Interstellar ice compositions

Observed abundances normalized to that of water ice (=100). W33A and NGC 7538-IRS 9 are two luminous protostars which span the observed range in interstellar ice composition. The abundances for the comets are an average of those observed for comets Hyakutake and Hale-Bopp. 

Although information on the chemical composition and reactivity of interstellar ices can be obtained only from remote observations and laboratory simulations, cometary ices and dusts are subject to direct studies, eg within Vega and Giotto (comet Halley), Stardust (comet Wild 2) and Rosetta (comet Churyumov-Gerasimenko) missions (Figure 8.12). 

Yamamoto et al. [2] made a condensation calculation to estimate tlie chemical composition of the ice in molecular clouds and cometaiy nuclei. They assumed tlie interstellar molecular composition for tlie abundance of gas. Inter-... 

The recent ai roach of large comets such as IP/Halley, C71996 B2 Hyakutake, and C/1995 Ol Hale-Bopp to the Earth provided a good opportunity to investigate the detailed composition of cometary ices by various methods such as mass spectrometry, infrared spectroscopy, and radio emission. The composition of interstellar ices is compared with that of the cometary ices in Table 9.3. It is striking that cometary and interstellar ices have quite comparable relative molecular abundances. 

The picture of mixed molecular interstellar ice described up to this point is supported by direct spectroscopic evidence (e.g. Figures 2, 3). The identities, relative amounts and absolute abundances of the ice species listed in Table I are sound (see references 6 and 7 and references therein for detailed discussions). However, this is not the entire story. Indeed, from a chemist s perspective, this is only the beginning of the story. As mentioned above, throughout the cloud s lifetime, processes such as accretion of gas phase species, simultaneous reactions on the surfaces involving atoms, ions, and radicals, as well as energetic processing within the body of the ice by ultraviolet photons and cosmic rays all combine to determine the ice mantle composition (5-7). Theoreticians are... 

Two types of models have been proposed that use this general picture as the basis for understanding volatile depletions in chondrites. Yin (2005) proposed that the volatile element depletions in the chondrites reflect the extent to which these elements were sited in refractory dust in the interstellar medium. Observations show that in the warm interstellar medium, the most refractory elements are almost entirely in the dust, while volatile elements are almost entirely in the gas phase. Moderately volatile elements are partitioned between the two phases. The pattern for the dust is similar to that observed in bulk chondrites. In the Sun s parent molecular cloud, the volatile and moderately volatile elements condensed onto the dust grains in ices. Within the solar system, the ices evaporated putting the volatile elements back into the gas phase, which was separated from the dust. Thus, in Yin s model, the chondrites inherited their compositions from the interstellar medium. A slightly different model proposes that the fractionated compositions were produced in the solar nebula by... 

Interstellar grains with ice mantles probably comprised a significant amount of the material that collapsed to form the solar nebula. Heating of this material caused the icy mantles to sublimate, producing a vapor that subsequently condensed as crystalline ices as the nebula cooled. By mass, H20 ice rivals rock in terms of potentially condensable matter from a gas of cosmic composition. The amount of water ice depends, of course, on the extent to which oxygen is otherwise tied up with carbon as CO and/or C02 (Prinn,... 

Figure 18 D/H ratios of several comets compared to the oceans (SMOW), planets, the solar nehula (PSN), and the interstellar medium. Low-temperature fractionation processes increase D/H. Jupiter and Saturn have compositions close to the original nehular composition, hut low-temperature formation of ice caused the enhancements seen in Uranus and Neptune (the ice giants) and comets. The discrepancy between the plotted LP comets and SMOW argues against these comets providing Earth with a major fraction of its water. Other comets, formed in warmer environments, near Jupiter, could he more similar to SMOW (source Huehner, 2002).

Ip and Fernandes [ 101] calculated that 6x lO to 6x lO s g of cometry material could liave been delivered to Earth at the time of the formation of the great Oort Cloud of comets. This amount is equivalent to 4-40 times the present mass of the oceans, assuming about 50% of the cometaiy mass is ice. Owen and Bar-Nun [102] examined tlie ability of amorphous ice formed at temperatures below lOOK to trap ambient gases. By comparison of the compositions of gases trapped by ice with tlie compositions of the interstellar medium, comets, and planetary atmospheres, Owen and Bar-Nun [102] concluded that icy comets delivered a considerable fraction of the volatiles to the imier planets. Owen [83] emphasized that Uie potential supply of cometaiy materials is more than adequate. 

The chemical composition of ices in space is inferred theoretically on the basis of condensation theory, which predicts tlie composition of solids condensed from gas of Uie cosmic abundance of elements. In Table 9.2, chemical compositions of ices and corresponding equilibrium condensation temperatures are shown in protosolar nebula [1] and interstellar molecular clouds [2],... 

Infrared spectroscopy enables us to obtain information on the chemical composition and structure of icy grains in interstellar molecular clouds [3], Table 9.3 summarizes the abundance of molecules identified [4]. Among these species, the predominance of H2O ice is clear, its abundance being one order of magnitude greater than that of all odier molecules. The molecules CO and CO2 are those next most abundant, following H2O. Small amounts of reduced molecules, hydrocarbons and NH3 are also observed. 

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