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A crater is a large, bowl-shaped hole found in solid, rocky surfaces. Terrestrial planets, moons, and asteroids all contain craters. Some craters form when meteoroids, asteroids, or comets smash into the surface of a planet or moon. These craters – called “impact craters” – are usually circular. They range in size from tiny pits to huge basins hundreds of meters across. Impact craters are the most common geological feature in the solar system. The Impact The appearance of an impact crater depends on many things. For example, the size and speed of the object causing the crater affects its width or depth. With smaller or slower impacts, the surface material is simply thrown out, like sand that is thrown when a rock hits the surface of a beach. However, when the impacting object is large and traveling at higher speeds, it hits the surface with enormous force. The extreme temperatures and pressures from the collision cause the object to melt and mix with the surrounding rock. Parts of a Crater After impact, a crater forms with a high rim and central peak (see the illustration below). Landslides may create terraces around the rim. The floor of the crater is often below the level of the surrounding terrain. Ejected materials, Craters: Links to the Past Craters can tell us about a planet or moon’s history. The more craters on a planet or moon’s surface, the older that part of the planet’s surface is. During the early formation of our solar system, many meteoroids bombarded the planets. The craters they caused can still be seen on the Moon and Mercury. This is because geological processes, such as wind and water erosion, stopped millions of years ago on these bodies. The craters remain much as they were at the time of their creation. Left – craters on Mercury. Above– craters on the far side of the Moon. Gaseous planets have little or no evidence of impact craters, even though meteoroids strike gaseous planets as often as they strike rocky planets. Craters leave only a temporary record in the gaseous atmosphere. Craters on Earth Earth also was heavily cratered during its formation, and it still receives impacts today. Many craters on Earth have been eroded by wind and water and destroyed by earthquakes and volcanism. About 120 impact craters have been identified on Earth. Some of Earth’s craters are relatively young; for example, Barringer (Meteor) Crater in Arizona (below) is only around 50,000 years old. Manicouagan Crater in Quebec, Canada, is much older; it was created about 214 million years ago. At 70 km in such as dust, sand, and – if the temperature is high enough diameter, Manicouagan Crater is one of the largest impact – liquid rock, fall back around the crater to form an area of craters debris that looks like spokes on a wheel. These spokes, on the called rays, radiate outward in all directions. Some rays can surface go for hundreds of kilometers beyond the point of impact. of Earth. Many smaller, secondary craters also form around the main crater. Craters on moons and planets can be seen most easily when long shadows are cast on their surfaces. Craters along the border between the Moon’s dark and light side are most visible from Earth when the Moon is in is quarter phase.