Comets and the Origin and Evolution of Life

pp 147-173

Comets as a Source of Prebiotic Organic Molecules for the Early Earth

  • C. F. Chyba
  • , C. Sagan

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Life on Earth originated during the final throes of the heavy bombardment, in which the Earth—Moon system, as well as the rest of the inner solar system, was subjected to an intense bombardment of comets and asteroids. This bombardment may have rendered the Earth’s surface inhospitable for life for hundreds of millions of years subsequent to terrestrial formation. It may also have delivered to the Earth’s surface the bulk of the current terrestrial volatile inventory, in the form of a late-accreting impact veneer. Delivering intact prebiotic organic molecules of interest for the origins of life is much more difficult. However, several mechanisms seem likely to have been delivering exogenous organics to the surface of the Earth, or shock-synthesizing them in impacts. In an early carbon dioxide-rich terrestrial atmosphere, these mechanisms would have quantitatively rivaled or exceeded terrestrial organic synthesis in situ. In an early reducing (methane-rich) atmosphere, the exogenous sources would have been quantitatively unimportant compared to atmospheric production.