About How Large (On Average) Is An Impact Crater Compared To The Size Of The Impactor?

Circular low on a solid astronomical body formed by a hypervelocity impact of a smaller object


Impact craters in the Solar Organisation: Top-left: 500-kilometre-wide (310 mi) crater Engelier on Saturn'due south moon Iapetus Top-right: Recently formed (between July 2010 and May 2012) impact crater on Mars showing a pristine ray やあっかかかっかksystem of ejecta^[1] Lesser-left: fifty,000-twelvemonth-one-time Meteor Crater e of Flagstaff, Arizona, U.S. on Earth Bottom-right: The prominent crater Tycho in the southern highlands of the Moon

An impact crater is a depression in the surface of a planet, moon, or other solid body in the Solar Organization or elsewhere, formed past the hypervelocity impact of a smaller body. In contrast to volcanic craters, which upshot from explosion or internal collapse,^[2] impact craters typically accept raised rims and floors that are lower in elevation than the surrounding terrain.^[3] Lunar touch craters range from microscopic craters on lunar rocks returned past the Apollo Plan^[4] and modest, simple, bowl-shaped depressions in the lunar regolith to large, complex, multi-ringed impact basins. Shooting star Crater is a well-known instance of a small impact crater on Earth.

Impact craters are 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 agile surface geological processes, such as Earth, Venus, Europa, Io and Titan, visible impact craters are less common because they go eroded, cached or transformed by tectonics over time. Where such processes accept destroyed most of the original crater topography, the terms impact structure or astrobleme are more ordinarily used. In early literature, before the significance of impact cratering was widely recognised, the terms cryptoexplosion or cryptovolcanic structure were oft used to describe what are now recognised as impact-related features on Earth.^[5]

The cratering records of very quondam surfaces, such as Mercury, the Moon, and the southern highlands of Mars, record a menstruum of intense early battery in the inner Solar System around 3.9 billion years ago. The rate of crater production on World has since been considerably lower, just it is appreciable still; Globe experiences from one to three impacts big enough to produce a xx-kilometre-diameter (12 mi) crater about one time every million years on average.^[6] ^[7] This indicates that there should exist far more than relatively immature craters on the planet than have been discovered then far. The cratering rate in the inner solar system fluctuates as a issue of collisions in the asteroid belt that create a family unit of fragments that are often sent cascading into the inner solar system.^[eight] Formed in a standoff 80 one thousand thousand years ago, the Baptistina family of asteroids is thought to take caused a large spike in the bear on rate. Note that the rate of bear on cratering in the outer Solar Organisation could exist unlike from the inner Solar Organisation.^[9]

Although World's active surface processes quickly destroy the touch on record, about 190 terrestrial impact craters have been identified.^[ten] These range in diameter from a few tens of meters up to about 300 km (190 mi), and they range in age from contempo times (e.g. the Sikhote-Alin craters in Russia whose creation was witnessed in 1947) to more than than two billion years, though most are less than 500 meg years erstwhile because geological processes tend to obliterate older craters. They are as well selectively plant in the stable interior regions of continents.^[eleven] Few undersea craters have been discovered because of the difficulty of surveying the sea flooring, the rapid rate of modify of the body of water bottom, and the subduction of the bounding main floor into Earth'southward interior past processes of plate tectonics.

Impact craters are not to exist dislocated with landforms that may appear like, including calderas, sinkholes, glacial cirques, ring dikes, common salt domes, and others.

History [edit]

Daniel M. Barringer, a mining engineer, was convinced already in 1903 that the crater he owned, Meteor Crater, was of cosmic origin. Yet almost geologists at the time assumed it formed as the consequence of a volcanic steam eruption.^[12] ^: 41–42

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

In the 1920s, the American geologist Walter H. Bucher studied a number of sites at present recognized equally bear upon craters in the United States. He ended they had been created past some bang-up explosive consequence, but believed that this forcefulness was probably volcanic in origin. However, in 1936, the geologists John D. Boon and Claude C. Albritton Jr. revisited Bucher's studies and ended that the craters that he studied were probably formed past impacts.^[13]

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 generally of touch origin. Around 1960, Gene Shoemaker revived the idea. Co-ordinate to David H. Levy, Cistron "saw the craters on the Moon as logical bear upon sites that were formed not gradually, in eons, but explosively, in seconds." For his Ph.D. degree at Princeton (1960), under the guidance of Harry Hammond Hess, Shoemaker studied the touch dynamics of Barringer Meteor Crater. Shoemaker noted Meteor Crater had the same form and structure as 2 explosion craters created from diminutive bomb tests at the Nevada Exam 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 affect generating extremely high temperatures and pressures. They followed this discovery with the identification of coesite inside suevite at Nördlinger Ries, proving its touch origin.^[12]

