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Transcript of Meteorites
of rock, iron and/or other
materials that come from space and reach the Earth’s surface.
They may have traveled from the distant reaches of the
Solar System or even beyond. There are three major classes of meteorites which are determined by their nickel-iron content. Stone meteorites are the largest group and make up approximately 92% of all meteorites. They are comprised chiefly of silicates, although they also contain small amounts of nickel-iron. Iron meteorites are nearly 100% nickel-iron and are therefore heavy and dense. Approximately 7% of all meteorites that fall to Earth are iron. Stoney-iron, the rarest type, are a combination of stone and metal. They make up a mere 1% of all meteorites. Types of Meteorites meteorites are classified into two major groups: chondrites and achondrites. STONE METEORITES Stony-iron meteorites are a combination of silicate, or stone materials, in an iron matrix. They are quite rare and only account for about 1% of all meteorites. There are two different types of stony-iron meteorites: pallasites and mesosiderites.
Pallasites contain green or golden olivine crystals embedded in a nickel-iron matrix. Like iron meteorites, they display the Widmanstatten pattern in the nickel-iron matrix when polished and etched. Scientists believe that these meteorites formed when a planet was forming. The material from the molten metal core of the planet mixed with the silicate magma, and the olivine crystallized out of the silicate as it cooled. These crystals were then forced into the metal “mold” where the mass solidified.
Mesosiderites consists of metal and fragments of rock 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 suggested that the silicate and metal portions were “smashed” together when they were partially molten. STONY-IRON METEORITES Iron meteorites are identified in two ways. First, they usually display a smooth black or oxidized surface often marked by pits called “thumbprints,” which are caused when some of the meteorite melts away during atmospheric entry. The second way to identify an iron meteorite is the slice, polish and etch it with a weak solution of nitric acid. This procedure will reveal a crisscross pattern called the “Widmanstatted Pattern,” named after its discoverer. This structure is unique to meteorites and is not found in any 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 iron-nickel alloys, taenite and kamacite. Iron meteorites are classified by their particular Widmanstatten structure.
Octahedrites contain about 6-17% nickel and re the most common of the 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.
Hexahedrites are made up of less than 6% nickel and contain kamacite but not taenite. When polished, the surface of a hexahedrite displays no features, but reveals “Neumann Lines” that are caused by impact shock.
Ataxites have a very high nickel content and are composed almost entirely of taenite, with a possible few microscopic plates of kamacite. Although they display no obvious structure, they do have a microscopic Widmanstatten structure which is not perceptible to the naked eye. IRON METEORITES Tektites, which are often mistaken for meteorites, are silicate-rich, impact-generated glass objects that are believed to have formed as a result of a meteorite impact. Some show signs of ablation, the unique melting action that is caused by friction that occurs when an object enters our atmosphere. Tektites may be formed when a large meteorite crashes into Earth and throws particles of terrestrial materials into space, where they re-melt as they descend through the atmosphere. TEKTITES Meteorites offer us a very wide variety of geological materials from our Solar System and beyond. Since they have not been subjected to the ravages of erosion and change like Earth rocks and are the only materials that have survived since the beginning of our Solar System, meteorites can provide us with insights into its origin and evolution. They can help to unlock the mysteries of our Solar System’s creation by providing physical evidence for scientists to study. Meteorites can give us an indication of the effects of the space environment because they have traveled in space. WHY THE STUDY OF METEORITES IS IMPORTANT... Traveling through space towards Earth, natural objects in space are defined as meteoroids. When a meteoroid collides with the Earth’s atmosphere, friction heats its surface producing a bright light. It is this light effect to which the term meteor is applied. If part of the object survives atmospheric passage without burning up and lands on the surface of the Earth, it becomes a meteorite. 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 are recovered. It is possible the millions of meteorites have struck the Earth during its history, yet we have only recovered several thousand specimens. That’s because meteorites fall unpredictably, and their fall is seldom physically witnessed. In addition, most fall into the ocean or onto land that is densely covered by vegetation which makes recovery nearly impossible. Meteorites are named after the place or locality where they fell to Earth or where they were found. For instance, they could be named for a city or local geographical feature. Those that are observed in the process of arriving on Earth are recovered soon thereafter are called falls; meteorites that fall to the ground unseen but are discovered are later called finds. Although the origin of these unique rocks is not absolutely certain, scientists believe that most meteorites originate in the asteroid belt located between Mars and Jupiter. In addition, scientists have recently determined that certain meteorites may have originated on the Moon and the planet Mars as well as from beyond our Solar System. Chondrites account for approximately 84% of all stone meteorites and are the most primitive in that they appear to be the only survivors of the early Solar System. Their composition shows little or no geological change since their formation over 4.5 billion years ago and is closest to the composition of pre-solar nebula. In fact, chondrites are believed to be the raw material that formed the planets.
