Crater

INTRODUCTION
Crater, bowl-shaped pit or depression on the surface of a planet, a moon, or another body in space. Natural craters are formed in two basic ways: by impacts from meteorites, asteroids, or comets; or by volcanic activity that causes an explosion or a collapse. Impact craters are of special interest to geologists, astronomers, and planetary scientists, who can use such craters to study the formation, composition, and history of planets and other bodies. Round pits and depressions that result from human-caused explosions such as nuclear weapons tests are also called craters.

IMPACT CRATERS
Impact craters are the result of the same process that built the solar system out of a rotating cloud of dust and gas about 4.6 billion years ago. The planets, moons, and other bodies orbiting the Sun formed out of material drawn together by gravitation. Larger and larger bodies grew through impacts and collisions, a process called accretion. The largest bodies became planets or moons. Meteoroids, asteroids, and comets represent material left over from this early planet-forming period of the solar system. These objects continue to strike the surfaces of planets and moons, pulled in by gravitation.
When a large object strikes the surface of a planet or a moon, the kinetic energy of the impact causes the impacting object to explode, producing a fireball of material heated to thousands of degrees. The force of the explosion can be equivalent to thousands of nuclear weapons. Some of the material in the object and on the surface can vaporize as hot gas or melt like glass. Dust, fragments, and debris are thrown out of the impact crater. Earthquake-like seismic shock waves are generated in the planet or moon.

Impact Craters in the Solar System
Space probes have found impact craters throughout the solar system on planets, moons, asteroids, and comets. The rocky planets Mercury, Venus, and Mars, as well as most moons, are partly or completely covered with impact craters.
The basic structure of impact craters can be seen clearly on the Moon. The Moon lacks an atmosphere and has been geologically inactive for billions of years. Its surface retains a long record of impacts from objects ranging in size from tiny micrometeorites to large asteroids and comets.
A typical crater has a round shape with a raised rim. The interior of a small crater may be a bowl-shaped depression. Larger craters may have a flat inner surface of melted material with a raised peak in the center, the result of material that rebounded after the impact. Material thrown out of the crater by the impact may lie in a rugged ring or in a raylike pattern around the crater. Small secondary craters may have formed from falling debris. Later events such as lava flows may have flooded or partly filled the oldest craters.

Impact Craters on Earth
Scientists have identified about 200 impact craters on Earth, although asteroids and comets must have struck our planet throughout its history. On Earth, active geology from plate tectonics and erosion caused by weather and other forces have removed, buried, or worn away most impact craters. Earth’s atmosphere also causes most smaller space objects to completely incinerate as meteors or to explode in midair before they reach the ground (see Tunguska Event).

Meteor Crater
The best known example of an impact crater on Earth is Meteor Crater in Winslow, Arizona. Meteor Crater is about 1.2 km (0.75 mi) in diameter and about 180 m (600 ft) deep. It was the first crater on Earth to be scientifically identified as an impact structure rather than as a volcanic feature. The walls of Meteor Crater provide an important clue to its origin: Its walls consist of sandstone and limestone, both of which are sedimentary rather than volcanic rocks. The small iron asteroid that produced Meteor Crater was about 50 m (150 ft) in diameter but was almost completely destroyed in the explosion. A number of small meteorite fragments, the only surviving remnants of the impacting body, have been found in the vicinity. Meteor Crater is only about 50,000 years old and is preserved in an arid desert.

Identifying Impact Craters
Remains of older and larger impact craters have been found around the world. Some craters have nearly been eroded away, and only circular scars, called astroblemes, remain. A number of these craters were first identified based on their round shape as seen from the air or from space, or from sonar studies of the sea bed. Mapping of underground gravitational and magnetic variations can also reveal buried craters. The special geology of impact sites gives scientists additional clues. The mineral suevite is a type of breccia (a mineral made up of rock fragments cemented together) formed out of debris created in the impact explosion. Minerals found in samples of impact breccia show signs of intense shock and melting not found in volcanic breccia. There may also be high levels of minerals such as iridium that are normally very rare on Earth. Unusual glassy minerals called tektites are often associated with impact sites. The pattern of debris thrown out by the explosion can also help identify impact structures.
Chicxulub Crater Linked to Dinosaur Extinction
Chicxulub Crater on the Yucatán Peninsula in Mexico was discovered in the 1980s based on drilling samples that contained impact breccias. Later gravitational and magnetic mapping revealed that the buried crater was about 180 km (110 mi) in diameter. Dating of the rocks put the impact event at about 65 million years ago at the end of the Late Cretaceous period, linking the crater to the extinction of the dinosaurs. Scientists think an asteroid about 10 km (6 mi) wide struck the Yucatán Peninsula, throwing huge amounts of debris into the atmosphere, igniting fires worldwide, and creating giant tsunamis that swept the coastlines. For a period of time, the climate on Earth would have been drastically changed by the dust and gas in the atmosphere. Greenhouse heating, acid rain, and blocked sunlight killed many plants and animals on land and in the oceans. A layer of iridium-enriched clay was deposited around the world at the boundary between the Cretaceous period and the Paleogene period.
Most scientists now accept that the Chicxulub impact was mainly responsible for the mass extinction that killed about 50 percent of plant species and 75 percent of animal species on Earth 65 million years ago, although other events such as volcanic eruptions and changing climate conditions may also have been involved. Some researchers have attempted to link other mass extinctions in Earth’s history to extraterrestrial impacts, but the evidence is not as strong. The search for large craters that coincide with other major extinctions continues.