Armed with the knowledge of stupor-metamorphic features, Carlyle South. 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 testify by recognizing the rate of impact cratering on the Moon.^[14] Because the processes of erosion on the Moon are minimal, craters persist. Since the Globe could be expected to have roughly the same cratering charge per unit every bit the Moon, information technology became articulate 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 germination

Affect cratering involves high velocity collisions between solid objects, typically much greater than the speed of sound in those objects. Such hyper-velocity impacts produce concrete effects such as melting and vaporization that do non occur in familiar sub-sonic collisions. On World, ignoring the slowing effects of travel through the atmosphere, the lowest touch velocity with an object from space is equal to the gravitational escape velocity of about eleven km/south. The fastest impacts occur at about 72 km/due south^[15] in the "worst case" scenario in which an object in a retrograde near-parabolic orbit hits Earth. The median impact velocity on Earth is nigh xx km/s.^[sixteen]

However, the slowing effects of travel through the atmosphere rapidly decelerate any potential impactor, especially in the lowest 12 kilometres where xc% of the world'south atmospheric mass lies. Meteorites of up to seven,000 kg lose all their cosmic velocity due to atmospheric drag at a certain distance (retardation point), and start to accelerate once again due to Earth's gravity until the trunk reaches its last velocity of 0.09 to 0.16 km/s.^[15] The larger the meteoroid (i.due east. asteroids and comets) the more than of its initial catholic velocity information technology preserves. While an object of 9,000 kg maintains about six% of its original velocity, ane of 900,000 kg already preserves about 70%. Extremely large bodies (about 100,000 tonnes) are not slowed past the temper at all, and impact with their initial cosmic velocity if no prior disintegration occurs.^[15]

Impacts at these loftier speeds produce shock waves in solid materials, and both impactor and the material impacted are rapidly compressed to loftier 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 touch crater. Impact-crater formation is therefore more closely analogous to cratering by high explosives than by mechanical deportation. Indeed, the free energy density of some textile involved in the formation of impact craters is many times higher than that generated by loftier explosives. Since craters are caused by explosions, they are nearly ever circular – just very low-angle impacts crusade significantly elliptical craters.^[17]

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.^[18]

It is convenient to divide the touch on procedure conceptually into 3 distinct stages: (1) initial contact and compression, (2) excavation, (3) modification and collapse. In practice, at that place is overlap betwixt the three processes with, for instance, the excavation of the crater continuing in some regions while modification and plummet is already underway in others.

Contact and compression [edit]

Nested Craters on Mars, twoscore.104° N, 125.005° Due east. These nested craters are probably caused by changes in the strength of the target material. This usually happens when a weaker fabric overlies a stronger material.^[19]

In the absenteeism of atmosphere, the touch process begins when the impactor offset touches the target surface. This contact accelerates the target and decelerates the impactor. Because the impactor is moving so quickly, the rear of the object moves a significant distance during the short-but-finite time taken for the deceleration to propagate beyond the impactor. As a result, the impactor is compressed, its density rises, and the pressure within it increases dramatically. Top pressures in large impacts exceed ane TPa to attain values more usually institute deep in the interiors of planets, or generated artificially in nuclear explosions.

In concrete 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 college-density phases by shock waves; for example, the mutual mineral quartz can be transformed into the higher-pressure forms coesite and stishovite. Many other shock-related changes take identify inside 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 past impact cratering.^[17]

Every bit the daze wave decays, the shocked region decompresses towards more usual pressures and densities. The damage produced by the stupor wave raises the temperature of the fabric. In all but the smallest impacts this increase in temperature is sufficient to melt the impactor, and in larger impacts to vaporize most of information technology and to cook big volumes of the target. Besides as being heated, the target near the impact is accelerated past the shock wave, and it continues moving away from the impact behind the decaying shock wave.^[17]

Excavation [edit]

Contact, compression, decompression, and the passage of the shock wave all occur within a few tenths of a 2d 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 digging, the crater grows as the accelerated target cloth moves abroad from the point of touch on. The target's motion is initially downward and outwards, but it becomes outwards and upwards. The flow initially produces an approximately hemispherical cavity that continues to grow, eventually producing a paraboloid (basin-shaped) crater in which the centre has been pushed down, a significant book of cloth 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.^[17]

The depth of the transient cavity is typically a quarter to a third of its diameter. Ejecta thrown out of the crater exercise not include textile excavated from the full depth of the transient cavity; typically the depth of maximum excavation is only virtually a third of the total depth. Every bit a result, about 1 third of the book of the transient crater is formed by the ejection of cloth, and the remaining two thirds is formed past the displacement of material downwards, outwards and upwards, to form the elevated rim. For impacts into highly porous materials, a meaning crater volume may also be formed past the permanent compaction of the pore space. Such compaction craters may exist important on many asteroids, comets and small moons.

