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Typical characteristics of a lunar impact

Impact Craters Teacher Page Exploring the Moon — A Teacher’s Guide with Activities, NASA EG-1997-10-116-HQ To determine the factors affecting the appearance of impact craters and ejecta. Background [also see “Teacher’s Guide” Pages 1, 2, photo on 8, 12, and photo on 13] The circular features so obvious on the Moon’s surface are impact craters formed when impactors smashed into the surface. The explosion and excavation of materials at the impacted site created piles of rock (called ejecta) around the circular hole as well as bright streaks of target material (called rays) thrown for great distances. Two basic methods forming craters in nature are: 1) impact of a projectile on the surface and 2) collapse of the top of a volcano creating a crater termed caldera. By studying all types of craters on Earth and by creating impact craters in experimental laboratories geologists concluded that the Moon’s craters are impact in origin. The factors affecting the appearance of impact craters and ejecta are the size and velocity of the impactor, and the geology of the target surface. By recording the number, size, and extent of erosion of craters, lunar geologists can determine the ages of different surface units on the Moon and can piece together the geologic history. This technique works because older surfaces are exposed to impacting meteorites for a longer period of time than are younger surfaces. Impact craters are not unique to the Moon. They are found on all the terrestrial planets and on many moons of the outer planets. On Earth, impact craters are not as easily recognized because of weathering and erosion. Famous impact craters on Earth are Meteor Crater in Arizona, U.S.A.; Manicouagan in Quebec, Canada; Sudbury in Ontario, Canada; Ries Crater in Germany, and Chicxulub on the Yucatan coast in Mexico. Chicxulub is considered by most scientists as the source crater of the catastrophe that led to the extinction of the dinosaurs at the end of the Cretaceous period. An interesting fact about the Chicxulub crater is that you cannot see it. Its circular structure is nearly a kilometer below the surface and was originally identified from magnetic and gravity data. Purpose Impact Craters 62 Teacher Page Exploring the Moon — A Teacher’s Guide with Activities, NASA EG-1997-10-116-HQ floor raised rim Aristarchus Typical characteristics of a lunar impact crater are labeled on this photograph of Aristarchus, 42 km in diameter, located West of Mare Imbrium. raised rim – rock thrown out of the crater and deposited as a ring-shaped pile of debris at the crater’s edge during the explosion and excavcation of an impact event. floor – bowl shaped or flat, characteristically below surrounding ground level unless filled in with lava. central – mountains formed because of the huge increase and rapid decrease in pressure during the uplifts impact event. They occur only in the center of craters that are larger than 40 km diameter. See Tycho crater for another example. walls – characteristically steep and may have giant stairs called terraces. ejecta – blanket of material surrounding the crater that was excavated during the impact event. Ejecta becomes thinner away from the crater. rays – bright streaks starting from a crater and extending away for great distances. See Copernicus crater for another example.