Grain Dust

Particles found in a diffuse cloud show a simpler chemistry consisting of silicate core, some carbonaceous particles and a thin ice mantel containing a small 

Mantle of refractory organic material Silicate core 

The primary process initiating dust surface chemistry is the collision of a molecule from the ISM with the surface. The sticking probability is a measure of how often molecules will stick to the dust surface but this depends on the collision energy, the temperature of the grain surface and the nature of the chemical surface itself. The silicate surface is highly polar, at least for a grain of sand on Earth, and should attract polar molecules as well as atoms. The adsorption process can also be reversed, resulting in thermal desorption, both as the reverse of adsorption and by new molecules as the product of surface reactions. 

Heterogeneous catalysis is also proposed for the formation of the ice mantels around the particles. Co-adsorption of H, O and N atoms leads to the formation of water and ammonia-water ice on the surface, as deduced from ISO spectra. Adsorption of CO onto the ice surface provides a carbon source to initiate organic synthesis, for example, in the simple sequence of reactions  

Surface chemistry increases the molecular diversity in the ISM, further enhanced by the presence of UV or far-UV radiation. Photolysis of molecules on the grain surfaces encourages radicals to be produced that undergo surface reactions but the rates of these fundamental processes are as yet unknown. Penetration of the radiation is controlled by the thickness and composition of the ice mantel and may protect larger molecules from photo-destruction. The dust surface is a fertile ground for organic synthesis. 

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In the dense interstellar medium characteristic of sites of star fonuation, for example, scattering of visible/UV light by sub-micron-sized dust grains makes molecular clouds optically opaque and lowers their internal temperature to only a few tens of Kelvin. The thenual radiation from such objects therefore peaks in the FIR and only becomes optically thin at even longer wavelengths. Rotational motions of small molecules and rovibrational transitions of larger species and clusters thus provide, in many cases, the only or the most powerfiil probes of the dense, cold gas and dust of the interstellar medium. 

What is the ultimate fate of the molecular material formed in the envelopes of carbon-rich stars as it heads out into space The dust grains will be processed only slowly by the interstellar radiation held and survive almost intact until they become part of an interstellar cloud. The survival of individual PAHs depends on their size the larger ones withstand radiation much better than do the smaller ones.115 By survival we are referring to the aromatic skeleton the interstellar radiation field will efficiently break H bonds and cause ionization so that unsaturated, ionized PAHs are likely to dominate those found in the diffuse interstellar medium. Such species have been suggested as a source of the DIBs.118,123 Small molecules photodissociate in the interstellar radiation field before the material becomes part of an interstellar cloud. 

Dust is probably present in the DLA and significantly affects the observed abundances. I counted 55 systems for which both Fe and Zn are measured, which are plotted in Fig. 1. The abundance of Fe is always found below that of Zn. By analogy with the interstellar medium, this behaviour is interpreted as the effect of some Fe being locked into dust grains. Other indicators for the presence of dust... 

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,... 

A complex chain of carbon insertion reactions is thought to initiate the formation of PAHs in the ISM and on the surface of dust grains by the formation of carbon chains ... 

The importance of organic synthesis within the ice mantel of dust grains remains poorly understood but there are some important laboratory reactions that point to... 

Decide on a network of chemical reactions gas-phase reactions surface of dust grains photochemical processing. 

Dust The role of dust grain surfaces in inducing different chemical processes enriching the ISM molecular inventory... 

Molecular inventory An extension of the idea of molecular processing on dust grains in the interstellar medium to the surface of a comet... 

The early period of the Earth s history, known geologically as the Hadean, is associated with huge bombardment of the surface of the planet by meteors and comets. The sequence of events in the Earth s formation is shown in Figure 7.3, starting with the Hadean. Any volatile materials on the surface of dust grains or planetesimals deposited on the Earth will be removed and become part of the atmosphere, or more generally the volatile component inventory of the planet. 

CHON Carbon, hydrogen, oxygen and nitrogen species found in the ice mantels of dust grains. 

Interstellar ice The formation of ice layers or mantles on the surface of interstellar dust grains formed by the adsorption of O and H separately, forming water ice on the surface. 

The lithium resonance doublet line X 6707 is fairly easy to observe in cool stars of spectral types F and later, and it has also been detected in diffuse interstellar clouds. There is thus an abundance of data, although in the ISM the estimation of an abundance is complicated by ionization and depletion on to dust grains. The youngest stars (e.g. T Tauri stars that are still in the gravitational contraction phase before reaching the main sequence) have a Li/H ratio that is about the same as the Solar System ratio derived from meteorites, Li/H = 2 x 10-9, which is thus taken as the Population I standard. 

Grain dust Grain processing Irritation eyes, mucous membranes, Sensitization Asthma Moderate (N/A)... 

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