In big impacts, as well as cloth displaced and ejected to form the crater, significant volumes of target fabric may exist melted and vaporized together with the original impactor. Some of this touch on cook stone may be ejected, but most of it remains inside the transient crater, initially forming a layer of impact cook blanket the interior of the transient cavity. In contrast, the hot dense vaporized fabric expands rapidly out of the growing cavity, carrying some solid and molten cloth within it as information technology does so. Every bit 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 rising to many times the calibration elevation of the temper, effectively expanding into free space.

Most cloth ejected from the crater is deposited within a few crater radii, but a minor 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 shut to the centre of bear on, and the slowest cloth is ejected close to the rim at low velocities to course an overturned coherent flap of ejecta immediately outside the rim. As ejecta escapes from the growing crater, information technology forms an expanding curtain in the shape of an inverted cone. The trajectory of individual particles within the curtain is thought to exist largely ballistic.

Small volumes of un-melted and relatively united nations-shocked fabric may be spalled at very high relative velocities from the surface of the target and from the rear of the impactor. Spalling provides a potential machinery whereby fabric 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 fabric is expelled from the convergence zone with velocities that may exist several times larger than the affect velocity.

Modification and collapse [edit]

Weathering may modify the attribute of a crater drastically. This mound on Mars' north pole may exist the consequence of an bear upon crater that was cached by sediment and subsequently re-exposed by erosion.

In most circumstances, the transient cavity is non stable and collapses under gravity. In pocket-size craters, less than about 4 km bore on Earth, in that location is some express collapse of the crater rim coupled with debris sliding down the crater walls and drainage of bear upon 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.

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

To a higher place a certain threshold size, which varies with planetary gravity, the plummet and modification of the transient cavity is much more extensive, and the resulting structure is chosen 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 issue of elastic rebound, which is a process in which a material with elastic force attempts to return to its original geometry; rather the collapse is a process in which a cloth with little or no forcefulness attempts to render to a land of gravitational equilibrium.

Complex craters have uplifted centers, and they have typically broad apartment shallow crater floors, and terraced walls. At the largest sizes, 1 or more exterior or interior rings may appear, and the structure may be labeled an impact bowl rather than an touch crater. Complex-crater morphology on rocky planets appears to follow a regular sequence with increasing size: pocket-sized complex craters with a central topographic peak are chosen primal summit craters, for example Tycho; intermediate-sized craters, in which the central height is replaced by a ring of peaks, are called peak-ring craters, for case Schrödinger; and the largest craters contain multiple concentric topographic rings, and are chosen multi-ringed basins, for example Orientale. On icy (as opposed to rocky) bodies, other morphological forms announced that may take fundamental pits rather than key peaks, and at the largest sizes may comprise many concentric rings. Valhalla on Callisto is an example of this type.

Identifying impact craters [edit]

Meteor Crater in the U.S. state of Arizona, was the world's first confirmed impact crater.

Non-explosive volcanic craters tin can unremarkably be distinguished from impact craters by their irregular shape and the clan of volcanic flows and other volcanic materials. Bear upon craters produce melted rocks as well, but usually in smaller volumes with unlike characteristics.^[v]

The distinctive mark of an touch crater is the presence of stone that has undergone shock-metamorphic effects, such equally shatter cones, melted rocks, and crystal deformations. The problem is that these materials tend to be deeply cached, at least for simple craters. They tend to exist revealed in the uplifted center of a complex crater, yet.^[xx] ^[21]

Impacts produce distinctive daze-metamorphic effects that permit bear upon sites to be distinctively identified. Such shock-metamorphic furnishings can include:

