SUMMARYExtraterrestrial organic compounds have been considered as important sources for the origin of life on the Earth. In order to study the formation of organic compounds found in extraterrestrial bodies, simulated interstellar media were irradiated with high-energy particles or photons. Particles used were protons from a Van de Graaff accelerator, protons and helium ions from a cyclotron, electrons from a synchrotron, and heavy ions from the "HIMAC" heavy ion accelerator. Photons used were UV light and soft X-rays from a synchrotron. Gamma-ray irradiation was performed by using a 60 Co source. Amino acid precursors were formed from a mixture of carbon monoxide (or methodanol), ammonia (or nitrogen), and water by high-energy particles, gamma rays, or X-rays. If nitrogen molecule was used as N-source, amino acid precursors could be formed by high-energy particles, X-rays and gamma rays, but not by UV light. When ammonia was used, amino acid precursors were detected in all cases, but energy yields (G-values) of amino acid precursors by high-energy particles were much higher than those by UV or X-rays photons. Possible formation of amino acid precursors in interstellar environments was discussed from the point of view of chemical evolution toward origins of life.

Various organic compounds including complex organic compounds have been detected in comets. Cometary organics are supposed to be formed in interstellar dust (ISD) environments. We examined possible formation of bioorganic compounds in ISDs through simulation experiments, and compared with that in primitive Earth atmosphere. It was shown that a wide variety of amino acids were formed in simulated ISD environments in the form of precursors. Pyrimidine bases (uracil, cytosine and thymine) were also formed from the possible interstellar media. These results suggest that comets can bring no less types of bioorganics to planets than those formed in primitive Earth atmosphere. Not only Earth, but also other planetary bodies like Mars and Europa received cometary organics during the late heavy bombardment era. They can be included in possible habitable areas whatever primitive planetary atmosphere they had.

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