This drawing represents a comet bringing atmospheric molecules and possibly primitive life forms to the Earth's surface.
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As well as being hit by debris and smaller, leftover planetismals, the planets in the inner solar system may have also been hit by comets. Since comets are made mostly of water, the impact of a comet with the primitive Earth would have deposited a large amount of water for the atmosphere and ocean. One comet the size of Halley's comet would bring enough water to form a large lake. Since the Earth had lost its original atmosphere, comets are a potential source of the replacement atmosphere.
Scientists believe it is possible that life may have begun on these distant, solar system traveling objects. Out in their distant home, the Oort Cloud, comets are exposed to the kind of energy needed to form the long, complex molecules of which life is thought to begin with. Thus, the crash of a comet onto the surface may have also brought the elements of life to the Earth.
Read more about comets by reading our comet section. Read more about comet crashes by taking the Exporatour of Spectacular Crashes in the Solar System, at the bottom of this page.