A layer of shattered or "brecciated" rock nether the floor of the crater. This layer is chosen a "breccia lens".^[22]
Shatter cones, which are chevron-shaped impressions in rocks.^[23] Such cones are formed most easily in fine-grained rocks.
High-temperature stone types, including laminated and welded blocks of sand, spherulites and tektites, or burnished spatters of molten rock. The impact origin of tektites has been questioned by some researchers; they have observed some volcanic features in tektites not plant in impactites. Tektites are likewise drier (contain less water) than typical impactites. While rocks melted past the touch on resemble volcanic rocks, they comprise unmelted fragments of bedrock, grade unusually big and unbroken fields, and have a much more mixed chemic composition than volcanic materials spewed up from inside the Earth. They too 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 "stupor" features, such as small shatter cones, which are often associated only with impact events, accept been found also in terrestrial volcanic ejecta.^[24]
Microscopic pressure deformations of minerals.^[25] These include fracture patterns in crystals of quartz and feldspar, and formation of high-pressure materials such equally 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, aeriform electromagnetic resistivity imaging, and airborne gravity gradiometry.^[26]

Economic importance of impacts [edit]

On Earth impact craters take resulted in useful minerals. Some of the ores produced from bear upon related effects on Globe include ores of iron, uranium, aureate, copper, and nickel. It is estimated that the value of materials mined from impact structures is 5 billion dollars/yr just for North America.^[27] The eventual usefulness of touch on craters depends on several factors especially the nature of the materials that were impacted and when the materials were afflicted. In some cases the deposits were already in place and the bear upon brought them to the surface. These are called "progenetic economic deposits." Others were created during the actual bear on. The bully energy involved caused melting. Useful minerals formed equally a result of this energy are classified as "syngenetic deposits." The third type, called "epigenetic deposits," is acquired past 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 Bowl is the largest goldfield in the world which has supplied nearly 40% of all the gold ever mined in an impact structure (though the aureate did non come up from the bolide).^[28] ^[29] ^[30] ^[31] The asteroid that struck the region was nine.7 km (6 mi) wide. The Sudbury Basin was caused past an impacting body over 9.7 km (half dozen mi) in diameter.^[32] ^[33] This basin is famous for its deposits of nickel, copper, and Platinum Group Elements. An impact was involved in making the Carswell structure in Saskatchewan, Canada; information technology contains uranium deposits.^[34] ^[35] ^[36] Hydrocarbons are common effectually impact structures. L percent of impact structures in Due north America in hydrocarbon-bearing sedimentary basins contain oil/gas fields.^[37] ^[27]

Martian craters [edit]

Because of the many missions studying Mars since the 1960s, there is good coverage of its surface which contains large numbers of craters. Many of the craters on Mars differ from those on the Moon and other moons since Mars contains ice under the basis, especially in the higher latitudes. Some of the types of craters that have special shapes due to touch into ice-rich ground are pedestal craters, rampart craters, expanded craters, and LARLE craters.

Lists of craters [edit]

List of impact craters on Earth
List of possible impact structures on Earth
List of craters on Mercury
List of craters on the Moon
Listing of craters on Mars
Listing of craters on Venus
List of geological features on Phobos
List of craters on Europa
List of craters on Ganymede
List of craters on Callisto
List of geological features on Mimas
List of geological features on Enceladus
List of geological features on Tethys
List of geological features on Dione
List of geological features on Rhea
List of geological features on Iapetus
List of geological features on Puck
List of geological features on Miranda
List of geological features on Ariel
Listing of craters on Umbriel
List of geological features on Titania
List of geological features on Oberon
List of craters on Triton

Impact craters on Earth [edit]

On Earth, the recognition of bear on 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 post-obit 20 are a sample of manufactures of confirmed and well-documented impact sites.

Barringer Crater, a.k.a. Meteor Crater (Arizona, United States)
Chesapeake Bay bear on crater (Virginia, United states of america)
Chicxulub, Extinction Event Crater (Mexico)
Clearwater Lakes (Quebec, Canada)
Gosses Bluff crater (Northern Territory, Australia)
Haughton impact crater (Nunavut, Canada)
Kaali crater (Estonia)
Karakul crater (Tajikistan)
Lonar crater (Bharat)
Manicouagan crater (Quebec, Canada)
Manson crater (Iowa, Usa)
Mistastin crater (Labrador, Canada)
Nördlinger Ries (Germany)
Pingualuit crater (Quebec, Canada)
Popigai crater, (Siberia, Russia)
Shoemaker crater (Western Australia, Australia)
Siljan Ring (Sweden)
Sudbury Basin (Ontario, Canada)
Vredefort crater (South Africa)
Wolfe Creek Crater (Western Australia, Commonwealth of australia)

See the World Bear upon Database,^[38] a website concerned with 190 (as of July 2019^[update]) scientifically-confirmed impact craters on Earth.

