The mineralogy of cosmic dust: astromineralogy

Abstract

Stardust is newly-formed in the ejected shells of gas that surround stars towards the end of their lives. Observations of the thermal emission from this dust, which is at relatively low temperatures ( T = 50–200 K), in the circumstellar shells around these stars indicate that the dust consists of both amorphous and crystalline materials. The observed solid phases include: almost pure crystalline Mg-rich silicates (forsterite and clinoenstatite), amorphous silicates, diopside, spinel, oxides (corundum and Fe 0.9 Mg 0.1 O), and also carbon-rich solids such as: (hydrogenated) amorphous carbons, aromatic hydrocarbons and silicon carbide. Crystalline grains with isotopic signatures that indicate that they formed around evolved stars, and that therefore pre-date the formation of the solar system ( e.g. , the pre-solar silicate, nanodiamond, silicon carbide, graphite, corundum, spinel, hibonite, titanium carbide and silicon nitride grains), have now been extracted from primitive meteorites. Pre-solar forsterite and amorphous silicate grains have also been extracted from interplanetary dust particles. The dust formed around evolved stars is ejected into the surrounding interstellar medium by the relatively benign effects of stellar winds, where it is subject to stochastic and violent processing in fast supernova-generated shock waves. In this medium between the stars all the silicate dust appears to be completely amorphous and it is generally thought that dust processing, via ion irradiation/implantation in shocks and/or by cosmic rays, leads to the amorphisation of the crystalline silicate grains that were formed around the evolved stars. The formation of the dust, in circumstellar environments and subsequently ejected into the interstellar medium, is thus balanced by destructive processes that erode and eventually destroy it. This paper introduces the subject of interstellar, circumstellar and pre-solar dust composition (Astromineralogy), discusses where the dust comes from, how it evolves and what its eventual fate might be.