Organic Matter in Cosmic Dust

Abstract

Organics are observed to be a significant component of cosmic dust in nearly all environments were dust is observed. In many cases only remote telescope observations of these materials are obtainable and our knowledge of the nature of these materials is very basic. However, it is possible to obtain actual samples of extraterrestrial dust in the Earth's stratosphere, in Antarctic ice and snow, in near-Earth orbit, and via spacecraft missions to asteroids and comets. It is clear that cosmic dust contains a diverse population of organic materials that owe their origins to a variety of chemical processes occurring in many different environments. The presence of isotopic enrichments of D and ¹⁵N suggests that many of these organic materials have an interstellar/protosolar heritage. The study of these samples is of considerable importance since they are the best preserved materials of the early Solar System available.

Keywords: Carbonaceous Matter; Cosmic Dust; Hydrocarbons; Microanalysis; Organics.

Figure 1

(a) SEM image of a stratospheric IDP; (b) corresponding high magnification images by transmission electronmicroscopy showing that the particle is an aggregate of smaller grains; (c) Compositional map corresponding to the TEM image in (b) (Thomas et al. 1993). Note that carbonaceous material, indicated by the light gray regions in (c), is present at the 40–50% level. By comparison, carbonaceous chondrites, meteorites rich in carbon, typically only contain 3–5% carbon.

Figure 2

Spectra of PAHs detected in Cap Prud’homme Antarctic micrometeorites using the L²MS technique in which an initial laser is used to vaporize material from the sample and a second laser ionizes selective molecular species before they are passed into a mass spectrometer (from Clemett et al. 1998). In this case the ionization laser is tuned to give good yields for PAHs. PAHs consist of fused benzene rings and example PAH structures are shown (moving from left to right) for peaks consistent with the PAHs phenanthrene, pyrene, and chyrsene.

Figure 3

Secondary (left) and Backscattered (right) electron micrographs of a fragment of an Ultracarbonaceous Antarctic Micrometeorite deposited on carbon tape. All dark grey patches in the right image are organic matter. Brighter particles are silicates and Fe-Ni sulfides.

Figure 4

Energy-filtered TEM images of a microtome slice of a Stardust particle. (Left) An image obtained using zero-loss filtering to provide contrast improvement. (Right) An image made using energy passbands above and below the 285 ev carbon edge; in such images the presence of carbon is indicated by light areas. The sulfide at the upper right of the images is C free, but regions of C are clearly seen in the fine-grained chondritic material below the sulfide grain (adapted from Matrajt et al. 2013).

Figure 5

Example of an agglomerate dust particle from comet 67P/Churyumov-Gerasimenko collected by COSIMA (Cometary Secondary Ion Mass Analyzer) (from Schulz et al. 2015). The image was obtained using grazing illumination from the right. The length of the shadows indicate that the altitude above the substrate reaches about 100 μm.