It is often said that the greatest stories are written in the stars. Yet there is an even greater story which can be found much closer. It is the story of the birth of this solar system. The clues to this amazing story are found in meteorites. Most meteorites come to Earth from the asteroid belt just beyond Jupiter. They are remnants of a time when this solar system was just coming into existence. They contain a lot of information about the formation of planets and the sun.
Meteorites are classified into three main groups: stony meteorites, iron meteorites, and stony-iron meteorites. Stony meteorites are the most common to fall to Earth, but they are relatively fragile and often burn up in the atmosphere. These meteorites come in a wide variety of colors and textures, and resemble rocks found on Earth. It is only through chemical means that scientists can determine whether a rock is a stony meteorite or not.
There are two sub-categories of stony meteorites: chondrites and achondrites. The word chondrite is derived from the Greek word for sand, because chondrites have a grainy texture composes of bits of silicate material, iron-nickel, and sulfides. These are the most primitive of the meteorites. They have a composition similar to the sun and are generally unaltered by freezing. Therefore, chondrites can be used to study the early solar system. The meteor that crashed in Antarctica was a chondrite bearing globules of silicate material from early solar nebulae. Some chondrites even carry water and other organic materials, and are thought to have played a major role in the development of life on Earth. The other sub-category of stony meteorites is the achondrites. These meteorites also bear similarities with other parts of the solar system and are used to study the formation and internal structure of planets. They are formed by a process called igneous differentiation, which means they consist of layers of hardened magma. Melting and freezing alters there composition and leaves them with a smooth texture.
The second category of meteorites is the iron meteorites. These meteorites, like achondrites, also form from igneous differentiation and feature concentric rings. However, iron meteorites contain large amounts of iron which collects at the center giving them an iron core, a mantle of silicate material, and possibly a silicate crust similar to that of planets. In fact, the differentiated structure of iron meteorites means that they can be used to study the formation of planets in the solar system. It is estimated that 5% of all meteorites are iron meteorites, and they are the most likely to cause craters. There are three sub-categories of iron meteorites: hexahedrites, octahedrites, and ataxites. The sub-category that a sample falls into depends on the amount of nickel it contains. The cross-section of an iron meteorite that contains nickel will display a Widmanstatten pattern when acid is applied to it. A Widmanstatten pattern is basically a lattice design that results from millions of years of cooling. They help scientists determine the size of the object from which the meteorite came.
The third of the categories of meteorites is the stony-iron meteorites. This category is the rarest of the three. It is estimated that only 2% of all meteorites are stony-iron meteorites. Stony-iron meteorites are separated into two subcategories: mesosiderites and pallasites. Mesosiderites are composed of small shards of various rocks held together by a fine cement-like material. They are formed from collisions between other meteors and may hold the histories of the destroyed bodies. Mesosiderites are used to study how the asteroids melted. Pallasites, which make up the other sub-category, contain green or olive-green crystals. This crystal is a magnesium-iron silicate material known as olivine. A pallasite with pure olivine crystals is a gemstone known as peridot. It is these meteorites that make the stony-iron category the most attractive category of meteorites. Because of the presence of these silicate crystals, some scientists think that pallasites come from the mantles of shattered asteroids. Others claim that there aren’t enough olivine-rich asteroids in the solar system to have formed all the pallasites that have been found.
Meteorites by themselves may not seem very interesting, other than the extravagant pallasites, but the stories they tell are truly fascinating. Meteorites may even hold the answer to the existence of life elsewhere in the universe. There can be no doubt that much of what is currently known about the solar system would still be a mystery if it weren’t for the study of meteorites.