Andhra Pradesh PSCComets As Parent Body of Meterorites Comets, meteorites and impact caartering are important topics for

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Comets as Parent Bodies of Meteorites

A Comet is a body that orbits around the Sun with an eccentric orbit. These orbits are not circular like those of the planets and are not necessarily within the same plane as the planets. Most comets have elliptical orbits which send them to the far outer reaches of the solar system and back toward a closer approach to the sun. As a comet approaches the sun, solar radiation generates gases from evaporation of the comets surface. These gases are pushed away from the comet and glow in the sun light, thus giving the comet its tail. While the outer surface of comets appear to composed of icy material like water and carbon dioxide solids, they likely contain a store rocky nucleus. Because of their eccentric orbits, many comets eventually cross the orbit of the Earth. Many meteor showers may be caused by the Earth crossing an orbit of a fragmented comet. The collision of a cometary fragment is thought to have occurred in the Tunguska region of Siberia in 1908. The blast was about the size of a 15 megaton nuclear bomb. It knocked down trees in an area about 850 square miles, but did not leave a crater. The consensus among scientists is that a cometary fragment about 20 to 60 meters in diameter exploded in the Earths atmosphere just above the Earths surface. Only small amounts of material similar to meteorites were found embedded in trees at the site. Other Sources: While the asteroid belt seems like the most likely source of meteorites. Some meteorites appear to have come from other places. Some meteorites have chemical compositions similar to samples brought back from the moon. Others are thought to have originated on Mars. These types of meteorites could have been ejected from the Moon or Mars by collisions with other asteroids or from Mars by volcanic eruptions.

Impact Events

When a large object impacts the surface of the Earth. the rock at the site of the impact. is deformed and some of it is ejected into the atmosphere re to eventually fall back to the surface. This results in a bowl shaped depression with a raised rim, called an Impact Crater. The size of the impact crater de pends on such factors as the size and velocity of the impacting object and the angle at which it strikes the surface of the Earth. Meteorite Flux and Size Meteorite flux is the total mass of extraterrestrial objects that strike the Earth. This is currently about 107 to 109 kg/year. Much of this material is dust-sized objects called micrometeorites. The frequency at which meteorites of different sizes strike the Earth depends on the size of the objects. Meteorites of larger sizes s trike the Earth less frequently. If they have a size greater than about 2 or 3 cm, they only partially melt or vaporize on passage through the atmosphere, and thus strike the surface of the Earth. Objects with sizes greater than 1 km are considered to produce effects that would be catastrophic, because an impact of such an object would produce global effects. Such meteorites strike the Earth relatively infrequently 1 km sized object strikes the Earth about once every million years, and 10 km sized objects about once every 100 million years Velocity and Energy Release of Incoming Objects: The velocities at which small meteorites have impacted the Earth range from 4 to 40 km/sec. Larger objects would not be slowed down much by the friction associated with passage through the atmosphere, and thus would impact the Earth with high velocity. Calculations show that a meteorite with a diameter of 30 m, weighing about 300,000 tons, traveling at a velocity of 15 km/sec (33,500 miles/hour) would re lease energy equivalent to about 20 million tons of TNT. Such a meteorite struck at Meteor Crater, Arizona (the Barringer Crater) about 49,000 years ago leaving a crater 1200 m In diameter and 200 m deep.

Cratered Surfaces: Looking at the surface of the Moon, one is impressed by the fact that most of the surface features of the moon are shaped by impact craters. The Earth is subject to more than twice the amount of impacting events than the moon because of its larger size and higher gravitational attraction. Yet, the Earth does not show a cratered surface like the moon. The reason for this is that the surface of the Earth is continually changing due to processes like erosion, weathering, tectonics, sedimentation. and volcanism. Thus, the only craters that are evident on the Earth are either very young, very large, or occurred on stable continental areas that have not been subject to intense surface modification processes. Currently, approximately 200 terrestrial impact structures have been identified, with the discovery rate of new structures in the range of 3-5 per year.

The Mechanics of lmpact Cratering: When a large extraterrestrial object enters the Earths atmosphere the initial impact with the atmosphere will compress the atmosphere, sending as hock wave through the air. Frictional heating will cause the object to heat and glow. Melting and even vaporization of the outer parts of the object will begin, but if the object is large enough, solid material will remain when it impacts the surface of the Earth.

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