Editing Impact crater (section)

==Crater formation==
[[File:Impact movie.ogg|thumb|A laboratory simulation of an impact event and crater formation]]

Impact cratering involves high velocity collisions between solid objects, typically much greater than the [[speed of sound]] in those objects. Such hyper-velocity impacts produce physical effects such as [[melting]] and [[Evaporation|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&nbsp;km/s. The fastest impacts occur at about 72&nbsp;km/s<ref name="ams-fireball-faq" /> in 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&nbsp;km/s.<ref>{{cite book |last1=Kenkmann |first1=Thomas |last2=Hörz |first2=Friedrich |last3=Deutsch |first3=Alexander |title=Large Meteorite Impacts III |issue=384 |publisher=Geological Society of America |page=34 |isbn=978-0-8137-2384-6 |url=https://books.google.com/books?id=QMwt9iaYA9gC&pg=PA34|date=2005-01-01 }}</ref>

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&nbsp;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&nbsp;km/s.<ref name="ams-fireball-faq" /> The larger the meteoroid (i.e. asteroids and comets) the more of its initial cosmic velocity it preserves. While an object of 9,000&nbsp;kg maintains about 6% of its original velocity, one of 900,000&nbsp;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.<ref name="ams-fireball-faq">{{cite web |url=http://www.amsmeteors.org/fireballs/faqf/#12 |title=How fast are meteorites traveling when they reach the ground |work=American Meteor Society |access-date=1 September 2015}}</ref>

Impacts at these high speeds produce [[shock wave]]s in solid materials, and both impactor and the material impacted are rapidly [[compression (physical)|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 the impact crater. Impact-crater formation is therefore more closely analogous to cratering by [[explosive material|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 [[explosion]]s, they are nearly always circular – only very low-angle impacts cause significantly elliptical craters.<ref name="Melosh89">Melosh, H.J., 1989, Impact cratering: A geologic process: New York, Oxford University Press, 245 p.</ref>

This describes impacts on solid surfaces. Impacts on porous surfaces, such as that of [[Hyperion (moon)|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.<ref>[http://www.space.com/4028-key-giant-space-sponge-revealed.html 'Key to Giant Space Sponge Revealed'], ''Space.com'', 4 July 2007</ref>

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.

===Contact and compression===
[[File:Nested Craters on Mars.jpg|thumb|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.<ref>{{cite web|url=http://hirise.lpl.arizona.edu/ESP_027610_2205|title=HiRISE – Nested Craters (ESP_027610_2205)|website=HiRISE Operations Center |publisher=[[University of Arizona]]}}</ref>]]

In the absence of [[atmosphere]], the impact process begins when the impactor first touches the target surface. This contact [[acceleration|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 a result, the impactor is compressed, its density rises, and the [[pressure]] within it increases dramatically. Peak pressures in large impacts exceed 1 [[Tera-|T]] [[Pascal (unit)|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 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.<ref name="Melosh89" />

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.<ref name="Melosh89" />

===Excavation===
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.<ref name="Melosh89" />

[[File:Mimas moon.jpg|thumb|[[Herschel (Mimantean crater)|Herschel Crater]] on Saturn's moon [[Mimas (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.

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 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.

Small volumes of un-melted and relatively un-shocked material may be [[spall]]ed 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.

===Modification and collapse===
[[File:Conical mound in trough on Mars' north pole.jpg|thumb|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]].]]

In most circumstances, the transient cavity is not stable and collapses under gravity. In small craters, less than about 4&nbsp;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 collapse [[breccia]], ejecta and melt rock, and a portion of the central crater floor may sometimes be flat.

[[File:Valhalla crater on Callisto.jpg|thumb|left|Multi-ringed impact basin Valhalla on Jupiter's moon [[Callisto (moon)|Callisto]]]]

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 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 the collapse is a process in which a material with little or no strength attempts to return to a state of [[Isostasy|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 a regular sequence with increasing size: small complex craters with a central topographic peak are called central peak craters, for example [[Tycho (lunar crater)|Tycho]]; intermediate-sized craters, in which the central peak is replaced by a ring of peaks, are called [[peak-ring craters]], for example [[schrodinger (crater)|Schrödinger]]; and the largest craters contain multiple concentric topographic rings, and are called [[multi-ringed basin]]s, for example [[mare orientale|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 (crater)|Valhalla]] on Callisto is an example of this type.

=== Subsequent modification ===
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 [[Palimpsest (planetary astronomy)|palimpsests]].<ref name=Barata_et_al_2012>{{cite journal
 | title=Characterization of palimpsest craters on Mars
 | last1=Barata | first1=T. | last2=Alves | first2=E. I.
 | last3=Machado | first3=A. | last4=Barberes | first4=G. A.
 | journal=Planetary and Space Science
 | volume=72 | issue=1 | pages=62–69 | date=November 2012
 | doi=10.1016/j.pss.2012.09.015 | bibcode=2012P&SS...72...62B }}</ref>