The Dust-Cloud Hypothesis

universe contains huge clouds made up of very
large amounts of dust and 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.

Category: Science