where e+, γ, and ν are a positron, a gamma ray, and a neutrino, respectively. The 12C/13C ratio resulting from the CNO cycle is in the range of 15 to 20.

The CNO cycle proceeds in the shells of the red giant stars that contain high levels of carbon, where the conversion of 12C to 13C, and to 14N and 15O, proceeds after the bulk of the hydrogen has been converted to He. The pulsation of the red giant during this energetic process disperses the elements formed into the interstellar medium, where they serve as the starting materials for the formation of a new star.

The first organic compounds were formed from the carbon injected into the interstellar medium under the influence of cosmic rays and ultraviolet light. Simple hydrocarbons and other compounds that contain nitrogen, oxygen, and sulfur were formed in this cloud of dust and molecules. This process proceeded for about 107 years, producing additional organics before the dust cloud collapsed to form stars and their associated planetary systems.

In the solar system the evolution of carbon compounds proceeded during planetary system formation. The existing compounds were subjected to the shock waves resulting from the collapse of the dust cloud to stars and protoplanetary disks. The intense ultraviolet and x-rays emitted by the new star effected changes in some of the organics. Carbon compounds ultimately derived from the interstellar medium were accreted onto planetesimals in the early solar system, where considerable thermal and aqueous modification may have occurred. These planetesimals then aggregated to form planets, a process that further modified some of their organic constituents. The organics present in the atmospheres of the newly formed planets were subjected to solar ultraviolet radiation as well. Organics on and below the surface of planets were further changed by energy sources including heat from volcanoes, heating by transport into planetary interiors where they were subjected to heat and pressure, contact with hydrothermal systems that initiated reactions with water at high temperatures and pressure, and reduction by minerals. Volcanoes also injected volatile organics into the atmosphere where solar ultraviolet radiation and x-rays changed them.



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