Prebiotic synthesis of biogenic molecules has traditionally been viewed as a planetary process. Unfortunately, the classic Miller-Urey Experiment (70, 71) requires a reducing atmosphere to efficiently synthesize amino acids and the early terrestrial atmosphere was at best just slightly oxidizing, consisting largely of N2 and CO rather than the CH4, NH3, and H2O that Urey originally hypothesized. Modern assessments are even more pessimistic for the abiotic synthesis of organic materials as CO2 may have been a major atmospheric constituent (72-74). Several workers (75, 76) pointed out that such an atmosphere makes abiotic synthesis of amino acids difficult to explain. Miller (77) asserted that although methane is the primary source of amino acid production, it can still occur when using CO and CO2. Clearly the latter are less favored than CH4 for amino acid synthesis, but may have contributed adequately. Undersea vents are now an integral part of the new Astrobiology Program. Such vents are reducing environments that might produce prebiotic organics on the primitive earth and might also exist on other planetary bodies such as Europa. However, these vents are not the only source of biogenic compounds available throughout the primitive Solar System.
Comets have been invoked as a source of organic materials needed for the evolution of life on Earth (78), and this idea has been revived recently (79). Comets are interesting because they may have delivered large volumes of water, along with the organics. Biogenic compounds like amino acids have not been identified in comets, but comets do contain simple volatile organic compounds, CH3OH, H2CO (68), C2H2 (80), C2H6 and CH4 (81), and H2CO2. Volatiles are observed in cometary comae. Laboratory experiments show that other organic compounds form from these molecules with an appropriate energy source, such as ion-bombardment or UV-irradiation. The overall suite of complex organics in comets is still unknown and may remain so until examined by a cometary lander. In situ sampling and sample return missions will be very important because large amino acids and primitive proteins are less volatile than water and would remain in the nucleus to temperatures well above 300 K.
The sources of organic materials in comets are unknown. They could represent unmelted, unprocessed ices and organic residues on presolar interstellar grains. Additional components may be low molecular weight organics synthesized in the nebula, or in higher pressure subnebulae of the giant planets. These hypotheses have been studied and both have had some success in explaining volatiles in comets (55). Gas-grain reaction experiments appropriate for such environments are in their infancy (82) and must be extended by using more realistic catalytic materials. Another new avenue for research is coupled accretion, chemistry, and dynamic evolution of planetesimals. Trapped radicals from presolar ices could react with liquid water in the interior of water-rich planetesimals subject to radioactive heating to form amino acids. Liquid water was present in many planetesimals and left an extensive record of hydrothermal alteration easily read in the meteoritic record. Water was also present on the early Earth and reaction of radicals produced via space irradiation of organics could occur following the fall of an icy planetesimal into a lake or pond.
A second aspect of this problem is the delivery of the organics, intact, to the primitive Earth. However, meteorites deliver a considerable quantity of organic material to the modern Earth each year. Some very fragile meteorites have survived passage through the atmosphere, as have meteorites containing a significant (>10%) mass of organics (83). Fragile organic components are preserved within the frozen bolide because of the inefficient transfer of thermal energy to the interior and heat dissipation by sublimation of its outer layers. If cometary ices are at least as coherent as the more fragile meteoritic specimens found in our museums, and if entry heating ablates the surface rather than conducting heat into the interior, then solid pieces of cometary ice could fall into primitive lakes or oceans. Recent studies (44, 84, ) have shown that significant fractions of amino acids in icy impactors survive to planetary surfaces. Infalling material could have had a considerable effect on the organic chemistry of the early oceans, as some models predict that a significant fraction of the Earth's oceans were acquired from c-type asteroids and comets (79).
Though comets do contain volatile organic compounds, their overall complement of organics is completely unknown. Even the volatile compounds detected by ground-based observational studies are not easily predictable by standard models of nebular chemistry (55). Most models invoke a combination of nebular processing and chemistry in the higher temperature-pressure environments of Giant Gaseous Protoplanets to explain observed ratios of reduced/oxidized cometary molecules (e.g., NH3/N2 or CnH2n+2/CO). Studies of catalytic activity on interstellar grain surfaces or on grains formed in situ are just beginning (62, 82, **). Much more work is needed to extend the work of Anders on Fischer-Tropsch-type reactions in the nebula (60).
Understanding the dynamic conditions in the Solar Nebula is key to successful prediction of the material found in comets. If gas and grains formed in cold molecular clouds take a one-way trip into the sun then comets will be dominated by interstellar species sparsely leavened with molecules that formed in Giant Gaseous Protoplanets. If there is large-scale circulation bringing processed dust and gas from the inner nebula back out into the region of comet formation, then the composition of comets will be determined by a chemical-kinetic reaction network closely coupled to the dynamic transport of dust and gas in the system (85). It is impossible to predict the output of this chemical reaction network without a comprehensive understanding of the dynamics of the nebula. Unfortunately, current models of protostellar nebulae are far from ready to supply the detailed environmental and dynamic parameters needed to construct a complete model. Without such a model, understanding chemical evolution leading to the origin of life on Earth and in the Solar System is an unconstrained problem with little prospect for a unique solution. As such, it is imperative that detailed models of the dynamics of protostellar nebulae and appropriate observational programs to test and constrain them must be accorded high priority by astrobiologists.
H.G.M.H. was supported by the National Academy of Sciences/National Research Council Resident Research Associateship Program. C.A.G. was supported by National Aeronautics and Space Administration Guaranteed Time Observer funding to the Space Telescope Imaging Spectrometer Science Team through the HST Project at Goddard Space Flight Center, and through the National Optical Astronomy Observatories. The National Optical Astronomy Observatories are operated by the Association of Universities for Research in Astronomy, under cooperative agreement with the National Science Foundation.
AU, astronomical unit.
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Nuth, J. A., Lunar and Planetary Science Conference, March 15-19, 1999, Houston, TX, abstr.
** Fegley, B. (1998) Bull. Am. Astron. Soc. 30, 1092 (abstr.).
Pierazzo, E. & Chyba, C. F. Lunar and Planetary Science Conference, March 15-19, 1999, Houston, TX, abstr.