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What are Meteorites?

Meteorites - Precious Celestial Wonders

For centuries meteorites have captured the imagination of man. In fact, throughout history "shooting stars" have been considered magical, sacred and mysterious. The Eqyptians often buried meteorites with their dead, a very sacred honor. They also forged tools from their "heavenly iron". In 1492 the residents of Alsatian (currently French) town of Ensisheim recovered a meteorite that remains enshrined in their church to this day. Another meteorite is displayed in the Suga Jinja Shinto Shrine in Nogata, Japan and has been kept as a treasure there for more than a thousand years.

Many of you have visited museums and have observed their collections of meteorites. Just looking at these peculiar visitors from outer space is enough to trigger the imagination and to conjure up questions about their origin and journey they took to reach the earth. Where did these "rocks" come from? What are they made of? What do they tell us about our universe?

Imagine holding a stone that traveled billions of miles through space. One that may have originally been part of the planet Mars! Or, part of an asteroid! Or, even part of an unknown body in another galaxy!

What is a meteorite?
What do meteorites look like?
Why are meteorites sliced and polished?
Are there different types of meteorites?
Stone Meteorites
Iron Meteorites
Stony-Iron Meteorites
Tektites
Why the Study of Meteorites is Important

What is a meteorite?

Meteorites are unique particles of rocks, nickel-iron metal and mixtures of the two that come from outer space and reach the earth's surface. They may have traveled from the distant reaches of the solar system or even beyond.

Most meteors, or "shooting stars," burn up in our atmosphere before reaching the ground. Only a few rare specimens actually land on the earth and earn the name meteorite. Furthermore, only a tiny portion of the meteorites that fall to earth is recovered. In fact, it is possible that millions of meteorites have struck the earth during its history, yet we have samples of less that 3000 falls. That's because meteorites fall unpredictably, and their fall is seldom physically witnessed. In addition, most fall into the oceans or on to land that is densely covered by vegetation, which makes recovery nearly impossible. The few specimens that are finally recovered can really be considered the "rarest material on earth."

Like rare gems, meteorites are expensive and difficult to discover. Some are found by accident. Others are seen as they make their descent to earth. Their unique beauty is unmatched by any terrestrial material. In fact, some substances found in the chemical composition of meteorites are not found anywhere on this planet.

Since so few meteorites actually reach the earthıs surface and earn the name "meteorite," the supply is limited. Most find their way into museums and laboratories for study and display, and very few are available to private collectors. Meteorites are named after the place or locality where they fall to earth or where they are found. For instance, they could be named for a city or local geographical feature.

Most meteorites originate in the asteroid belt located between Mars and Jupiter. In addition, scientists have determined that certain meteorites originated on the Moon and the planet Mars.

What do meteorites look like?

Meteorites are usually irregular in shape and are composed essentially of the same elements that are found on earth, although some substances found in the overall chemical compositions of meteorites are different from terrestrial rocks. Meteorites are typically covered with a thin black or dark gray fusion crust, which is formed by surface melting during entry into the atmosphere. Inside, meteorites can be very different and unique from each other.

Why are meteorites sliced and polished?

Meteorites are sliced and polished primarily to help scientists to identify minerals and textures and to help them to learn about the composition of meteorites. In addition, by slicing meteorites, more pieces are available for scientific study, for museum display and for private collectors. In this way, many people may "share" one meteorite. Slicing also allows us to enjoy the unique, gem-like beauty of meteorites. For example, only when you slice one can you reveal the unique Widmanstatten pattern of an iron meteorite, or the chondrules in a chondritic stone meteorite, or the crystals in a pallasite.

Are there different types of meteorites?

There are three main classes of meteorites.

Stone meteorites make up the largest group accounting for approximately 92% of all meteorites. They are comprised chiefly of silicates, although they contain 5-15% nickel-iron and iron sulfide.

Iron meteorites are nearly 100% nickel-iron and are therefore heavy and dense. Iron meteorites make up about 6.5% of all meteorites.

Stony-iron meteorites, the rarest type, are a combination of stone and metal. They make up a mere 1.5% of all recovered specimens. While the outside looks like an iron meteorite, the inside is quite unique. There are two types of stony-iron meteorites, pallasites that display crystals of olivine, and mesosiderites that show irregular grains of nickel-iron in a stone matrix.

Stone Meteorites

92% of all meteorites that travel to earth are stone meteorites. However, since many of them closely resemble earth rocks on the outside, they are often not identified as meteorites.

