Meteorites

When a meteorid (that can be a small NEO or even a dust particle) enters the atmosphere, friction tends to burn it up (generating the phenomena of meteors), completely vaporizing it if the body is small enough. If the body is larger, it will survive its descent through the atmosphere (although its initial size will be reduced), and will impact the ground, being called in this case meteorite.
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 classification

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). 
This meteorite was collected in Antartica and 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%)
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 ones.
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

 

Meteorites classes' properties

Dating of chondrites 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 (click here to know more about the solar system formation).
Chondrites can be classified in many subgroups, each with different properties that represent a hint to deduce the provenience of the meteorite's formation. Enstatite chondrites contain the most refractory elements and are believed to have formed in the inner solar system. Ordinary chondrites, containing both volatile and oxidized elements, are thought to have formed in the inner asteroid belt. Carbonaceous chondrites, which have the highest proportions of volatile elements and are the most oxidized, are thought to have originated in even greater solar distances.
Achondrites 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.
Pallasites are stony iron meteorites composed of olivine enclosed in metal.
Iron meteorites 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.

A meteorite from Mars

An example of meteorite whose provenience has been identified is the meteorite ALH84001, a martian meteorite. The strongest evidence for its martian origin is the fact that it contains traces of gas similar to the martian atmosphere (which composition has been analyzed by the Viking lander).
An analysis of this rock revealed unique structures called micromagnetites with a characteristic shape (hexagonal when viewed from one side, rectangular when viewed from another) that has never been found with a non-biological origin. A sign of life on Mars?

Photo: courtesy of NASA