Silicate Mineralogy of the Dust in the Inner Coma of Comet C/1995 01 (Hale-Bopp) Pre- and Postperihelion

We present 7.5-13.4 μm infrared (IR) spectrophotometry (R≃180-360) of the 10 μm silicate emission from dust in the inner coma (i.e., within a diameter of 3'') of comet C/1995 O1 (Hale-Bopp) at four temporal epochs from 1996 October through 1997 June during Hale-Bopp's approach to, arrival at, and recession from perihelion. Hale-Bopp's silicate feature is the strongest observed from any comet to date: the flux-to-continuum ratio at 10.0 μm is 2.5 at 2.8 AU preperihelion, increases to 3.0 at 0.93 AU, and then decreases to 2.4 at 1.7 AU postperihelion, dropping more rapidly in strength than expected from preperihelion spectra and indicating a diminishment in the relative abundance of submicron sized grains by perihelion passage. The silicate feature also evolves with heliocentric distance. When far from perihelion, at ~2.8 AU, the High Efficiency Faint Object Grating Spectrometer (HIFOGS) 10 μm silicate feature contains a 9.3 μm shoulder attributable to amorphous pyroxene, broad emission from amorphous olivine (9.7 μm), and the 11.2 μm peak associated with crystalline olivine. Concurrent with the HIFOGS spectra at 2.8 AU, the Infrared Space Observatory (ISO) Short Wavelength Spectrometer (SWS) spectrum also shows the strong far-IR peaks of crystalline olivine at 18 μm, 23 μm, and 33 μm. When close to perihelion the HIFOGS spectra include a newly discovered 9.3 μm peak, identifiable as Mg-rich crystalline pyroxene. We hypothesize that the crystalline pyroxenes are cooler than the other silicates, too cool to be detected at 9.3 μm at 2.8 AU but warm enough to be detected at 1.2 AU. The best-fit empirical model for the temporal evolution of Hale-Bopp's silicate feature constrains the Mg-rich crystalline pyroxenes to be dominated by ortho-pyroxene rather than clino-pyroxene. The crystalline pyroxenes have a color temperature that is 0.6 times cooler than and an abundance that is about 9 times greater than the other silicates at all epochs, assuming equivalent particle size distributions for all components. Comparing crystalline ortho-pyroxene and crystalline olivine in radiative equilibrium, both with Mg number 0.9, the ortho-pyroxene crystals are cooler because they are less absorbing at visible and near-IR wavelengths. Thus, it is the high Mg content of the crystalline pyroxenes that accounts for their cooler temperature. The dominance of Mg-rich pyroxenes among Hale-Bopp's cometary silicates is consistent with PUMA-1 mass spectrometer measurements of comet P/Halley 1986 III and with cometary interplanetary dust particles. The high Mg content of the pyroxenes in comet Hale-Bopp implies that they are either pristine solar nebula condensates or presolar grains such as the Mg-rich crystals recently discovered by ISO around asymptotic giant branch stars.