Stone meteorites are classified into two major groups: chondrites and achondrites.

Chondrites

Chondrites account for approximately 84% of all stone meteorites recovered and are the most primitive of all meteorites in that they appear to be only survivors of the early solar system. Their composition shows little or no geologic change since their formation over 4.5 billion years ago. In fact, chondrites are believed to be the raw material that formed the planets.

Chondrites were named because of the existence of unique chondrules in them. The chondrules are composed of silicate materials that have melted and resolidified and are sometimes found whole and sometimes found shattered. Chondrules do not appear in any other type of rock, and, although there are many theories regarding how they formed, the reasons behind their creation remain a mystery.

There are three classes of chondrite meteorites: ordinary, enstatite and carbonaceous.

Ordinary chondrites are the most common class of stone meteorites. These meteorites were created at low pressures and temperatures in space. They are further classified by the amount of iron found in their composition. H type chondrites contain high metallic iron and low oxidized iron in the silicate minerals; L types contain lower metallic iron and higher oxidized iron; and the LL type contain the lowest metallic iron and the greatest oxidized iron.

The second type of chondrite, the enstatite chondrites, are so named because enstatite is the most abundant mineral. These stones are very rare and often contain minerals not found on the earth. Theory has it that these meteorites have been heated and have, therefore undergone a metamorphosis.

The third type of chondrite, the carbonaceous chondrites, are the most primitive of all meteorites. It appears that these meteorites formed in the pre-solar nebulae at low pressures. They contain carbon, partially in the form of organic molecules, carbon and hydrogen, as well as tiny diamonds (nanodiamonds), various forms of graphite, and fullerenes or "Buckyballs" that originated in stellar and molecular cloud environments. The study of carbonaceous chondrites is very exciting because of the discovery of certain organic molecules in their composition. This means that the "building blocks of life" can form in space. Could life have been "planted" on earth by a meteorite? Did other meteorites "seed" life on other planets in our solar system, or beyond?

Achondrites

Achondrites have been named as such because they do not contain chondrules. They are believed to have formed from planetary processes and display signs of igneous (melting) activity. There are a number of achondrite subclasses:

Eucrites look so much like earth basalt that they are rarely recovered unless someone sees them fall. As a result, eucrite achondrite meteorites are very rare and valuable.

Diogenites are calcium-poor basaltic achondrites and are similar enough to eucrites to suggest a common parent body. Because diogenites have a coarse-grained texture with large interlocking crystals, they must have cooled more slowly than the eucrites.

Aubrites are calcium-poor achondrites consisting mostly of enstatite as their pyroxene. They are sometimes referred to as enstatite achondrites and might be related to the enstatite chondrites. Less than twenty different aubrites are known.

Ureilites, are a unique type composed mostly of olivine and pyroxene and contain diamonds and carbonaceous matter. The diamonds probably originated from very heavy shock metamorphis. Scientists are still very puzzled by these peculiar and mysterious meteorites. Are they very primitive or have they been greatly modified since their origin?

Martian Meteorites

Shergotittes, nahklites and chassignites display very young crystallization ages as well as intense shock metamorphism. This group of meteorites, often referred to as the SNC group, is one of the youngest groups of meteorites with an estimated age of approximately 1.3 billion years. However, what makes this group extremely exciting is the fact that their composition closely resembles that of the planet Mars. In fact, the resemblance is so remarkable, scientists are all but sure that these meteorites had their origin on the red plant!

Lunar meteorites

Less than twenty meteorites from the Moon have been found. While in recent years a few have been discovered in the deserts of northern Africa and the Middle East. Most have been recovered in Antarctica,Lunar meteorites are of great scientific importance because they come from areas of the Moon that were likely not sampled by the Apollo or Luna missions.

Iron Meteorites

Because of their physical composition, iron meteorites usually survive the intense heat and friction when entering the earth's atmosphere much better than stone meteorites. Iron meteorites are also easier to find because they can more readily be distinguished from ordinary rocks and can be located using a metal detector. As a result, although they only account for 6.5% of all meteorite falls, they are "over-represented" in most museum collections, and people are more familiar with them than they are with the other types of meteorites.

Iron meteorites are identified mainly in two ways. First, they usually display a common, black or oxidized surface often marked by pits called "thumbprints" or regmaglypts. These thumbprints are caused when some of the meteorite melts away during atmospheric entry.

