What Are Meteorites?
Meteorites are pieces of other
bodies in our solar system that make it to the ground when a
meteor or "shooting star" flashes through our
atmosphere at speeds of 15 to 70 kilometers per second (roughly 32,000 to
150,000 miles per hour). The majority originate from asteroids shattered by
impacts with other asteroids. In a few cases they come from the Moon and,
presumably, comets and the planet Mars. Meteorites that are found after a
meteoric event has been witnessed are called a "fall," while those found by
chance are called a "find." Meteorites are usually named after a town or a
large geographic landmark closest to the fall or find, collectively termed
localities. The word "meteorite" can refer to an individual specimen, to those
collected within a strewnfield, or to a specific locality.
The recovered mass of meteorites
represents some of the scarcest material on Earth, much rarer than gold, yet
presents us with some of the original material from which the entire Earth was
formed. Because of their scarcity and their primitive origins, meteorites are
sought after by collectors and researchers alike.
There are three major types of
meteorites: stone, iron and
Parnellee, India, an LL3
meteorites consist of two groups: chondrites and
achondrites. Most chondrites have
remained unchanged since their formation 4.56 billion years ago,
shortly after the formation of the Sun. Almost all chondrites contain
chondrules - small, gaseous, spherical
inclusions that formed during the solar
Achondrites appear to have been
chondritic before being altered by a heating or impact event. These meteorites
are much rarer than chondrites and include the HED group from asteroid 4 Vesta
and SNCs from the planet Mars.
Ghubara, Oman, an L5
are thought to be pieces of the shattered cores of
differentiated asteroids and contain
varying amounts of nickel. This type comprises the three basic groups:
Hexahedrites that contain 4.5 to 6.5
percent nickel, Octahedrites containing
6.5 to 13 percent nickel and Ataxites with
nickel amounts of 16 to 30 percent. Iron meteorites are the most common group
of meteorites by weight.
A stunning display of
chondrules in this image of Axtel, Texas, USA. Axtel is a carbonaceous
Mesosiderites and Pallasites are
the two main groups of the stony-iron meteorites.
Mesosiderites consist of broken angular fragments of mantle rock and
nickel-iron that have been fused together by impacts with another body.
Pallasites, some of the most attractive meteorites, are believed to have formed
at the core-mantle boundary of asteroids and contain crystals of olivine
Camel Donga, Australia. 633
gram end piece. Camel Donga is a eucrite achondrite.
New England Meteoritical