,[22] a website concerned with 188 (as of 2016) scientifically-confirmed impact craters on Earth.

Some extraterrestrial craters[edit]

Unnamed crater in Caloris Basin, photographed by MESSENGER, 2011

Largest named craters in the Solar System[edit]

Tirawa crater straddling the terminator on Rhea, lower right.
  1. North Polar Basin/Borealis Basin (disputed) - Mars - Diameter: 10,600 km
  2. South Pole-Aitken basin - Moon - Diameter: 2,500 km
  3. Hellas Basin - Mars - Diameter: 2,100 km
  4. Caloris Basin - Mercury - Diameter: 1,550 km
  5. Imbrium Basin - Moon - Diameter: 1,100 km
  6. Isidis Planitia - Mars - Diameter: 1,100 km
  7. Mare Tranquilitatis - Moon - Diameter: 870 km
  8. Argyre Planitia - Mars - Diameter: 800 km
  9. Rembrandt – Mercury – Diameter: 715 km
  10. Serenitatis Basin - Moon - Diameter: 700 km
  11. Mare Nubium - Moon - Diameter: 700 km
  12. Beethoven - Mercury - Diameter: 625 km
  13. Valhalla - Callisto - Diameter: 600 km, with rings to 4,000 km diameter
  14. Hertzsprung - Moon - Diameter: 590 km
  15. Turgis - Iapetus - Diameter: 580 km
  16. Apollo - Moon - Diameter: 540 km
  17. Engelier - Iapetus - Diameter: 504 km
  18. Mamaldi - Rhea - Diameter: 480 km
  19. Huygens - Mars - Diameter: 470 km
  20. Schiaparelli - Mars - Diameter: 470 km
  21. Rheasilvia - 4 Vesta - Diameter: 460 km
  22. Gerin - Iapetus - Diameter: 445 km
  23. Odysseus - Tethys - Diameter: 445 km
  24. Korolev - Moon - Diameter: 430 km
  25. Falsaron - Iapetus - Diameter: 424 km
  26. Dostoevskij - Mercury - Diameter: 400 km
  27. Menrva - Titan - Diameter: 392 km
  28. Tolstoj - Mercury - Diameter: 390 km
  29. Goethe - Mercury - Diameter: 380 km
  30. Malprimis - Iapetus - Diameter: 377 km
  31. Tirawa - Rhea - Diameter: 360 km
  32. Orientale Basin - Moon - Diameter: 350 km, with rings to 930 km diameter
  33. Evander - Dione - Diameter: 350 km
  34. Epigeus - Ganymede - Diameter: 343 km
  35. Gertrude - Titania - Diameter: 326 km
  36. Telemus - Tethys - Diameter: 320 km
  37. Asgard - Callisto - Diameter: 300 km, with rings to 1,400 km diameter
  38. Vredefort crater - Earth - Diameter: 300 km
  39. Kerwan - Ceres - Diameter: 284 km
  40. Powehiwehi - Rhea - Diameter: 271 km

There are approximately twelve more impact craters/basins larger than 300 km on the Moon, five on Mercury, and four on Mars.[23] Large basins, some unnamed but mostly smaller than 300 km, can also be found on Saturn's moons Dione, Rhea and Iapetus.

See also[edit]


  1. ^ Spectacular new Martian impact crater spotted from orbit, Ars Technica, 6 February 2014.
  2. ^ Basaltic Volcanism Study Project. (1981). Basaltic Volcanism on the Terrestrial Planets; Pergamon Press, Inc.: New York, p. 746.
  3. ^ Consolmagno, G.J.; Schaefer, M.W. (1994). Worlds Apart: A Textbook in Planetary Sciences; Prentice Hall: Englewood Cliffs, NJ, p.56.
  4. ^ French, B.M. (1998). Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures; Simthsonian Institution: Washington DC, p. 97.
  5. ^ Carr, M.H. (2006) The surface of Mars; Cambridge University Press: Cambridge, UK, p. 23.
  6. ^ Grieve R.A.; Shoemaker, E.M. (1994). The Record of Past Impacts on Earth in Hazards due to Comets and Asteroids, T. Gehrels, Ed.; University of Arizona Press, Tucson, AZ, pp. 417-464.
  7. ^ a b Bottke, WF; Vokrouhlický D Nesvorný D. (2007). "An asteroid breakup 160 Myr ago as the probable source of the K/T impactor". Nature. 449 (7158): 48–53. Bibcode:2007Natur.449...48B. doi:10.1038/nature06070. PMID 17805288. 
  8. ^ Zahnle, K.; et al. (2003). "Cratering rates in the outer Solar System". Icarus. 163: 263. Bibcode:2003Icar..163..263Z. doi:10.1016/s0019-1035(03)00048-4. 
  9. ^ Grieve, R.A.F.; Cintala, M.J.; Tagle, R. (2007). Planetary Impacts in Encyclopedia of the Solar System, 2nd ed., L-A. McFadden et al. Eds, p. 826.
  10. ^ Shoemaker, E.M.; Shoemaker, C.S. (1999). The Role of Collisions in The New Solar System, 4th ed., J.K. Beatty et al., Eds., p. 73.
  11. ^ Grieve, R.A.F. (1990) Impact Cratering on the Earth. Scientific American, April 1990, p. 66.
  12. ^ a b c "How fast are meteorites traveling when they reach the ground". American Meteor Society. Retrieved September 2015.  Check date values in: |access-date= (help)
  13. ^ Kenkmann, Thomas; Hörz, Friedrich; Deutsch, Alexander. Large Meteorite Impacts III. Geological Society of America. p. 34. ISBN 0-8137-2384-1. 
  14. ^ a b c d Melosh, H.J., 1989, Impact cratering: A geologic process: New York, Oxford University Press, 245 p.
  15. ^ 'Key to Giant Space Sponge Revealed',, 4 July 2007
  16. ^ Nested CratersESP_027610_2205 at HiRISE Operations Center, University of Arizona
  17. ^ Randall 2015, p. 157.
  18. ^ Randall 2015, pp. 154-155.
  19. ^ Randall 2015, p. 156.
  20. ^ Randall 2015, p. 155.
  21. ^ US Geological Survey. "Iowa Meteorite Crater Confirmed". Retrieved 7 March 2013. 
  22. ^ Impact Cratering on Earth
  23. ^ USGS Astrogeology: Gazetteer of Planetary Nomenclature


Further reading[edit]

External links[edit]