Comets, organic compounds

Star formation and the formation of star systems with planets around them, constantly takes place in dense interstellar clouds. The material present in these clouds is incorporated into the objects that are formed during this process. Pristine or slightly altered organic matter from the cloud from which our solar-system was formed is therefore present in the most primitive objects in the solar system comets, asteroids, and outer solar-system satellites. Pieces of asteroids (and perhaps comets) can be investigated with regards to these components through the analyses of meteorites (and eventually in samples returned from these bodies by spacecraft) in laboratories on Earth. The infall of asteroid and comet material from space may have contributed to the inventory of organic compounds on primordial Earth. 

As rich as the Murchinson and other meteorites are, comets are perhaps the most generous source of organic compounds. They are particularly rich in HCN (Miller, 1998) which appears to be very common in space and it has for this reason been investigated by several authors (Ord, 1961 Ferris and coworkers, 1973,1974, and 1978 Matthews, 1975). 

Hydrate clathrates of organic compounds are thought to be responsible for the behavior of ice in the heads of comets and in wet methane under pressure.22 Unless methane is carefully dried, high-pressure lines may become clogged with the ice-like gas hydrate. There may be large deposits of methane hydrates, the ice that burns. beneath the ocean floor. 

It appears that conditions in the solar nebula were appropriate for the FTT but not the Miller-Urey reaction. Kinetic calculations (Lewis and Prinn, 1980) as well as observations on comets (Delsemme, 1977) show that CO and COj, not CH, were the principal forms of carbon. And the dust-laden solar nebula was opaque to UV, precluding any photochemical reactions. It seems best, however, to approach the problem empirically, by examining the meteoritic organic compounds themselves for clues to their formation. We shall review these compounds class by class, looking for the signatures of the FTT or Miller-Urey reactions. 

Other researchers suggest that organic precursors to life arrived on Earth aboard meteors or comets. Once here, these organic compounds arranged themselves into molecules that eventually led to the development of life. 

As briefly reviewed above, various types of apparatus have been used according to each experimental purpose, but few methods allowed for the collection of materials produced, without incotporating surrounding contamination. Recently, we developed a simplified system for the shock technique, which can be applied to any form of material and which enables us to recover and examine shocked products witliout contamination [134,135]. Furthermore, this system can be used at extremely low temperatures to simulate reactions in space such as those caused by icy comet impacts. In tlris section, we describe chemical reactions disclosed by the new technique developed in our laboratory. These studies provide us witli useful infonnation on the means of creating the organic compounds found in the cosmos. 

Carbon compounds in the solar system can be traced in planetary atmospheres, on planetary surfaces, comets, asteroids, meteorites and in the interplanetary medium. Gas and solid state chemistry within the solar nebula was responsible for new organic material (49,50). Many organic compounds have been identified on outer solar system bodies, some of them originating in the surface ices through energetic processing (4). 

The importance of exogenous delivery of organic matter to the early Earth is critically dependent on the survivability of organic compounds during the delivery process. It is presently unclear exactly how much organic material would escape destruction during asteroid, comet and interplanetary dust particle infall to the Earth s surface. 

Hydrocarbons are also found in outer space. Asteroids and comets contain a variety of organic compounds. Methane and other hydrocarbons are found in the atmospheres of Jupiter, Saturn, and Uranus. Saturn s moon Titan has a solid form of methane—water ice at its surface and an atmosphere rich in methane. Whether of terrestrial or celestial origin, we need to understand the properties of alkanes. We begin with a consideration of their shapes and how we name them. 

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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