Impact crater

An impact crater is a bowl-shaped dent on the surface of a solid space object, created when a smaller object smashes into it at extremely high speed. Unlike volcanic craters, which form from explosions or internal collapse, impact craters usually have raised rims and floors that sit lower than the surrounding ground. Most are round, but they can be oval or oddly shaped if landslides or other events distort them. Their sizes vary wildly—from tiny pits on Moon rocks brought back by Apollo astronauts to simple bowl-like holes and enormous, complex basins with multiple rings. A famous small example on Earth is Meteor Crater. These craters dominate the landscapes of many solid bodies in the Solar System, such as the Moon, Mercury, Callisto, Ganymede, and most small moons and asteroids. On planets and moons with more active surfaces—like Earth, Venus, Europa, Io, Titan, and Triton—visible craters are rarer because erosion, burial, or tectonic and volcanic activity wears them down over time. When a crater’s original shape is mostly destroyed, scientists call it an impact structure or astrobleme. In older writings, before people understood impact cratering, such features were often labeled cryptoexplosion or cryptovolcanic structures. Very old surfaces—like those on Mercury, the Moon, and Mars’s southern highlands—show signs of a heavy bombardment period about 3.9 billion years ago. Since then, Earth’s cratering rate has dropped, but it’s still noticeable. On average, Earth gets one to three impacts every million years that are big enough to leave a crater 20 kilometers across. That suggests many more young craters should exist than have been found. The inner Solar System’s cratering rate changes when collisions in the asteroid belt create families of fragments that sometimes rain inward. For instance, the Baptistina family of asteroids, formed in a collision 80 million years ago, is thought to have caused a big spike in impacts. The outer Solar System might have a different cratering rate. Earth’s active surface quickly erases most impact evidence, but about 190 terrestrial impact craters have been identified. They range from a few dozen meters to about 300 kilometers wide, and from very recent (like the Sikhote-Alin craters in Russia, seen forming in 1947) to over two billion years old, though most are younger than 500 million years because older ones get wiped out. They tend to be found in stable continental interiors. Few undersea craters have been spotted because the seafloor is hard to survey, changes rapidly, and gets swallowed by plate tectonics. **History**

In 1903, mining engineer Daniel M. Barringer was sure that the crater he owned, Meteor Crater, came from space. Most geologists back then thought it was a volcanic steam blast. In the 1920s, American geologist Walter H. Bucher studied several sites now known as impact craters in the United States. He decided they were caused by a huge explosion, but figured the force was volcanic. Then in 1936, geologists John D. Boon and Claude C. Albritton Jr. looked at Bucher’s work again and concluded the craters were probably from impacts. Grove Karl Gilbert suggested in 1893 that the Moon’s craters came from big asteroid hits. Ralph Baldwin wrote in 1949 that most lunar craters were impact-made. Around 1960, Gene Shoemaker revived that idea. David H. Levy noted that Shoemaker saw the Moon’s craters as logical impact sites formed explosively in seconds, not slowly over eons. For his PhD at Princeton in 1960, guided by Harry Hammond Hess, Shoemaker studied Meteor Crater’s impact dynamics. He saw that it matched two atomic bomb test craters at the Nevada Test Site—Jangle U in 1951 and Teapot Ess in 1955. In 1960, Shoemaker and Edward C. T. Chao found coesite (a form of silicon dioxide) at Meteor Crater, proving the crater came from an impact with extreme heat and pressure. They then found coesite in suevite at Nördlinger Ries, confirming its impact origin too. Armed with knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Astrophysical Observatory in Canada, and Wolf von Engelhardt at the University of Tübingen in Germany, began a systematic search for impact craters. By 1970, they had tentatively identified over 50. Their work was controversial, but the ongoing Apollo Moon landings provided support by showing the Moon’s cratering rate. Since erosion is minimal on the Moon, its craters last. Because Earth likely gets hit at about the same rate, it became clear that our planet had suffered far more impacts than the visible craters suggested.