Fact Sheet on Meteorites
Western Australian Museum
A
Meteorites are. travellers through time and space that plunge to Earth, sometimes trailing a
brilliant light and with a noise like thunder, at the end of a journey that began in the farthest
reaches of the solar system as long ago as 45,000 million years.
They are solid bodies of crystalline matter thought to have originated deep inside planet-like
bodies that were later fragmented. Unlike Earth rocks, the oldest of which date back about
3,800 million years, they have been isolated in deep space since the birth of the solar system.
While in space, they are known as meteors. Though their origins cannot be known for
sure, there are indications that they are associated with comets, and that they originate from
the asteroid belt between Mars and Jupiter. There they remain orbiting the Sun, but any that
stray too close to Earth find themselves caught in its gravitational pull and begin to plummet
towards its surface. Tearing through the atmosphere they are heated and their surfaces melted
by friction. Between 100 and 159 kilometres above Earth they become incandescent and the
larger ones are visible as bright lights trailing luminous tails. Some explode with a brilliant flash
and a roar like thunder.
Most meteors burn up completely on entering the atmosphere, but some survive the journey
and land on Earth's surface. They then become known as meteorites.
Meteorites vary in size. Some are the size of small pebbles, others weigh many tonnes.
Scientifically, they are extremely valuable. Studying their chemical and mineralogical
composition helps us to interpret the origins and nature of the solar system, as well as of Earth
itself.
B
The study of meteorites is a comparatively young science, but meteorites. have been known to
man since the·earliest days of prehistory.
Meteorite falls were described by writers of the Han Dynasty in China and by the
philosophers of ancient Greece. In Mecca, Moslems pay homage to the sacred stone of Kaaba,
which is apparently a meteorite. American· Indians are also known to have paid homage to
meteorites and members of two tribes made yearly pilgrimages to a hill in southern Alberta
where the Iron Creek meteorite lay.
During the Iron Age, many meteorites were destroyed to make implements and weapons -
among some peoples, these weapons were believed to confer supernatural powers on the
bearer.
In the Middle Ages meteorites were regarded with awe as signs of God's wrath, so they were rarely preserved. It was· not until the early 19th century that scientists became convinced that they were perfectly natural bodies; the scientific study of them is only about 150 years old.
C
There are three main types of meteorite - irons, stones and stony-irons.
Irons are mainly composed of nickel-iron alloy and have a characteristically dense and
metallic appearance. They originate from the 'core' of their parent body and because of
their metallic nature are more resistant to atmospheric friction than other meteorites. The
largest meteorites are usually irons. The first meteorites found in Western Australia were irons
discovered by a policeman named Alfred Eaton towards the end of the 19th century.
Stones consist largely of magnesium-rich silicate minerals, with varying amounts of nickel-iron. There are two main classes of stony meteorites - those that contain substances called
chondrules, and those that do not. The chemical composition of the first group - known as
chondrites - is very similar to that of the Sun.
Stony-irons contain about equal amounts of metallic nickel-iron alloy and silicate material.
They are less common than the other two types of meteorite. The stony portion generally
occurs as fragments welded by metal. They are probably the result of a high-speed collision in
space between a body of iron and one of stone - the metal would have melted at the time of impact.
D
A meteorite 'fall' is a meteorite recovered after a witnessed fireball. A meteorite 'find' is a meteorite found by chance long after it fell to Earth.
Seeing a meteorite fa11.. can be quite spectacular. For,example; residents and visitors in Wiluna, Western Australia, for the Annual Race Meeting festivities on 2 September 1967 saw
the night sky lit by a flash 'like a welding arc - white and blue'. One man reported seeing 'an
object about 20 feet [six metres] long throwing out balls of fire' ;Others reported 'a terrific
rumbling noise' and 'six or seven bangs' . These explosive noises were caused by atmospheric
shock waves as the meteorite fragmented on its journey to Earth.
E
The surface of a meteorite is quite different from that of most ordinary rocks. This·is a result of entering the atmosphere at high speed and the outer portions of the meteorite being melted or burned off.
Stony meteorites have a glassy, dull black to deep brown fusion crust. This coating is only a few millimetres thick and if part of the surface is broken, the interior looks quite different.
Iron and stony-iron meteorites also have fusion crusts, but they are not quite so obvious.
Most meteorites contain some metallic iron. This can be recognised as silvery areas or grains on broken surfaces. It means that meteorites are usually heavier than ordinary rocks of the same size. Also, they will attract a magnet.
Making a ·positive identification and classifying a meteorite requires expert knowledge and sophisticated equipment.
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