This drawing represents a comet bringing atmospheric molecules and possibly the building blocks of life to the Earth's surface.
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Comet Origin of Life on Earth

Some scientists believe it is possible that life may have begun on comets. The Miller Urey experiment showed that amino acids may form from ammonia and other molecules under certain conditions of energy from lightning or ultraviolet light. Out in their distant home, the Oort Cloud, comets are exposed to the kind of cosmic radiation which provides the energy needed to form these long, complex molecules. Spectra taken of recent comets reveal the presence of molecules such as amino acids, proteins, and even polycyclic aromatic hydrocarbons (PAH's) .

There is no atmosphere to speak of in comets in the Oort cloud, but when they are disturbed from their homes and begin traveling toward the sun for a perihelion passage, they develop a transient atmosphere. Because comets making a perihelion passage lose a portion of their friable surface, it is thought that comets may have brought some of life's elementary molecules to the inner solar system and deposited them on planets, where they could better thrive. Every year the Earth passes through the Leonids, an area of debris left in the wake of comet Temple 2. This debris often creates spectacular meteor showers as the particles which enter the Earth's atmosphere can be seen from the ground.

More importantly however, comets have even hit a planet's surface, such as the case when comet Shoemaker-Levy 9 hit and plunged into the planet Jupiter in 1995. Craterchains found on other moons show that comets have hit planets before.

Comets range in size from 10 km in diameter to 100 km in diameter, which implies a sizeable volume of material. Since comets are made mostly of water, the impact of a comet with the primordial Earth would have deposited a large amount of water for the atmosphere and ocean. Since comets are only loosely held together by gravity, it is not required for such an object to hit the surface of the Earth to disintegrate. It could burn up at high altitude and deposit its water into the atmosphere. One comet the size of Halley's comet would bring enough water to form a large lake. In recent years some evidence has surfaced that many small comets may be hitting the Earth all the time, bringing water and other molecules.

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