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The Dust-Cloud Hypothesis
The universe contains huge clouds made up of very large amounts of dustand
gas. About 6,000,000,000 (billion) years ago, one of these clouds began to
condense. Gravitation--the pull that all objects in the universe have for
one another--pulled the gas and dust particles together. As the dust cloud
condensed, it began to spin. It spun faster and faster and flattened as it
spun. It became shaped like a pancake that is thick at the centre and thin
at the edges.
The slowly spinning centre condensed to make the sun. But the outer parts
of the pancake, or disk, were spinning too fast to condense in one piece.
They broke up into smaller swirls, or eddies, which condensed separately to
make the planets.
The forming sun and planets were made up mostly of gas. They contained
much more gas than dust. The earth was far bigger than it is now and
probably weighed 500 times as much.
The large body of dust and gas forming the sun collapsed rapidly to a much
smaller size. The pressure that resulted from the collapse caused the sun
to become very hot and to glow brightly.
The newly born sun began to heat up the swirling eddy of gas and dust that
was to become the earth. The gas expanded, and some of it flowed away into
space. The dust that remained behind then collected together because of
gravity. Although the shrinking earth generated a lot of heat, most of
this heat was lost into space. Therefore, the original earth was most
likely solid, not molten.
This hypothesis was developed by a scientest, Harold C. Urey in 1952. It is
also known as the Urey's hypothesis. He showed that methane, ammonia, and
water are the stable forms of carbon, nitrogen, and oxygen if an excess of
hydrogen is present. Cosmic dust clouds, from which the earth formed,
contained a great excess of hydrogen.
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