3.3. Interstellar Depletions
"Where have all these atoms gone?"
-- J. Mayo Greenberg 
Derivations of the relative abundances of the elements in our Galaxy are one of the principal needs for understanding the chemical evolution in interstellar space - and ultimately its memory in comets. A major factor in developing consistent dust models was the observation of the "depletion" in low density clouds (atoms locked up in grains are "depleted" from the gas phase) using the UV absorption line spectroscopy as a probe of the gas-phase abundances and assuming a reference abundance (abundances of atoms both in gas and in dust).
The deduced possible dust composition was initially only constrained to the extent that silicates alone could not be responsible for the interstellar extinction (Greenberg 1974). But in recent years, the problem of grain modelling has been exacerbated by the apparent decrease of the available condensible atoms (O, C, N, Si, Mg, Fe) by about 30% (Snow & Witt 1996) since the solar system was born. (25) This implies that the heavy elements are being consumed more than they are being created. However, if one goes back far enough in time, there were no condensible atoms because their initial production must follow the birth of stars. This brings us to the cosmological question of what do high-z galaxies look like and when and how was dust first found in them?
25 But see also Sofia & Meyer (2001), who argue that interstellar abundances are approximately solar. It is also possible that the solar system formed out of material with a higher metallicity than the average ISM at that time (there is evidence that stars with planets have higher metallicities than equal age stars without planets). So the metallicity of the ISM may not have declined since the solar system was formed (Draine, private communication). Back.