About meteors, meteorites and asteroids...
by Livia Giacomini -Copyright Tumbling Stone 2001

 

Some definitions

The term meteor comes from the Greek word meteoron, meaning phenomenon in the sky. It is used to describe the streak of light produced as matter coming from the solar system falls into Earth's atmosphere creating temporary trails of light.
The word meteroid refers generally to matter revolving around the Sun (or any object in interplanetary space) that is too small to be called an asteroid or a comet and that can produce the phenomena of meteors when falling into the Earth's atmosphere.  Even smaller particles, like dust grains or any interstellar material that happens to enter our solar system can be called micrometeoroids. When the meteorid is big enough to reach the surface of Earth without being completely eaten up by friction, it is called a meteorite.

Why trails of light?


But why are meteors accompanied by trails of light? When the body enters the atmosphere friction causes ablation of its surface, burning it up. If the meteor is small, it vaporizes before hitting the ground. If the meteor is larger, it survives the impact on the ground (see T.S. number 4 about impact structures), although it will be reduced in size during its passage into the atmosphere.   Whether or not the meteor survives to this passage, friction with the atmosphere makes the body loose its initial energy, transferring it partly to the air in the form of heat, and partly irradiating it  as light. For this reason, meteors are associated with very brilliant trails of light.

 

Metorites study and classification

One of the primary goals of studying meteorites is to determine the history and origin of their parent bodies: the majority of meteorites are believed to be fragments of asteroids, but in some cases meteorites coming from the Moon or from Mars have been detected and recognized.
Meteorites aren't easy to classify, but the three broadest groups are stony (both chondrites and achondrites), stony iron, and iron.     

Stony meteorites
Chondrites are the most common meteorites (85,7% of what falls on Earth). Dating has placed them at the age of 4.55 billion years, which is the approximate age of the solar system. These stones are therefore considered samples of early solar system matter, although in many cases their properties have been modified by thermal metamorphism or icy alteration.

This meteorite, collected in Antartica, is thought to have formed in the primordial nebula, about 4.55 billion years ago. Photo: NASA/JPL

Achondrites are the second type of meteorides that falls on Earth (7.1%). Achondirtes are also stony meteorites, but they are considered differentiated or reprocessed matter. They are formed by melting and recrystallization on (or within) meteorite parent bodies. As a result of this process of formation, achondrites have distinct textures and mineralogies.

This sample, found in Antartica, has a basaltic composition. Photo: NASA/JPL

Stony iron meteorites
1,5 % of what falls on earth is made of stony-iron meteorites .

Photo: NASA/JPL

Iron Meteorites
5.7 % of meteorites are Iron meteorites, which are classified into thirteen major groups and consist primarily of iron-nickel alloys with minor amounts of carbon, sulfur, and phosphorus. These meteorites formed when molten metal segregated from less dense silicate material and cooled, showing another type of melting behavior within meteorite parent bodies.

This meteorite was found in Antartica and is mostly made of iron and Nickel. It is probably a small piece that broke up from the core of a larger asteroid. Photo: NASA/JPL

 

Chemical classification of asteroids

The chemical classification of asteroids, made thanks to spectral observations (see T.S. special issue about "Observational techniques") finds some correspondences with the above meteorites classification, since there is a tight bound between asteroids and meteorites.
Anyhow, this classification must be taken very carefully, because of biases involved in the observations. For example, the experimental difficulty of detecting dark asteroids (e.g. the dark C-types ) could make the percentages of the different classes not representative of the true distribution of asteroids.

C-type ( where C stands for carbonaceous)

This class includes more than 75% of known asteroids. They are extremely dark (albedo 0.03), similar to carbonaceous chondrite meteorites. Approximately these asteroids have the same chemical composition as the Sun, minus hydrogen, helium and other volatiles. The spectra of these asteroids have relatively blue colors and are fairly flat and featureless.

S-type ( the S stands for silicaceous)

This type of asteroids represents about 17% of the totality. They are relatively bright objects (albedo .10-.22). They have a metallic composition (basically made of nickel, iron and magnesium-silicates).
The spectra relative to this class are reddish and similar to those of stony-iron meteorites.

The C and S types include most of the asteroids but there are a few smaller classes including :

M-type
(M for metallic). This class includes most of the rest: they are bright asteroids (albedo .10-.18), made of pure nickel-iron.
E-Type (E for enstatite)
R-Type (R for red)

There is also a number of other very rare types that grows as more scientific information about asteroids is collected ....

 


Adapted from Spaceguard Science pages (
http://spaceguard.ias.rm.cnr.it)...

 

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