Silicate Mineralogy

Small silicate grains (submicron to micron size) exhibit spectral features in thermal emission at 8-13 nm and 16-33 ^m. These silicate features were stronger - and more structured - in Hale-Bopp than in any previously observed comet, allowing us to infer the mineralogy of the silicate grains.

The 8-13 |im spectral region was well observed from the ground from r > 4 AU to 0.92 AU [47,48,49,44,50,51], An example is displayed in Figure 2. One sees 3 maxima at 9.2, 10.0, and 11.2 |im and minor structure at 10.5 and 11.9 |im. The sharp peak at 11.2 |im is attributed to crystalline olivine, (Mg,Fe)2Si04, based on the good spectral match with the measured spectral emissivity of Mg-rich olivine; it was first detected in comet P/Halley [52,53], and subsequently observed in several new and long-period comets [54,55]. The 11.9 |im shoulder is also due to crystalline olivine. The broad 10 |im maximum is characteristic of amorphous olivine [56], Crystalline olivine has a secondary maximum at 10 |im as well.

Wavelength (^m)

Figure 2. The silicate feature in Hale-Bopp (points) versus Halley (solid line), total flux/continuum. Spectral structure is marked [47],

Wavelength (^m)

Figure 2. The silicate feature in Hale-Bopp (points) versus Halley (solid line), total flux/continuum. Spectral structure is marked [47],

The 9.2 nm maximum, first recognized in Hale-Bopp, is a signature of pyroxene, (Mg,Fe)Si03. Both amorphous and crystalline pyroxenes can exhibit a peak near 9.2 ^m [56,57,58], Crystalline pyroxenes have considerable variety in their spectral shape [59] and can contribute to the 10 nm maximum, the structure near 10.5 nm, and the overall width of the observed silicate feature. Thus, the major silicate minerals appear to be present in both crystalline and glassy or amorphous form.

The full 7-45 |im spectral region was observed with the ISO SWS spectrometer at r = 2.9 AU [60], This spectrum shows several strong peaks that correspond to laboratory spectra of

Mg-rich crystalline olivine [58]. Minor structure is attributed to crystalline pyroxene [48]. In contrast, airborne spectra of P/Halley at 1.3 AU (the only other 16-30 jam spectra of a comet) show only weak olivine peaks at 28.4 and 23.8 nm [61,62], That the silicates are Mg-rich is consistent with the elemental composition of the dust measured during the Halley spacecraft encounters [63],

The relative abundances of the various silicate components are difficult to determine from the spectra, because the strength of an observed feature will depend on the temperature of the emitting grains (as well as on their size and shape). The temperature of small silicate grains in a comet coma is determined by the amount of sunlight they absorb at visual wavelengths. Dorschner et al. [57] showed that the absorptivity depends on the Mg/Fe ratio. Pure Mg-pyroxene grains have extremely low absorption at visual wavelengths and, consequently, would be quite cold. For grain radius 0.5 nm, the difference between a pure Mg-pyroxene and an olivine grain with Mg/Fe = 1 can be hundreds of degrees at 1 AU [47]. However, the dust composition data from the Halley encounters showed that the silicates were usually associated with carbonaceous material [64], Any absorbing material adhering to silicate grains will heat them.

Spectral models to match the Hale-Bopp spectra with a mixture of silicate minerals have been presented by Brucato et al. [65], Hanner et al. [47], Wooden et al. [48,66], and Hayward et al. (2000). Wooden et al. proposed that observed changes in spectral shape with heliocentric distance could be explained by temperature differences between more transparent (cooler) Mg-rich crystalline pyroxene and less transparent (warmer) olivine grains. In their model, the crystalline pyroxene grains produce the 9.2 |im maximum in 1997 and constitute ~ 95% of the small silicate grains in the coma, in order to produce observable emission at the cold temperatures.

Hayward et al. [44] fit their spectra with a composite silicate model plus a size distribution of absorbing grains; they fit the 9.2 jxm maximum with amorphous Mg-pyroxene. The same silicate mixture matched all of the 1997 spectra of the central core, the jets, and the background coma. They concluded that, even assuming all silicate components have the same temperature, small pyroxene grains (amorphous and crystalline) must be more abundant than silicate grains of an olivine composition.

Thus, while there is not a single, unique model for the silicates in Hale-Bopp, one can say that pyroxenes (either glassy or crystalline) were more abundant than grains of olivine composition, with an abundance ratio of about 2:1 or higher. Because of its strong emissivity at resonances, olivine produces a strong peak at 11.2 jim with an abundance < 20% of the total small silicate grains.

Not all comets exhibit the strong, structured silicate emission features characteristic of comets Hale-Bopp, P/Halley, C/1990 K1 Levy, and C/1993 Al Mueller (e.g., [54,67]). Whether the silicate grains in other comets are simply clumped into larger, optically thick particles or whether there is a real difference in the silicate mineralogy is not known.

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