Upper panel: spectra of several comets, for comparison with 8P/Tuttle. The data are from Crovisier et al. (1997) for Hale–Bopp, from Lisse et al. (2006) and Kelley & Wooden (2009) for 9P/Tempel 1 (pre- Deep Impact), and from Reach et al. (2010) for 17P/Holmes (soon after the November 2007 explosion). The spectrum of 17P/Holmes has been divided by 10 for clarity. Dashed lines correspond to a blackbody continuum of 220 K for Hale–Bopp, 240 K for 17P/Holmes, and 270 K for 9P/Tempel 1. The continuum for 8P/Tuttle is not shown for clarity, but it is identical to that of Fig. 8. Middle panel: continuum-divided spectra, using the same color code as in the upper panel. Error bars are indicated. For 8P/Tuttle, this is the same spectrum as in Fig. 10. The contrast of the spectrum of 8P/Tuttle is far lower than that of other comets. Lower panel: continuum-divided spectra, scaled to facilitate comparison, so that they all have the same value at 11 μm. Same color code as in the other panels. The black spectra at the bottom are those of crystalline forsterite (solid line) and amorphous pyroxene (dash-dotted line), extracted from Reach et al. (2010) for grains with a diameter of 2 μm. The vertical gray lines highlight the position of the amorphous pyroxene emission around 18 μm observed on comet 8P/Tuttle compared with that of forsterite at 19.2, 23.7, and 27.8 μm observed on the other comets.
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