Critical Intervals in Earth History
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containing complex organic compounds like amino acids, “polyols,” and carbonaceous matter such as hydrogenated amorphous carbon or kerogen-like material. The inner rim is covered by an outer rim consisting of “dirty” ice. In that icy outer rim, due to the heavy ultraviolet radiation, radicals are formed and H2 is dissociated in monoatomic H that may react with other radicals to form organic molecules (Sorrell, 2001; Pearson et al., 2002). An important finding obtained with the Hubble Space Telescope was the observation of protoplanetary discs in the Orion cloud (Megeath et al., 2005). The size of these discs is more or less similar to our solar system. During solar system formation, an interstellar cloud becomes the building block, from which planets, asteroids, and comets form. Comets and carbonaceous chondrites are chemically very similar to star-forming interstellar clouds and are relics of them (Busemann et al., 2006; Wickramasinghe et al., 2009). They carry pristine primitive organic matter and represent a plausible source of prebiotic organic matter on the late Hadean early Earth. The stable isotope compositions of organic matter, e.g., from the Murchison carbonaceous chondrite (CM2), provide clear evidence that the organic matter was formed under conditions in stellar clouds and not on Earth (Sephton et al., 2003, see entry “Meteoritics” for further reading). External delivery of water and organic compounds is widely accepted as the origin of pristine earth-related organic matter (Wickramasinghe, 2010) and may have pushed the start of early life with the first microbial organism – LUCA – Last Universal Common Ancestor (Hoenigsberg, 2003).
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