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The surface of every planet and moon in our Solar System is scarred with craters formed by the collision of asteroids and comets. These circular structures lay testiment to the bombardment of planets over the four and a half billion year history of our Solar System and remind us that collisions can still occur today.
Crater Formation
Impact craters form when an asteroid or comet collides with the surface of a planet, moon or even another asteroid at speeds of many kilometres per second. The energy of the impact is equivalent to the kinetic energy of the object due to its motion, which is equal to half its mass times the square of its velocity, and is enormous. It enables rocks in the target to flow out of the crater and form a depression. The larger the kinetic energy of the impactor the larger the resulting crater. The size of the crater is thus more sensitive to the speed of the impactor than its mass.
Rocks ejected from a crater are thrown out in a curtain of debris that falls back to blanket an area with a radius of around ten times the diameter of the crater. In large impacts some of the blocks of debris can be larger than a car and produce secondary craters where they fall. The last rocks to be ejected are often thrown out of a crater more or less intact and land upside down to form a raised crater rim.
Some of the energy generated in an impact is converted to heat which causes melting and can even turn rocks into gas by vaporisation at a temperature of several thousand degrees. Most of the colliding object is usually vaporised along with a proportion of the target rocks and the resulting gases rise above the crater in a plume. When these gases start to cool they condense into droplets of magma which rain down around the crater.
The initial formation of the crater generally happens in less than a few seconds.
Volcanoes versus impacts
Before the late 1960’s many impact craters were thought to have been formed by volcanic eruptions, however, impact and volcanic craters are now known to be very different. Volcanic craters are usually smaller than impact craters and are often irregular in shape. Impact craters are almost always circular in outline and can be hundreds of kilometres across.
Types of Crater
Craters change nature with size. Small craters (> 2-4 km diameter on Earth) are simple bowl-shaped depressions surrounded by raised rims and ejecta deposits. Large craters are known as complex craters and have flat, smooth floors and are often surrounded by terraces, where blocks of rocks have slide back into the crater on curved fractures. Complex craters form due to the fluid-like behaviour of rocks during impacts and display features similar to a droplet of water falling onto a smooth pond. Like the surface of the pond the target rocks rebound upwards and then fall back. With increasing size this rebound produces central-peak craters, ring-peak craters and finally multi-ring basins.
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