Irradiation interstellar grains

The detection of diamino acids in the Murchison meteorite (41) has stirred up the speculation that these compounds are perhaps direct products of interstellar synthesis processes. In particular, laboratory experiments aimed at the ultraviolet (UV)-irradiation of interstellar ice analogs leads to the formation of refractory material which, after hydrolysis with 6 M HCl, has been shown to contain mono-amino acids as well as diamino acids (42). Although it is evident that some organic compounds that were formed in the interstellar medium, either in the gas phase or in interstellar grains, survived exposure to the interstellar... 

Laboratory investigations confirm that crystalline silicates form in stellar outflows and in protoplanetary disks. In contrast, dust grains in the ISM are dominated by amorphous materials less than 2.2% of the grains are crystalline silicates (Kemper et al 2005). Laboratory simulations of the harsh interstellar radiation fields demonstrate that ion irradiation of crystalline silicates quickly leads to their amorphization (e.g. Jager et al. 2003 Brucato et al. 2004). 

In order to investigate the primary processes responsible for photocatalytic reactions Moesta and Breuer (1968) performed experiments in which simple gases, some examples of which are given later, were adsorbed on clean metal surfaces and subsequently irradiated by UV light. The different metals used in these experiments certainly do not resemble the chemical and physical composition of interstellar dust grains, but they at least offer well defined surfaces and thus avoid introducing errors due to impurities which may lead to a misunderstanding of the primary processes. 

Solid icy surfaces are observed both in the interstellar medium as mantles on silicatic or carbonaceous grains and on many objects in the Solar System." In space, these icy targets are continuously bombarded by energetic ions from solar wind and flares, planetary magnetospheres, stellar winds and galactic cosmic rays. When an energetic ion collides with an icy target produces physico-chemical modifications in the latter. The study of those effects is based on laboratory ion irradiation experiments carried out under physical conditions as close as possible to the astrophysical ones. 

Figure 4 Molecular formation on interstellar dust grains. 1 )An ice layer is formed by accretion ontodustgrains. 2) The icy layer is exposed to irradiation from the ISM. 3) Further ice is accreted on top of the irradiated layers. 4) Icy material and molecules are ejected from the ice into the gas phase through heating events such as shocks and grain-grain collisions.

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