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Frozen solids
- The Molecules that Fell to Earth

The temperature of dense molecular clouds (10-40K) is usually too low for chemical reactions to take place amongst those molecules in the gas phase. However, UV light can provide the energy needed to overcome the activation barriers, through the production of ions. Most of the gas-phase chemistry taking place in dense molecular clouds is, therefore, the result of ion-molecule reactions.

By using radio and infrared (IR) telescopes, astronomers have so far identified over 100 molecular species in dense molecular clouds and they think that there are bound to be many more (see Box 2). However, a great number of these species could not have been produced simply by gas-phase chemistry - other processes must also be involved.

Laboratory experiments have shown that another class of process involved would be gas-grain reaction. Dense molecular clouds are so cold that any gas hitting a grain of solid matter (such as the silicates and silicon carbide detected in interstellar dust) will freeze out of the gas phase onto the grain, forming an icy mantle. This freezing-out process is so efficient that the products of gas-phase reactions spend most of their lifetime as icy grain mantles and any species observed in the gas phase, including PAHs, should also be present in the grain mantles.

The grain provides a nucleus onto which those molecular species already synthesised in gas-phase reactions are brought into close contact with one another and react to form species that could not be created by gas-phase chemistry alone.

The same UV light that powers gas-phase reactions will also irradiate the constituents of the icy mantles, instigating further reactions. Laboratory experiments, including some undertaken by Sandford and his team, have shown that when the kinds of ices thought to exist in the dense molecular clouds undergo such irradiation, a whole host of new molecules is created.

Astronomers have identified two basic types of ice mantle that should exist in dense molecular clouds, their composition being dictated by the local H:H2 ratio (see Fig 1). Hydrogen is the major constituent of interstellar clouds, being three to four times more abundant than heavier elements such as carbon, nitrogen and oxygen.

Fig 1. Grain mantle growth and evolution in dense molecular clouds
(Source: Scott Sandford)

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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