Interstellar medium carbon surfaces

Another approach to the study of the interstellar medium includes experiments carried out here on the Earth s surface. In such experiments, researchers attempt to simulate the conditions found in various regions of the ISM and to determine if chemical reactions hypothesized for those regions actually do occur. The study by scientists at SETI and NASA on amino acids, reported earlier in this chapter, is an example of such experiments. Researchers general approach in such experiments is to enclose certain fundamental substances (such as hydrogen, oxygen, and carbon monoxide) within a sealed container at the low temperatures and pressures common to the ISM and then expose those substances to the type of radiation that may be found in some particular region of the ISM, such as ultraviolet radiation or cosmic-ray-like radiation. The substances formed in such experiments can then be compared with those actually observed in the ISM. 

The interstellar medium constitutes 10 % of the mass of the galaxy. It can be subdivided into environments with very low-density hot gas, environments with warm intercloud gas, and regions with denser and colder material (23). H and He gas are the major components of interstellar clouds molecules and submicron dust particles are only present in small concentration (22). Through gas phase reactions and solid-state chemistry, gas-grain interactions can build up complex organic molecules. Silicate and carbon-based micron-sized dust particles provide a catalytic surface for a variety of reactions when they are dispersed in dense molecular clouds 24). In cold clouds such dust particles... 

Cosmic abundances in the interstellar medium are derived by measuring elemental abundances in stellar photospheres, the atmospheric layer just above the stellar surface. Such measurements indicate the amount of elements available for the formation of molecules and particles. Cosmic dust models indicate that up to 80% of the carbon in the photon-dominated diffuse interstellar medium is incorporated into solid aromatic macromolecules and gaseous polycylic aromatic hydrocarbons (41,30). CO gas and C-based ice species (such as CO, CO2, CH3OH and others) may be responsible for up to -25 % of the carbon in cold dense interstellar regions. 

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