THE 8-13 MICRON SPECTRA OF COMETS AND THE COMPOSITION OF SILICATE GRAINS

We have analyzed the existing spectra of seven comets which show an em ission feature at 7.8-13 mum. Most have been converted to a common cal ibration, taking into account the SiO feature in late-type standard st ars. The spectra are compared with spectra of the Trapezium, interplan etary dust particles (IDPs), laboratory mineral samples, and small par ticle emission models. The emission spectra show a variety of shapes; there is no unique ''cometary silicate.'' A peak at 11.20-11.25 mum, i ndicative of small crystalline olivine particles, is seen in only thre e comets of this sample, P/Halley, Bradfield 1987 XXIX, and Levy 1990 XX. The widths of the emission features range from 2.6 to 4.1 mum (FWH M). To explain the differing widths and the broad 9.8 mum maximum, gla ssy silicate particles, including both pyroxene and olivine compositio ns, are the most plausible candidates. Calculations of emission models confirm that small grains of glassy silicate well mixed with carbonac eous material are plausible cometary constituents No single class of c hondritic aggregate IDPs exhibits spectra closely matching the comet s pectra. A mixture of IDP spectra, particularly the glass-rich aggregat es, approximately matches the spectra of comets P/Halley, Levy, and Br adfield 1987 XXIX. Yet, if comets are simply a mix of IDP types, it is puzzling that the classes of IDPs are so distinct. None of the comet spectra match the spectrum of the Trapezium. Thus, the mineralogy of t he cometary silicates is not the same as that of the interstellar medi um. The presence of a component of crystalline silicates in comets may be evidence of mixing between high- and low-temperature regions in th e solar nebula.