What is a Meteorite?
Most people are familiar with the term “shooting star,” but few know its importance. Actually, it is not a star shooting across the sky, but a small piece of solid matter called a meteoroid colliding with the atmosphere. As the meteoroid enters the Earth’s atmosphere, the friction created by its incoming velocity causes its surface to heat up, and the brilliant flash of light records the passage of a meteor. Should the object survive this firey plunge through the atmosphere and hit the ground, it then becomes a meteorite. On very rare occasions when an extremely bright meteor is observed, it is referred to as a fireball. It is from these fireballs that most meteorites of recoverable size originate. Meteorites recovered in this manner are termed falls, indicating that the specimen was observed while falling. The majority of meteorites are recorded as finds, those specimens which were not observed to fall.
As compared to Earth rocks, meteorites have several features which can be used to establish their extraterrestrial origin. The surface of a meteorite is generally very smooth and featureless, but often has shallow depressions and deep cavities resembling clearly visible thumbprints in wet clay. Meteorites which have fallen recently may have a black “ash-like” crust on their surface. This provides evidence of their flaming entry through the atmosphere. However, this crust weathers to a rusty brown color after several years of exposure on the Earth’s surface and will eventually disappear altogether.
The Three Main Types of Meteorites. Although there are a large number of sub classes, meteorites are divided into three main groups: irons, stones and stony-irons. Almost all meteorites contain extraterrestrial nickel and iron, and those that contain no iron at all are so rare that when we are asked for help and advice on identifying possible space rocks we usually discount anything that does not contain significant amounts of metal. The three types of meteorites are: Nickel Iron, Pallasite, and Stone Meteorites.
It is thought that when asteroids melted, iron, being dense, sank to the centre to form a metallic core. These melted asteroids are known as differentiated since they have separated into concentric shells, with an iron core surrounded by a silicate mantle and perhaps even a silicate crust.
As this concentric, differentiated structure is similar to that of the terrestrial planets (Mercury, Venus, Earth and Mars), iron meteorites can tell us a great deal about how the metallic cores of planets formed. Around 5 out of every 100 meteorites that fall are iron. However, because iron is a tough material, they are more likely than stony meteorites to survive the fall to Earth.
This means that most meteorite craters are likely to have been caused by iron meteorites. The overwhelming bulk of these meteorites consists of the Fe,Ni-alloys kamacite and taenite. Minor minerals, when occurring, often form rounded nodules of troilite or graphite, surrounded by schreibersite and cohenite. Schreibersite and troilite also occur as plate shaped inclusions, which show up on cut surfaces as cm-long and mm-thick lamellae.
The troilite plates are called Reichenbach lamellae. The chemical composition is dominated by the elements Fe, Ni and Co, which make up more than 95%. Ni is always present; the concentration is nearly always higher than 5% and may be as high as about 25%.
The majority of meteorite falls are stony meteorites consisting mainly of silicate minerals. There are two main types of stony meteorite. Chondrites have never significantly melted and have compositions similar to the sun and the solar system as a whole. Achondrites have melted and are similar to igneous rocks on Earth.
These include rocks from Mars and the moon as well as from melted asteroids. Achondrites have compositions different from the sun as they have been changed by melting and crystallisation. Both chondrites and achondrites are split into many sub-groups based on their compositions, minerals and structures.
Pallasites contain big, beautiful olive-green crystals, a form of magnesium-iron silicate called olivine , embedded entirely in metal. Pallasites can show big variations. Sometimes the olivine does not occur as a single crystal but as a cluster and elsewhere it can create a pattern of veins through solid metal.
There are differing opinions as to how pallasites formed. Some scientists believe that they formed from melted asteroids, like iron meteorites. When an asteroid melts, the dense iron metal sinks toward the centre to form an iron core. Pallasites are thought to be samples of the boundary between this metal core and the silicate, olivine-rich mantle around it.
If this is the case, pallasites could potentially tell us a lot about the formation of the Earth and other terrestrial planets because they all have a similar structure. However, other scientists think that there are very few olivine-rich meteorites in the asteroid belt, and too many pallasites for them all to have come from a core-mantle boundary.
Achondrites include meteorites from asteroids, Mars and the moon. They are igneous, meaning that at some point they melted into magma. When the magma cools and crystallises, it creates a concentric layered structure.
This process is known as igneous differentiation. The rocky planets of Mercury, Venus, Earth and Mars, have also been formed by igneous differentiation, giving them planetary crusts, mantles and cores. Achondrites can, therefore, tell us a lot about the internal structure and formation of the planets.
Stony Meteorites without chondrules. Scientists believe that some of these meteorites originated on the surface of the Moon or Mars. 7.8 percent of meteorite falls are achondrites.
Chondrites are some of the most primitive rocks in the solar system. These 4.5-billion-year-old meteorites have not changed much from the asteroid they came from. Because they have never really got hot, they have not melted. This means that they have a very distinctive appearance made from droplets of silicate minerals, mixed together with small grains of sulphides and iron-nickel metal.
This structure of millimetre-sized granules also gives chondrites their name, which comes from the Greek for sand grains ‘chondres’. Chondrites are the materials from which the solar system formed and have been little changed compared with rocks from the larger planets, which have been subjected to billions of years of geological activity. They are very similar in composition to the sun and can tell us a lot about how the solar system formed.
Stony Meteorites are characterized by chondrules–small spheres (average diameter of 1 mm) of formerly melted minerals that have come together with other mineral matter to form a solid rock. Chondrites are believed to be among the oldest rocks in the solar system. 82 percent of meteorite falls are chondrites.
These meteorites are mixtures of iron-nickel alloy and non-metallic mineral matter. Scientists believe that they are like the material that would be found where the Earth’s core meets the mantle. 1.2 percent of meteorite falls are stony irons.
Irons (structural classification)
These meteorites are made of a crystalline iron-nickel alloy. Scientists believe that they resemble the outer core of the Earth. 4.8 percent of meteorite falls are irons.