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Infrared spectra of dust, showing the main spectral lines.The emission features at 3.3, 6.2, 7.7, 8.6 and 11.3 ^m are probably from PAHs. [ESA/ISO, SWS, F.J. Molster et al.]

Infrared spectra of dust, showing the main spectral lines.The emission features at 3.3, 6.2, 7.7, 8.6 and 11.3 ^m are probably from PAHs. [ESA/ISO, SWS, F.J. Molster et al.]

PAHs. They can be identified through some of their emission lines, like those shown in Fig. 14.7. They are stable at high temperatures, and were originally proposed to explain diffuse emission from hot extended clouds.

It also seems likely that many grains, especially the larger ones, are not of uniform composition. Rather, they have layers, like the schematic grain in Fig. 14.7. There could be a core of silicates and carbon. Outside of that are various mantles, one with water and ammonia ice and another with O, N and CO (carbon monoxide) all in solid form. There may even be a thin outer layer of hydrogen.

Inner Mantle

Inner Mantle

Outer Mantle

Water Ammonia

Outer Mantle

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