Irradiation solar nebula

V2 Dt, where t is time, and D is the diffusion coefficient) with penetration depth that, except for He and Ne, other noble gas atoms can hardly diffuse out from the target material if the irradiated materials were kept at 0°C for more than 107 years. The latter conditions are likely to approximate those in the preaccretionary solar nebula. Therefore, considering all these experimental results, we may conclude that implantation is unlikely to have played a major role in acquiring noble gases by the Earth. 

In the case of molecular clouds, it has been assumed that a cloud is irradiated by ambient ultraviolet starlight, so that CO molecules in the periphery of the cloud are dissociated in proportion to their original isotopic abundances. However, photodissociation in the cloud interior favors the rarer, unshielded isotopic species and In the case of the solar nebula,... 

Since hydrogen cyanide and ammonia are two of the most important parent compounds present in comets and since these bodies are intensely irradiated when they pass througih their closest proximity to the Sun, it is not unreasonable to think that some of the organic compounds described above may also be found in comets. The chemical composition of these bodies is considered to be representative of the composition of the solar nebula from which the primitive earth was formed. Therefore, it offers a solar-system model for studies on prebiological chemistry.< >... 

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