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Stardust's seven year mission is to collect samples of the interstellar dust …
Fig 1. Grain mantle growth and evolution in dense molecular clouds
Polar ices are produced when the H:H2 ratio is greater than one and excess atomic hydrogen is free to react with other elements. The main component is water, but polar ices also contain methanol (which drives much of the interstellar ice photochemistry and gas phase chemistry), carbon monoxide, ammonia, carbon dioxide, formyl radicals, formaldehyde, carbonyl sulphide and dihydrogen. Non-polar ices are created when the H:H2 ratio is less than one and elements other than hydrogen are able to react together. So non-polar ices comprise O2, N2, CO and CO2, with minimal amounts of H2O. These two types of ice mantle may well exist simultaneously in different parts of the same cloud.
Not surprisingly, perhaps, the photochemistry in non-polar ices, which is driven by hot oxygen atoms liberated by photolysis of O2 or CO, produces no interesting products, at least in terms of organic molecules. The principal species produced are carbon dioxide, nitrous oxide, ozone, carbon trioxide, with some evidence for minor amounts of nitrogen oxide and nitrogen dioxide. Formyl radicals and formaldehyde may also be produced if there is some water in the ice.
Polar ice irradiation results in the destruction of some species (such as methanol) and the synthesis of others, such as dihydrogen, formyl radicals, formaldehyde, methane, carbon dioxide, and an as yet unidentified isonitrile, termed XCN. Of these products, only XCN synthesis specifically needs irradiation; all the others could also be, and probably are, produced by gas-phase or gas-grain chemistry.
Things really start to get interesting, however, at least in the laboratory, when the polar ices are warmed up. The additional energy allows new bonds to be formed as reactive species become mobile.
At a temperature of 200K, many of the parent species, as well as the new photoproducts, sublime out of the ice and, at this point, moderately complex organic molecules, such as ethanol (CH3CH2OH), formamide (HC(=O) NH2), acetamide (CH3C(=O)NH2) and nitriles and isonitriles (R-C(integral)N and R-N(integral)C), including XCN, are detectable spectroscopically.
At room temperature, species with even greater complexity are produced in the ice, such as hexamethylenetetramine (HMT, C6H12N4), polyoxymethylene-related species (POM, (-CH2O-)n), ketones (R-C(=O)-R) and amides (R-C(=O)NH2). On Earth many of these species are biologically important, with formaldehyde, nitriles and ethanol all identified as necessary precursors in the production of proteins, phospholipids, and RNA and DNA. Encouragingly, several of these complex organic molecules have recently been detected in clouds by IR telescopes.
Furthermore, experiments performed by Sandford and his co-researchers have shown that when HMT is hydrolysed in acid, amino acids are spontaneously produced (a simpler method for forming amino acids than the extended exposure of methane, ammonia and water vapour to an electrical discharge in the presence of excess hydrogen as described by Miller and Urey in 1953). And when the residue left over from warming polar ices to room temperature is placed in water, insoluble lipid-like droplets are formed, which show self-organising, membrane-forming behaviour.
This is what happens in the laboratory, but how might the molecular cloud be warmed in space? Well, short lived, transient heating may occur as a result of cosmic shock-waves or grain collisions. A more continuous source of heat might be provided by a protostar forming as the dense molecular cloud undergoes gravitational collapse.
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