Ice mantles

It is highly unlikely that all organic molecules have gas-phase formation routes and many may be formed on the surface of dust grains, probably with ice mantles,... 

Giant molecular clouds the GMCs have a lifetime of order 106—10s years and are the regions of new star formation. The Orion nebula (Orion molecular cloud, OMC) is some 50 ly in diameter and 1500 ly from Earth. The temperature within the cloud is of order 10 K and the atomic density is 106 cm-3. The chemical composition is diverse and contains small diatomic molecules, large polyatomic molecules and dust particles covered with a thick ice mantle. 

The cocktail of molecules, however, depends on the composition and condition of the cloud and the allowed chemistry on the surface or within the ice mantle. A complex network of chemical reactions exists, taking in energy from the... 

Ice mantles are important constituents of interstellar grains in molecular clouds, and icy bodies dominate the outer reaches of the solar system. The region of the solar system where ices were stable increased with time as the solar system formed, as accretion rates of materials to the disk waned and the disk cooled. The giant planets and their satellites formed, in part, from these ices, and probably also from the nebular gas itself. 

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,... 

Concerning ices, it has been discussed that they must be amorphous (Smoluchowski 1983) in the interstellar medium and not crystalline. This implies that the adsorbed H atoms are localized in deep traps so that their wavefunctions have a limited spatial extent. This fact reduces significantly their mobility and hence the interaction with another H atom absorbed on another site is slow as compared to the residence time unless the two atoms happens to be localized near each other. This phenomenon reduces the rate of H2 formation by several orders of magnitude when compared to the situation on crystalline surfaces. Computational simulations on soft and hard ice model surfaces have shown that for a cross-section of 4,000 nm2 the reaction probability is 1 (Takahashi et al. 1999). Furthermore, the H2 formed, due to the high amount of energy liberated is rapidly desorbed in an excited state from the ice mantle in timescales of 500 fs (Takahashi et al. 1999). 

The ice mantles formed on the surface of dust particles can be divided into two main categories according to the chemical composition, which in turn result from... 

Figure 7.2 The relation between the particle growth in the disk mid-plane traced by the millimeter opacity index and that of the inner disk surface traced by the 9.7 pm silicate emission feature. The star symbols represent individual disks. Data points are from van Boekel etal. (2003), Natta etal. (2004),Furlanc/ al. (2006), Rodmann et al. (2006), and Lommen el al. (2007). Typical errors are 10-30% in both /3 and silicate band strength. Note also that differences in how the silicate band strengths were derived may introduce slight systematic offsets for the different data sets. The circle symbols represent dust opacity models calculated for the interstellar medium at a range of densities. From top to bottom the circles are Ry = 3.1 and Ry = 5.5 from Weingartner Draine (2001), a Spitzer-constrained dust opacity for dense clouds from Pontoppidan et al. (in preparation) and the particle growth simulation for protostellar envelopes [thin ice mantles, Ossenkopf Henning (1994)].

This is relevant for calculating extinction of dust grains coated by an ice mantle, e.g. for dust in molecular clouds or in the cool (T < 150 K) parts of accretion disks. 

Figure 9 Comparison of silicate mass fractions. Two assumptions for interior strueture are shown (i) differentiated—rock core, ice mantle, and (ii) homogeneous—uniformly mixed ice and roek. Also shown are silicate mass fractions for the Jupiter and Saturn systems and expected values for two models of the early solar nebula carbon chemistry (see text) (after Johnson et aL, 1987) (reproduced by permission of Ameriean Geophysieal Union from /. Geophys. Res. Space Phys. 1987, 92, 14884-14894).

This summarizes the present state of our knowledge about Triton, hi size, mass, and mean density, Triton appears to be a larger and more massive variant of Pluto, since their mean densities are both nearly 2.1 grams/ cm. A plausible model for Triton s interior is one with a rocky core of about 621 mi (1,000 km) radius surrounded by a 217 mi (350 km) thick water ice mantle, above which there is a crust of nitrogen, methane, carbon monoxide, and carbon dioxide ices which is only a few miles thick. 

Fig. 41. Schematic diagram of ice calorimeter, A, outer jacket B, cap C, Dewar vessel F, tube for addition of reagents J, semi-microburette M, protrusions to hold ice mantle N, ice mantle.

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... 

Plate 6. The Greenberg model of interstellar ice mantle formation and chemical evolution. The mantle grows by condensation of gas phase species onto the cold dust grains. Simultaneously, surface reactions between these species, ultraviolet radiation and cosmic ray bombardment drive a complex chemistry. These ice-mantled grains are thought to be micron sized at most. Plate reproduced with permission from (37). (See page 6 of color inserts.)... 

The formation of stars occurs through the gravitational collapse of individual galactic molecular cloud cores within dense clouds. As cold interstellar gas and ice-mantled dust grains collapsed onto the protosolar nebula. 

Mathis et al (1976) fitted the observed extinction curve O.ll A/p 1 by a least square fit with size distribution and chemical composition (within plausible restrictions) as free parameters. Graphite was found to be a necessary component, no ice mantles were needed for the fit and the derived particle size distribution can be approximated by n(a) aa within the size range l a/y O.OOl. 

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