Meteorites are pieces of stony or metallic material that reach the Earth from outer space. They are mostly left-over debris from the formation of the Solar System, and may well contain CHONDRULES: smallish spherical grains that formed by aggregation of the dust in the solar disc five billion years ago. Broadly speaking, Meteorites are classified according to their composition and structure, as follows:
Eucrite, possibly from Vesta!
Achondrites are stony Meteorites that contain no chondrules. Scientists believe that some of these meteorites originated on
the surface of the Moon or Mars, while others may have
originated on the Asteroids Vesta, Angelina, Nysa and others.
7.8 % of meteorites are achondrites. It has been suggested that
some angrites and enstatites could have originated on the
planet Mercury.
Chondrite from Oman
82% of all Meteorites are Chondrites: these contain
chondrules, and are therefore among the oldest rocks in the
Solar System. Chondrites contain varying amounts of nickel &
iron, and are generally attracted by a strong magnet. They may display evidence of their fiery passage through the atmosphere
in the form of dark fusion crust and surface 'thumb-printing'
This is a slice of a Carbonaceous Chondrite, showing the
typical dark matrix, large chondrules, coloured clasts and
whitish calcium/aluminium inclusions of this type of stone.
Over seventy amino acids (including eight of the twenty amino
acids found in terrestrial life) have been identified in these
meteorites, causing many biologists to suggest that life may
have been 'seeded' on Earth in this way!
Pallasite from Brahin, Russia
Pallasites are widely believed to have originated at the core /
mantle boundary of small planets. They consist of a nickle-iron
matrix, in which are suspended crystalline fragments of
minerals such as olivine and pyroxine. Pallasites are very rare,
and make up around 1% of all falls.
The origin of Mesosiderite stony irons is, as yet, a mystery!
They consist of a stony matrix with varying amounts of metal
suspended randomly throughout. It is assumed that they were
formed during a massive impact between two well-differentiated
bodies.
Iron from Chinge, China
Iron meteorites make up around 5% of all falls. They largely
consist of Nickel and Iron with varying amounts of rarer metallic elements such as Niobium and Iridium. When cut and polished,
irons often display the characteristic Widmanstatten pattern.
The Earth, like all the planets and their satellites, was heavily bombarded with meteorites and planetoids for a billion years
after the formation of the Solar System. This process has,
thankfully, slowed down, but massive impacts do still occur:
one such 65 million years ago probably hastened the extinction
of the Dinosaurs and drew the Mesozoic Era to an end!
Several hundred craters have been discovered on Earth: this
one is at Tenoumer in Mauretania.