Comets are rich in water, carbon, nitrogen, and complex organic molecules that originate deep in space from radiation-induced chemical processes. Many of the organic molecules in the coma of comets originated in the dust of the solar nebula at the time and location where the comets initially formed in the early history of the solar system. Comets are relatively small bodies that have preserved these early organic molecules in a cold, relatively pristine state. This has led many scientists to speculate that life may have come to Earth on a comet, early in the history of the planet. Clearly, comets both delivered organic material to the early Earth and also destroyed and altered organic material with the heat and shock from impacts. Numerical models of the impact of organic-rich comets with Earth show that some of the organic molecules could have survived the force of impact. The organic molecules in comets may be the source of the prebiotic molecules that led to the origins of life on Earth.
Studies of the chemistry and origin of the atmosphere and oceans suggest that the entire atmosphere, ocean, and much of the carbon on Earth, including that caught up in carbonate rocks like limestone, originated from cometary impact. The period of late impacts of comets and meteorites on Earth lasted about a billion years after the formation of Earth, before greatly diminishing in intensity. Life on Earth began during this time, hinting at a possible link between the transport of organic molecules to Earth by comets, and the development of these molecules into life. The early atmosphere of Earth was also carbon dioxide-rich (much of which came from comets), however, and organic synthesis was also occurring on Earth.
In addition to bringing organic molecules to Earth, the energy from impacts certainly destroyed much of any biosphere that attempted to establish itself on the early Earth. Even the late, very minor K-T impact at Chicxulub had major repercussions for life on Earth. Certainly the early bombardment characterized by many very large impacts would have had a more profound effect on life. Any life that had established itself on Earth would need to be sheltered from the harsh surface environment, perhaps finding refuge along the deep sea volcanic systems known as black smokers, where temperatures remained hot but stable, and nutrients in the form of sulfide compounds were used by early organisms for energy.
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