Saturday, September 8, 2012

All About Meteorites

Today's blog posting includes information about meteorites.

A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids. When a meteoroid enters the atmosphere, frictional, pressure, and chemical interactions with the atmospheric gasses cause the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting/falling star. The term bolide refers to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface.

More generally, a meteorite on the surface of any celestial body is a natural object that has come from elsewhere in space. Meteorites have been found on the Moon and Mars. Meteorites that are recovered after being observed as they transited the atmosphere or impacted the Earth are called falls. All other meteorites are known as finds. As of February 2010, there are approximately 1,086 witnessed falls having specimens in the world's collections. In contrast, there are over 38,660 well-documented meteorite finds.

Meteorites have traditionally been divided into three broad categories:

1. Stony meteorites are rocks, mainly composed of silicate minerals

2. Iron meteorites are largely composed of metallic iron-nickel.

3. Stony-iron meteorites contain large amounts of both metallic and rocky material.

Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy. Meteorites are always named for the places they were found. Most meteoroids disintegrate when entering Earth's atmosphere. However, an estimated 500 meteorites ranging in size from marbles to basketballs or larger do reach the surface each year, but only 5 or 6 of these are typically recovered and made known to scientists.

Few meteorites are large enough to create large impact craters. Instead, they typically arrive at the surface at their terminal velocity and, at most, create a small pit. Even so, falling meteorites have reportedly caused damage to property, and injuries to livestock and people.

Large meteoroids may strike the ground with a significant fraction of their cosmic velocity, leaving behind a hypervelocity impact crater. The kind of crater will depend on the size, composition, degree of fragmentation, and incoming angle of the impactor. The force of such collisions has the potential to cause widespread destruction. The most frequent hypervelocity cratering events on the Earth are caused by iron meteoroids, which are most easily able to transit the atmosphere intact. One of the most famous examples of craters caused by iron meteoroids is the Barringer Meteor Crater in Northern Arizona. Fifty thousand years ago, a giant fireball streaked across the North American sky. It struck the earth in what is now northern Arizona, exploding with the force of 2 ½ million tons of TNT. The crater is named for Daniel Moreau Barringer, a Philadelphia mining engineer. Barringer was one of the first people to claim that the crater was the result of an impact, contradicting the most eminent scientists of his time. Though he never found the fortune in meteoritic iron he was convinced lay beneath the floor of the crater, Barringer’s theory of the crater’s origin was eventually vindicated and accepted by the scientific community. Below is the diagram that Barringer drew of the crater in 1909, as well as an aerial shot of the crater.

Here is a picture of Daniel Barringer.

The meteorite that formed the crater was a chunk of nickel iron about 150 feet (50 meters) wide. The meteorite weighed 300,000 tons and traveled at a speed of 26,000 miles per hour (12 kilometers per second). When it struck the earth in what is now northern Arizona, it exploded with the force of about 150 times the force of the atomic bomb that destroyed Hiroshima.

Most of the meteorite was melted by the force of the impact, and spread across the landscape in a very fine, nearly atomized mist of molten metal. Millions of tons of limestone and sandstone were blasted out of the crater, covering the ground for a mile in every direction with a blanket of shattered, pulverized and partially melted rock mixed with fragments of meteoritic iron.

When the dust settled, what remained was a crater three-quarters of a mile (about 1 kilometer) wide and 750 feet deep. The impact occurred during the last ice age, a time when the Arizona landscape was cooler and wetter than it is today. The plain around it was covered with a forest, where mammoths, mastodons and giant ground sloths grazed. The force of the impact would have leveled the forest for miles around, hurling the mammoths across the plain and killing or severely injuring any animals unfortunate enough to be nearby.Over time, the landscape recovered.

A lake formed in the bottom of the crater, and sediments accumulated until the bowl was only 550 feet deep. Then, with the ending of the ice age, the climate changed and dried. The desert that we see today has helped to preserve the crater, by limiting the erosion that might otherwise have blurred or erased the traces of the ancient impact.

One interesting fact about Barringer's hypothesis about the source of the crater is that it was not proven until 1960 when Eugene Shoemaker, Edward Chao and David Milton were responsible for the discovery of a new mineral at the Barringer crater. This mineral, a form of silica called “coesite”, had first been created in a laboratory in 1953 by chemist Loring Coes. Its formation requires extremely high pressures and temperatures, greater than any occurring naturally on earth. Coesite and a similar material called “stishovite” have since been identified at numerous other suspected impact sites, and are now accepted as indicators of impact origin.

Finally, in 1963, Eugene Shoemaker published his landmark paper analyzing the similarities between the Barringer crater and craters created by nuclear test explosions in Nevada. Carefully mapping the sequence of layers of the underlying rock, and the layers of the ejecta blanket, where those rocks were deposited in reverse order, he demonstrated that the nuclear craters and the Barringer crater were structurally similar in nearly all respects. His paper provided the clinching arguments in favor of an impact, finally convincing the last doubters.

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