ASTEROIDS, METEORITES AND COMETS
(i)
Asteroids
Asteroids are small rocky bodies that obit the sun. They come in sizes from just a
few feet across to several miles. Some of the largest asteroids are so big
they've been called planetoids. The largest asteroid known so far is named Ceres
and is 950 km wide.
There are millions of asteroids in the universe. The majority of them a
found in the asteroid belt between Mars and Jupiter. Most believe asteroids are
leftover pieces of matter from the formation of our solar system that are kept
in the belt area due to Jupiter's gravitation pull. Others think the asteroid
belt is a planet that was broken apart during a collision. There are other
small groups of asteroids in the solar system, too; one of them is near Earth.
Asteroids need to be very big and close enough to Earth to be seen with
binoculars or a small telescope. Since Earth was first formed, asteroids have
slammed into it. Fortunately for us, though, dangerous asteroids, like the one
that probably killed off the dinosaurs, are extremely rare.
When an asteroid, or a part of it, crashes into
Earth, it is called a meteorite. An estimated 500 meteorites reach the surface
of the Earth annually.
(ii)
Meteorites
They are extra terrestrial bodies resulting
from the destruction of asteroids and they swim in space and occasionally fall
to the earth surface when captured by the earth gravitational pull. They are
usually very small but large fragments of several hundreds of them in size are
common.
When
meteorites or meteoroids enter the earth’s atmosphere, they are heated by
friction as they collide with atmospheric molecules and atoms. This collision
converts some of the kinetic energy to heat energy which vaporizes matter from
the surface of the meteorite. If this phenomenon is taking place at night when
the skies are clear, one can be able to see a trace of light commonly called a “shooting
star”. It is the hot evaporated
matter from the surface of the meteorite that produces the trail of light. This lovely phenomenon is called a
meteor. If the falling meteoroid is very
small, the trail of light will be brief because the object quickly evaporates
completely in the air. But if it is
large enough, it will fall unto the Earth and is called a meteorite. Most are only
a few centimetres in size.
Types of Meteorites
Meteorites are classified
into four main groups according to their composition and structure as:
-
Iron meteorites (siderite)
-
Stony meteorites (Aerolite)
-
Stony iron meteorites(siderolite)
-
Tektites
a)
Stony meteorite (Aerolite)
Stony
meteorites are composed of silicate compounds with olivines and pyroxenes. They
are made up of rounded chunks of rocky materials stuck together. Two types of
stony meteorites are: chondrites and
achondrites.
Ø Chondrites or Chondritic stony meteorites are made up of rounded chunks of
rocky masses of pyroxene and olivine called chondrules. The chondrules contain
traces of radioactive material which can be used to measure their age.
Ø Achondrites or Achondritic meteorites are made up of
crystallized grains of pyroxenes and olivine, resembling some igneous rocks
both in texture and composition.
b)
Iron meteorite or siderite
These types
of meteorites are composed of mostly Ni-Fe (nickel-iron) alloys. Their composition is similar to that of the
earth’s core.
c)
Stony iron
meteorites(siderolite)
This type of
meteorite composes of mostly Ni-Fe (nickel-iron) alloy and silicates. On the
bases of their structure, they are:
-
Pallasite meteorites: which have continuous
array of Ni-Fe surrounded by silicates
-
Misosiderolite: which consist of Ni-Fe
mixed with silicate in no definite order.
d)
Tektites:
These are
meteorites composed entirely of glass.
They usually occur around volcanic areas and are rich in silica. They resemble obsidian.
No comments:
Post a Comment