The Importance Of Variables Pure Ice Vs Icy Mixtures

Irradiation studies related to cometary analogs are diverse. For example, the composition of the sample to be analyzed, the temperature of irradiation, and the kind of radiation employed are features that can be monitored and modeled. For this reason, experiments can be performed with pure or mixed ices. Evidently, the selection of the components to be incorporated into the model depends on the pursued objectives. Studying simple ice may help us to understand basic processes and to elucidate the reaction mechanism of species involved. On the other hand, icy mixtures contribute to the recreation of more complex interactions, and are closer to reality, but isolating phases is not easy.

3.1.1. Models Related to Pure Ices

As previously stated, most of the experiments that have been conducted used water ice as the main component. The reason for this is that cometary nuclei are composed mostly of water ice (Ehrenfreund et al., 2002). Moreover, water ice is the most abundant icy component in astrophysical environments and in planetary systems; many bodies in the solar system (Kuiper Belt objects, satellites, the nuclei of comets, and some planetary rings) are composed of abundant amounts of water ice (Zheng et al., 2006).

A whole body of research has been conducted in order to understand the processing of water ice by energetic particles and ultraviolet photons. Zheng et al. (2006) present a good review of this theme; they indicate that an ample range of conditions has been explored. The variables include temperature, film thickness, type of radiation employed, and, in some cases, the observed products.

Many authors have studied the irradiation of pure icy water systems with ions of different energies. The first research studies were those of Brown et al. (1978, 1980a, 1980b) who studied films of different thicknesses made of water at different temperatures (15-110 K) irradiated with He+ and H+. Other authors have employed the same particles, although the ion energy was different (Bar-Nun et al., 1985; Shi et al., 1995b; Moore and Hudson, 2000; Baragiola et al., 2003; Leto and Baratta, 2003; Gomis et al., 2004a; Gomis et al., 2004b; Baragiola et al., 2005; Loeffler et al., 2006). Many other radiation sources have been tried; this includes F+ (Cooper and Tombrello, 1984), Ne+ (Bar-Nun et al., 1985; Christiansen et al., 1986; Shi et al., 1995a); N+ (Christiansen et al., 1986; Shi et al., 1995b; Orlando and Sieger, 2003); Ar+ (Christiansen et al., 1986; Baragiola et al., 2003; Orlando and Sieger, 2003; Loeffler et al., 2006); Ar++ (Letto and Baratta, 2003; Gomis et al., 2004b); O+ (Shi et al., 1995a; Shi et al. 1995b; Gomis et al. 2004b); UV radiation (for example, Matich et al., 1986; Gerakines et al., 1995; Leto and Baratta, 2003) and electrons (Christiansen et al., 1986; Kimmel and Orlando, 1995; Sieger et al., 1998; Orlando and Sieger, 2003; Pan et al., 2004, Baraggiola et al., 2005; Zheng et al., 2006).

Despite the wide set of conditions of raw materials and the variarity of experimental conditions explored, the final products in those simulations are common. These include products such as H2, O2, H2O2 (Bar-Nun et al., 1985). The presence of some of these products could be temperature dependent; for example, Moore and Hudson (2000) found no H2O2 production at 80 K, but the product was identified at 16 K when water ice had been irradiated under the very same conditions. Other species, such as radicals are rarely reported; an exception is the work of Gerakines et al. (1995), in which the authors mention the presence of HO2 and OH. The lack of this kind of information could be a consequence of the technical difficulties in reference to the detection of this kind of species (that are extremely reactive) at low temperatures and while irradiation is occurring.

The molecular products formed coincide with those observed for the radiolysis of liquid water (H2, O2, H2O2). The radiolysis of water has been widely discussed by many authors; for a detailed treatise, see Draganic and Draganic (1971), Spinks and Woods (1990), and references therein. When a sample is irradiated, the first effects are excitations and irradiation events. It has been pointed out that radicals and ions that are produced are condition-dependent. For example, the formation of the HO2 radical is dependent on the presence of oxygen and the source of radiation. It is common when irradiating with particles of high Linear Energy Transference (LET). The abundance of other species is dependent on pH; this is the case with e- and H\ which can interconvert from one into the other. The aq '

presence of these intermediates can finally modify the yield of formation of the products. On the other hand, there are other processes that can be followed through sputtering (whose main consequence is mass lost) and amorphization.

Other efforts are directed at the irradiation of other components, such as carbon monoxide. In this context, Loeffler et al. (2005) performed the irradiation of CO, comparing the effect of different sources of energy, UV and ions, and found that the main product in both experiments was CO2. Hudson and Moore (2001) affirm that, in general terms, the species produced by ionizing and UV irradiation are almost the same, but there are differences in terms of yields; that depends on the rate of energy deposition (Linear Energy Transfer, LET).

Brucato et al. (1997a) performed an experiment in which CO2 was irradiated, using the technique of ion implantation (H+). This resulted in the production of carbonic acid (H2CO3), which is an indicator of the incorporation of ions implanted into parent molecules to generate new ones.

3.1.2. Multi-component Systems

Mixed molecular ices are so common, not only in cometary nuclei, but also in grains in the ISM, planets, and satellites (Ehrenfreund and Charnley, 2000). Interstellar and cometary ices include not only pure ice, but also non-polar (dominated by CO, N2, O2 and CO2) and polar (dominated by H2O, and containing CO, CO2 and CH3OH) ices (Bernstein et al., 1997). As can be seen, carbon monoxide can be included in the group of non-polar or polar ices, depending on the presence of other components: water and its particular characteristics make the difference.