The second way to positively identify an iron meteorite is to slice, polish and etch it with a weak solution of nitric acid. This procedure will reveal a very unique criss-cross pattern call the Widmanstatten Pattern, named after its discoverer. This pattern is unique to meteorites and is not found in any other terrestrial rock. It is caused by the slow cooling of metals with different nickel contents and is actually the result of the growth of crystals composed of two different iron-nickel alloys, taenite and kamacite.

Iron meteorites are classified by their particular Windmanstatten structure. The first classification of iron meteorites is the Octahedrites. They contain about 6-13% nickel and are the most common iron meteorites. Octahedrites are further classified into three main groups: Coarse, medium and fine, which describe the width of the bands in the crystalline pattern. The coarser the pattern the greater the amount of iron. The finer the pattern, the higher the nickel content.

The second class, Hexahedrites, is made up of less than 6% nickel and contains kamacite but not taenite. When polished, a hexahedrite displays no surface features, but reveals "Neumann Lines" that are caused by impact shocks.

The third class, Ataxites, are the finest of the iron meteorites. They have a very high nickel content and contain taenite. Their Widmanstatten pattern can only be seen under a microscope.

Sometimes iron meteorites contain silicate inclusions, which give them a very different and beautiful appearance. Silicated irons are much more rare than even the stony-iron pallasites.

Stony-Iron Meteorites

Stony-iron meteorites are a combination of silicate, or stone materials, in a nickel-iron matrix. They are quite rare and only account for about 1.5% of all meteorite falls.

There are two major types of stony-iron meteorites: Pallasites and mesosiderites.

Pallasites are the most beautiful of all meteorites. They contain green to golden olivine crystals embedded in a nickel-iron matrix. Like iron meteorites, they display the unique Widmanstatten pattern in the nickel-iron matrix when polished and etched.

Scientists believe that pallasites were created when their parent bodies were formed. The material from the molten metal core of the planet mixed with the silicate magma, and the olivine crystallized out of the silicate as the molten mass cooled. These crystals were then formed into a metal "mold" where the mass solidified, forming a unique matrix.

Mesosiderites consist of metal and fragments of rocks. While pyroxene is the main stone element, no single crystals are found in the matrix. Instead, these pyroxene crystals are fragmented and scattered throughout the metal. One theory suggests that the silicate and metal portions were smashed together when they were partially molten. In fact, there may have been repeated impacts.

Tektites

Tektites are silica-rich, impact generated glass objects that are believed to have formed as a result of an asteroid impact. Some show signs of ablation, the unique melting action that is caused by friction that occurs when an object enters our atmosphere. Scientists believe that tektites were formed when a large asteroid crashed into earth, melted the rocks and ejected molten masses, some up and through the atmosphere. On their decent, they were re-melted, e.g., "flanged" australites. Those that did not reach or re-enter the atmosphere are characterized by irregular shapes.

Why the Study of Meteorites is Important

Consider for a moment the brave endeavor or our astronauts as they visited the Moon to recover manıs first "bit of space." The Apollo program, which cost the United States over $24 billion, yielded approximately 381 kilograms of lunar rocks and soil. Just a little math reveals that those lunar samples cost $28.5 million per pound or $63,000 a gram! (And thatıs in 1969-1972 dollars!)

Meteorites, on the other hand, are natural "space ships" and are quite a bargain in comparison. The costs to recover a meteorite are extremely modest when compared to the lunar rock recovery and even more costly ventures to Mars. In addition, meteorites provide a much wider range of materials than what is technologically possible for us to collect today. However, there are drawbacks, since the meteorites that reach earth are recovered are done so by chance. That is, we have no control over the types of meteorites we have for study, or a choice regarding their source. Nevertheless, because meteorites have traveled in space and have been affected by the space environment, they can provide us with insights into the origin and evolution of our solar system as well as an indication of the effects of the space environment.

Some meteorites are also the oldest rocks known to man. Some, like the Allende meteorite, predate our solar system. They formed before the planet earth. In fact, meteorites are the only rocks that have survived the beginning of our solar system. They help to unlock the mysteries of our universe and help to tell the story about its origin because meteorites have not been subjected to the ravages of erosion like earth rocks. They also tell us about the composition of other bodies in our solar system. For instance, there are a number of meteorites that are believed to have come from the Moon and Mars.

Meteorites may have also affected the history of planet earth. They may have had a direct impact on evolution. For example, the Murchison meteorite contains specific amino acids that are the "building blocks of life." Is it possible that a meteorite such as this landed on the primordial earth and planted the first seeds of life? In addition, some theories hold that a giant meteorite impact may have altered the earth's climate that caused the extinction of the dinosaurs.

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