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Asteroids are currently classified into three types:-


C-Types -  Carbonaceous - these asteroids are derived from upper crustal material. They are the oldest of asteroids and the most common type.

S-Types - Silicaceous - these asteroids are derived from lower crustal material and mantle material which was molten.

M- Types - Metallic - these, usually smaller asteroids are derived from the core material and hence are composed of iron and nickel.  


The molten iron core explodes into smaller units which then cool down to form meteoric material which later may fall to Earth as a meteorite - like the one in the following image.










The largest iron meteorite to have been discovered on Earth is at Hoba in Namibia, South-west Africa. It weighs about 60 tons.


Hot, silicaceous magma material from the mantle region of the old planet can form a dumb-bell shape before it cools and accretes - much in the same way molten silica from volcanic plinian eruptions forms the same shaped tektites. Cleopatra, a dumb-bell shaped (bilobated) asteroid which lives in the asteroid belt, is an example of these types of asteroids.










It is thought that the shape formed when the material cooled during a stage when it was spinning. An even faster spin would have caused the separation of the globular shapes into two individual globular pieces.


The asteroid belt between Mars and Jupiter holds the greatest concentration of asteroid material. However the total mass of these asteroids amounts to less than one previous planet.


Some asteroids clearly show differentiation and stratification and hence must be fragments of the outer crust of the previous planet. It is in these larger asteroids that the basest life-forms (such a methanogens) can sustain an existence and possibly transfer to other planets when a subsequent impact with it takes place. In this way, life which had evolved on a previous planet 20 billion years ago may have infected our young planet, nearly four billion years ago. Perhaps the seventeenth previous planet had evolved basic life and exploded 67 my + (17 x 212my) ago = 3.72 billion years ago.


It is more likely that it took 4 billion, not just 1 billion years, for Methanogens to evolve from inorganic materials. Despite being the most primitive of life, they are still very elaborate and complex organisms.








I Halliday The variation in frequency of Meteorite Impact with Geographical Latitude. 1964 . Meteoritics Vol 2, No 3, p271.

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