The volatile fraction should include mixtures in different proportions and combinations and other materials such as minerals and amorphous carbon. All of these ingredients can be combined and tested in different proportions, temperatures, and by employing different sources of energy. For this reason, the number of experimental simulations is vast. There are experiments in which the volatile fraction is the sole component; in others, besides this, other constituents are involved.

There are a lot of examples which reveal the significance of the synthesis of new molecules by irradiation; molecules that are produced match the signatures observed. Laboratory astrophysics is mainly devoted to the characterization of interstellar molecules and cosmic dust; this allows the comparison of laboratory spectra with astronomical data (Altwegg et al. 1999).

The next paragraphs do not pretend to be exhaustive, but are presented to illustrate the ample assortment and nature of the material formed from irradiation of simple ices.

Carbon monoxide is the second most abundant type of ice after water in the ISM. This and carbon dioxide are important molecules. CO is commonly dispersed through the solar system and the ISM (Hudson and Moore, 2001). In fact, many of the observed lines that contain H2O ice also contain CO (Bernstein et al., 1997). Hudson and Moore (1999) have shown the formation of CO2 and methanol during ion irradiation of water and carbon monoxide mixtures. The source of energy is not a constraining factor in the synthesis of methanol, which is formed both from ion (Hudson and Moore, 1999) and UV irradiation (Schutte et al., 1996) of the same mixture. In reference to H2O:CO mixtures, ion irradiation produces different molecules, many with increased volatility in respect to the raw material, others with less volatility, and some with intermediate volatility (Strazzulla, 1997). Hudson and Moore (1999) propose that the H' and OH' produced from the radiolysis of water are added to CO to form HCO, H2CO, HCOOH, and CH3OH. They also point out one characteristic that has to be taken into account: the yields. In addition, they found higher yields compared to other previous experiments. This demonstrates that ice composition and other characteristics are critical factors in laboratory simulations.

Methane, the most reducing form of carbon, has been extensively studied. Hudson and Moore (1997) irradiated icy mixtures of methane and water at 16 K, and identified the formation of methanol and ethane; the first one was also formed when irradiating water, but, this time, with another molecule, acetylene (Hudson and Moore 1997).

Strazzulla and Baratta (1992) demonstrated that the irradiation of isolated volatile carbon molecules (benzene, butane and methane) or their mixtures with water ice produced a refractory residue. This means that the production of an organic crust formation on comets through irradiation was possible, and the evolution of the residue as a function of irradiation dose was also followed. At low doses (10 eV/C-atom) they observe the conversion of the frozen film into a solid; but, at higher doses (10-25 eV-C-atom), they observed properties of polymer; at ever-higher doses, the compounds changed to an amorphous carbon film.

More than one or two single component models have been studied. In another approximation, Bernstein et al. (1997) simulated the photo processing of polar and non-polar ices. The experiments simulating non-polar ices included irradiation of CO mixtures with different amounts of O2, N2, and CO2. These experiments produced CO2 N2O O3, CO3, HCO, H2CO, and, maybe, NO and NO2. It is thought that hot O atom production dominates the reaction mechanisms;

in this system, the destruction of N2 is difficult. On the other hand, the radiolysis of polar ices with H2O, CH3OH, CO, and NH3 generated small molecules, such as H2, H2CO, CO2, and CH4, as well as the formyl radical (HCO). After photolysis, the warming produces complex species, such as ethanol, formamide, aceta-mide, isonitriles, and/or nitriles, amides, ketones, and polyoxymethylene. So, the differences in initial composition and polarity are clues to the production of more complex molecules.

Cottin et al. (2001) irradiated a mixture containing H2O, CO, CH3OH and NH3 with both UV and proton sources. They identified the presence of hexame-thylenetetramine (HMT) in organic residues. For the first time, HMT was detected after proton irradiation of an interstellar or cometary ice analog. In fact, it was thought that this molecule was a characteristic signature of UV processing.

Probably one of the most significant examples of evidences of important compound synthesis is the work of Kobayashi et al. (1995). They reported that, when irradiating propane (methane or carbon monoxide) in the presence of water and ammonia, and after an acid hydrolysis, amino acid production was detected by ion chromatography.

In reference to this point, the importance of the production of more refractive materials, when irradiating simple icy molecules, is fundamental in explaining the emission of other molecules, for example CO (Brucatto et al., 1997b) and in order to explain the abundance of detected molecules. This means that the rate of production and the stabilization of species in the irradiated samples need to be considered.

As has been revealed in those representative examples, organic material is formed when irradiating simple ices. Other experiments with bulk irradiation of simple molecules (H2O, HCN) reveal the formation of complex organic compounds, which can be used as raw materials for the synthesis of bioorganic compounds. Among the products detected after the gamma irradiation of a frozen solution of HCN were CO2, ammonia, urea, some simple amines, an oligomeric material that upon hydrolysis yields amino acids, carboxylic acids and other products such as adenine. (Colin-Garcia et al., 2008).

In addition, most of the material formed is consistent with those supposed to be in the nucleus of a comet; many studies are focused on matching the signatures obtained from studies in situ and those done on the basis of observation. But production is not the most important feature; the volatility, stability, and reactivity of raw compounds and those formed by irradiation need to be considered. So, experiments of thermal stability and observations of change in the structure in ice and other components have to be considered for a better understanding of phenomena occurring in comets and in all the icy surfaces in the interstellar medium.

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