Modeling the dust composition

The grains that formed in the solar nebula are believed to have been produced by direct condensation as the nebular gas cooled (see also Chapter 4). If the thermodynamic properties of the reactants and mineral products are known, it is possible to model the condensation of solids by allowing a gas of any initial composition to cool. Larimer (1967) pioneered the application of this technique to the condensation of minerals in the solar nebula. A limitation of it is that minerals whose thermodynamic properties have not been determined cannot be included in the modeling, and computer limitations generally require that the number of minerals that are included in any model must be restricted. 

A detailed comparison of the predictions of these models with direct observations of the minerals present in dust from the primitive asteroids and comets provides a critical test of the accuracy of the presumed starting composition. For example, the effect of raising the C/O ratio in the initial composition results in some of the oxide and silicate minerals being replaced by carbides and carbonates (as discussed by Lodders 2003 Ebel 2006 and references therein). 

Gail (2002, 2004) has set up a series of large-scale simulations predicting the abundances of minerals formed in the early Solar System as based on assumptions on the input dust materials from the interstellar medium (ISM), and condensation, 

Spectroscopy allows one to see not only the elements in the gas phase, but also those in the dust grains. Using bright X-ray binaries as background sources, Ueda et al. (2005) were able to determine the total abundances of the elements in the diffuse ISM and found that most of them are approximately solar, with the exception of oxygen. From the details of the X-ray absorption spectra it could be determined to some extent what fraction of the elements were in the solid phase and also to a lesser extent the lattice structure could be determined. From this, they found the silicates to be predominantly rich in magnesium and poor in iron, in agreement with the infrared absorption spectroscopic study of the diffuse ISM by Min et al. (2007). 

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