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Crater counting is a method for estimating the age of aplanet or moon's surface based upon the assumptions that when a piece of planetary surface is new, then it has noimpact craters; impact craters accumulate after that at a rate that is assumed known. Consequently, counting how many craters of various sizes there are in a given area allows determining how long they have accumulated and, consequently, how long ago the surface has formed. The method has been calibrated using the ages obtained byradiometric dating of samples returned from theMoon by theLuna andApollo missions.[1] It has been used to estimate the age of areas onMars and other planets that were covered by lava flows, on the Moon of areas covered by giantmares, and how long ago areas on theicy moons of Jupiter andSaturn flooded with new ice.
The crater counting method requires the presence of independent craters. Independent craters represent the primary impact point on a planets surface, while secondary craters represent the second impact on the surface of a planet.[2] Secondary craters ('secondaries') are craters formed by material excavated by a primary impact that falls back to the surface seconds or minutes later.[2] A way to distinguish primary and secondary craters is to consider their geometric arrangement; for example, large craters often haverays of secondary craters.[2] Secondaries can sometimes also be recognized by their particular shape different from primary craters; this is due to the fact that the excavated material is slower and impacts at a lower angle than asteroids that arrive from space to create the primary crater.[2]
The accuracy of age estimates of geologically young surfaces based on crater counting onMars has been questioned due to formation of large amounts ofsecondary craters. In one case, the impact that createdZunil crater produced about a hundred secondary craters, some more than 1000 km from the primary impact.[3] If similar impacts also produced comparable amounts of secondaries, it would mean a particular crater-free area of Mars had not been "splattered by a large, infrequent primary crater", as opposed to suffering relatively few small primary impacts since its formation.[4] High speedejecta generated from independent craters generates secondary craters which can resemble independent craters as well, contaminating counting processes as the secondary craters appear more circular and less cluttered than typical secondaries.[5] Secondaries will inevitably contaminate independent crater counts leading to some who may question its effectiveness (see criticism heading for further information).
The earliest scientist to study and produce a paper using crater counting as an age indicator wasErnst Öpik, an Estonian astronomer and astrophysicist.[6] Ernst Öpik utilized the crater counting method to date the Moon'sMare Imbrium to be approximately 4.5 billion years of age, estimating the maria to be about 1000 years younger than thecontintentes.[6] The method was also utilized byGene Shoemaker and Robert Baldwin, and further improved by Bill Hartman.[7] Hartman's work includes dating theLunar Mare to be approximately 3.6 billion years old, an age that was in accordance with isotopic samples.[7] In later years,Gerhard Neukum advanced the method by proposing a stable impacting population over the period of 4 billion years due to unchanged shape of crater size-frequency distribution.[8] More recent work has seen the transition from Lunar surface to Martian surface cratering, including work done by Neukum and Hartman.[9] Within the past ten years, the Buffered Crater Counting approach has been used to date geologic formations.[10] The calibration provided by the Lunar samples brought back during the six Apollo missions between 1969 and 1972 has remained invaluable to further refining and advancing the crater counting method to this day, but new work is being done to computerize the crater counting technique using Crater Detection Algorithms which uses high resolution imagery to detect small impact craters.[11][12]
While crater counting has been refined in past years to be an accurate method of determining surface age of a planet despite a lack of isotopic samples, there is dissension in the planetary scientific community concerning the acceptance of crater counting as a precise and accurate form ofgeochronology. Shallow surface mechanism such asaeolian deposition, erosion, and diffusional creep can also alter crater morphology, making the surface appear younger than it truly is.[13] Planets heavily covered by water or dense atmosphere would also impede the accuracy of this method, since observational efforts would be hampered. Planets with dense atmospheres will also cause incoming meteors to burn up due to friction before impacting the surface of the planet.[14] The Earth is bombarded with approximately 100 tons of space dust, sand, and pebble particles every day; however, most of this material burns up in the atmosphere before ever reaching the surface of the planet.[15] This is common for space material that is smaller than 25 meters, burning up due to friction in the atmosphere.[15] While resulting observational values dating the Lunar surface from Hartman and Öpik do illustrate ages that correspond to isotopic data, they are potentially hampered by observational bias and human error. New advances continue to improve upon the original method.
Below is a list of studies which utilize or concern crater counting:
