Abstract
Thermochemical equilibrium calculations predict gas phase, gas-grain,
and solid phase reactions as a function of pressure and temperature
in the solar nebula. However, chemical reactions proceed at different
rates, which generally decrease exponentially with decreasing temperature.
At sufficiently low temperatures (which vary depending on the specific
reaction) there may not have been enough time for the predicted equilibrium
chemistry to have taken place before the local environment cooled
significantly or before the gaseous solar nebula was dispersed. As
a consequence, some of the high temperature chemistry established
in sufficiently hot regions of the solar nebula may be quenched or
frozen in without the production of predicted low temperature phases.
Experimental studies and theoretical models of three exemplary low
temperature reactions, the formation of troilite (FeS), magnetite
(Fe3O4), and hydrous silicates, have been done to quantify these
ideas. A comparison of the chemical reaction rates with the estimated
nebular lifetime of 0.1-10 million years indicates that troilite
formation proceeded to completion in the solar nebula. Magnetite
formation was much slower and only thin magnetite rims could have
formed on metal grains. Hydrous silicate formation is predicted to
be even slower, and hydrous silicates in meteorites and interplanetary
dust particles probably formed later on the parent bodies of these
objects, instead of in the solar nebula.
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