Chondrites were named because of the existence of unique, tiny, round objects called chondrules in them. The chondrules are composed of silicate materials that have melted and resolidified and may be found whole or shattered. There are three classes of chondrites: ordinary, enstatite and carbonaceous. Chondrites Ordinary chondrites make up the most common class of stone meteorites. They are further classified by the level of iron found in their composition. High-iron, H-Group, approximately 27% total iron; Low-iron, L-Group, approximately 23% total iron; and Low iron, low metal, LL-Group, approximately 20% total iron. Enstatite chondrites are so called because enstatite is the most abundant mineral. They contain less oxygen than the other groups and are metal-rich and magnesium-poor.
Carbonaceous chondrites are the most primitive of all meteorites. It appears that these meteorites condensed in the pre-solar nebulae at low pressures. As a result, they contain carbon, partially in the form of organic molecules, as well as carbon and hydrogen. The study of carbonaceous chondrites is very exciting because of discovery of certain carbon molecules in their composition. This means that the “building blocks of life” can form in space. In fact, amino acids the basis of DNA and RNA found in the human body, have been found in certain carbonaceous chondrites. Achondrites are stone meteorites without chondrules. They are believed to have formed from planetary processes and display signs of igneous melting and crystallization activity. There are seven types of achondrites: Aubrites; Digoenites; Eucrites: Howardites; Shergottites, including Nakhlitesa and Chassignites; Ureilites; and Lunar Meteorites.
Diogenites are achondrites composed primarily of cumulative pyroxenes. They consist of larger interlocking crystals than eucrites. Aubrite is an alternative name for enstatite achondrite. They are differentiated stone meteorites consisting predominantly of enstatite with very low iron content.
Howardites are achondrite breccias containing rock and mineral fragments of eucrites and diogenites.
Eurites are a class of achondrites that formed as basaltic flows on their parent body. They consist mostly of plagioclase and pyroxene.
Ureilites are a unique type of achondrite composed mostly of olivine and pyroxene. Some display very heavy shock metamorphism.
Shergottites, nakhlites and chassignites are very different from the other achondrite groups. Unlike other meteorites, they contain iron-rich silicates and iron oxides which indicate they formed in a rather oxygen-rich environment. They also contain small amounts of water-bearing minerals. This group of meteorites, often referred to as the SNC group, is one of the youngest groups of meteorites with an estimated crystallization 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 planet!
Lunar meteorites have also been classified as a subgroup of the achondrites because they show evidence of the igneous process that took place on the Moon. Achondrites At the same time, they also offer us important information about the composition of the other planets and bodies within our Solar System. Meteorites may have also directly affected the history of planet Earth and may have had an impact on evolution. For instance, some meteriories have been found to contain specific amino acids that are considered to be the “building blocks” of life. It is possible that a meteorite may have landed on the primordial Earth and helped begin the process of creating life on the planet. Ironically, life for the dinosaurs may have been ended as the result of the impact of a huge meteorite which altered the Earth’s climate and resulted in their extinction.