The thermodynamics of dust formation: Evidence from meteorites

A substantial fraction of interstellar dust probably formed in the nebulae around protostars, a setting similar to that envisioned for meteoritic material. From studies of the mineralogy and composition of meteorites it is possible to obtain quantitative information on the conditions that prevailed in the nebula. For example, pressures in the range 10^–3 to 10^–6 atm are indicated. At these pressures the kinetics of nucleation and grain growth are favorable.The fact that the gas associated with interstellar dust has solar H/S ratios indicates that FeS, which forms at 680 K, independent of pressure, is not present in the dust. Since iron only becomes oxidized at even lower temperatures, also via pressure-independent reactions, oxidized iron is not expected in the dust. If most interstellar dust forms in nebulae and is ejected back into space, a relatively high temperature is implied, 700K. Dust formation around stars with high C/O ratios is expected to produce minerals found in the highly reduced enstatite chondrites.High-temperature fractionation processes ( 1000 K) played an important role in the nebula. Much of the Al, Ca, Ti, etc., evidently condensed and accreted into cm-sized objects, some of which are found in carbonaceous chondrites. These objects are explicable in terms of formation from a cooling neutral gas with cosmic composition. Their most important distinguishing characteristics are low volatile and low Si contents, coupled with high refractory element contents constrains formation via isothermal compression to grain temperature 1000 K.Invited contribution to the proceedings of a workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.