Studying Impact Craters
Impact craters on planets, moons, and other bodies in space are important tools scientists can use to study the history of the solar system. The manned Apollo missions to the Moon confirmed that nearly all the craters on the surface of the Moon resulted from impacts; some earlier theories had proposed volcanism as an important crater mechanism. Space probes that photographed Mercury, Mars, and the moons of the outer planets Jupiter, Saturn, Uranus, and Neptune showed that such bodies had large numbers of impact craters, as well.
Scientists now think that bodies throughout the solar system were struck by large numbers of asteroids and comets during a period known as the “Late Heavy Bombardment” from 4.1 to 3.8 billion years ago. Since that time the impact rate has been much lower and probably a regular process on average. Smaller impacts likely happen more often than larger impacts since the population of small meteoroids is much greater. Areas with only small impacts are likely younger than areas with a mix of large and small impacts. Sharply preserved impacts on top of other impacts or geological features are younger than the underlying features.
Counting large and small craters preserved on the surface of a planet or moon can allow scientists to determine how old that area of the surface must be. For example, there are very few impact craters on the tops of the giant volcanoes on Mars, indicating that the volcanoes there may have been active in the past few million years. The surface of Jupiter’s icy moon Europa has very few impact craters, indicating that some heating process is causing the icy surface to smooth out over time. Such techniques can be used to produce a history of a planet or moon. One potential complication, however, arises from the secondary craters created by large impacts. Such secondary impacts from a single event might be mistaken for many small individual impacts over time, making an area of the surface look older than it really is.
Impacts can also excavate deeper layers on the surface of a body in space, revealing its composition. Some of the rocks collected by Apollo astronauts were dug out of older layers on the Moon by impacts. Scientists used the robotic Mars exploration rovers to examine deeper layers on the surface of Mars exposed as the walls of impact craters. These older layers provided important information about the early history of water and rock-formation on Mars that the rovers could not have gained otherwise.
In some cases, space scientists have deliberately crashed objects into bodies in space to study the debris thrown up by the impact. A special impacting device on the Deep Impact comet probe was smashed into comet Tempel 1 in 2005 to excavate a crater and a cloud of debris that could be studied by the probe and by telescopes on Earth. A number of space probes have been crashed into the Moon at the end of their missions to study the debris clouds caused by the impacts.

VOLCANIC CRATERS
The term crater is often used for the bowl-like depression that surrounds a vent on a volcano. Larger types of volcanic craters can be formed either by explosion or by collapse, or by both events in sequence. The term caldera is commonly used for large volcanic craters, usually where collapse has occurred, or for circular structures containing groups of smaller volcanic craters. Crater Lake in Oregon is a caldera left when a volcano called Mt. Mazama exploded and collapsed about 7,700 years ago.
In a volcanic explosion, hot gases associated with volcanic activity build to a high pressure and may blow away the rocky material that forms the top or side of the volcano, leaving behind a crater.
Collapse craters are formed when the pressure of the molten rock, or magma, inside a volcano can no longer support the weight of the rock or lava above it. The collapse may happen after most of the magma has been drained away during an eruption of the volcano. The material that is no longer supported collapses into the space previously occupied by the magma, thus forming a crater. Well-known examples of collapse craters are found on the summit of Mauna Loa in the Hawaii Volcanoes National Park.

CRATERS CREATED BY HUMANS
Human-made explosives can also form craters. Exploding bombs from aircraft or shells from heavy artillery can produce craters. The force of the explosion may compress the soil, creating a deep bowl-like depression. Tests of nuclear devices conducted underground can produce much larger craters. If the nuclear explosion does not break the surface, a crater may form when material collapses or subsides because of a hollow chamber left below ground. If the explosion breaks the surface, the blast can throw material out of the crater.
The Sedan Crater at the Nevada Test Site northwest of Las Vegas was created in 1962 when a 100-kiloton thermonuclear device was tested underground. The explosion broke the surface and left a crater 390 m (1,280 ft) in diameter and 194 m (635 ft) deep. Researchers and astronauts used the craters at the Nevada Test Site to study the formation of impact craters and to prepare for the Apollo Moon missions.