[edit]

Caloris Basin (Mercury)
Hellas Basin (Mars)
Herschel crater (Mimas)
Mare Orientale (Moon)
Petrarch crater (Mercury)
Southward Pole – Aitken bowl (Moon)

Largest named craters in the Solar Organization [edit]

North Polar Basin/Borealis Bowl (disputed) – Mars – Bore: 10,600 km
Southward Pole-Aitken basin – Moon – Diameter: 2,500 km
Hellas Basin – Mars – Bore: ii,100 km
Caloris Basin – Mercury – Diameter: 1,550 km
Imbrium Basin – Moon – Diameter: ane,100 km
Isidis Planitia – Mars – Bore: ane,100 km
Mare Tranquilitatis – Moon – Diameter: 870 km
Argyre Planitia – Mars – Bore: 800 km
Rembrandt – Mercury – Diameter: 715 km
Serenitatis Basin – Moon – Diameter: 700 km
Mare Nubium – Moon – Diameter: 700 km
Beethoven – Mercury – Diameter: 625 km
Valhalla – Callisto – Diameter: 600 km, with rings to iv,000 km diameter
Hertzsprung – Moon – Diameter: 590 km
Turgis – Iapetus – Bore: 580 km
Apollo – Moon – Diameter: 540 km
Engelier – Iapetus – Diameter: 504 km
Mamaldi – Rhea – Diameter: 480 km
Huygens – Mars – Diameter: 470 km
Schiaparelli – Mars – Diameter: 470 km
Rheasilvia – iv Vesta – Diameter: 460 km
Gerin – Iapetus – Bore: 445 km
Odysseus – Tethys – Diameter: 445 km
Korolev – Moon – Bore: 430 km
Falsaron – Iapetus – Diameter: 424 km
Dostoevskij – Mercury – Diameter: 400 km
Menrva – Titan – Diameter: 392 km
Tolstoj – Mercury – Diameter: 390 km
Goethe – Mercury – Diameter: 380 km
Malprimis – Iapetus – Diameter: 377 km
Tirawa – Rhea – Diameter: 360 km
Orientale Basin – Moon – Diameter: 350 km, with rings to 930 km diameter
Evander – Dione – Bore: 350 km
Epigeus – Ganymede – Bore: 343 km
Gertrude – Titania – Diameter: 326 km
Telemus – Tethys – Diameter: 320 km
Asgard – Callisto – Bore: 300 km, with rings to one,400 km diameter
Vredefort crater – World – Diameter: 300 km
Kerwan – Ceres – Bore: 284 km
Powehiwehi – Rhea – Diameter: 271 km

There are approximately twelve more bear upon craters/basins larger than 300 km on the Moon, five on Mercury, and iv on Mars.^[39] Big basins, some unnamed but mostly smaller than 300 km, can also exist found on Saturn'southward moons Dione, Rhea and Iapetus.

References [edit]

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Bibliography [edit]

Baier, Johannes (2007). Die Auswurfprodukte des Ries-Impakts, Frg. Documenta Naturae. Vol. 162. Verlag. ISBN978-three-86544-162-1.
Bond, J. W. (Dec 1981). "The development of central peaks in lunar craters". The Moon and the Planets. 25 (4): 465–476. Bibcode:1981M&P....25..465B. doi:x.1007/BF00919080. S2CID 120197487.
Melosh, H. J. (1989). Impact Cratering: A Geologic Process. Oxford Monographs on Geology and Geophysics. Vol. 11. Oxford Academy Printing. Bibcode:1989icgp.book.....M. ISBN978-0-nineteen-510463-9.
Randall, Lisa (2015). Dark Matter and the Dinosaurs. New York: Ecco/HarperCollins Publishers. ISBN978-0-06-232847-ii.
Wood, Charles A.; Andersson, Leif (1978). New Morphometric Data for Fresh Lunar Craters. ninth Lunar and Planetary Scientific discipline Conference. thirteen–17 March 1978. Houston, Texas. Bibcode:1978LPSC....9.3669W.

External links [edit]

Media related to Touch on craters at Wikimedia Commons
The Geological Survey of Canada Crater database, 172 impact structures
Aerial Explorations of Terrestrial Meteorite Craters
Touch on Meteor Crater Viewer Google Maps Page with Locations of Meteor Craters around the world
Solarviews: Terrestrial Impact Craters
Lunar and Planetary Institute slidshow: contains pictures