Subaru/COMICS Mid-Infrared Spectroscopic Observations of the Dust Plume from Comet 9P/Tempel

We carried out mid-infrared observations of the Deep Impact (DI) collision with a Jupiter-family (JF) comet, 9P/Tempel, using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the 8.2 m Subaru Telescope. The N-band spectrum (8–13 μm; $R ∼ 250$) of the dust ejecta from the comet nucleus 3.5 hours after the DI event shows a strong crystalline silicate feature. The Subaru/COMICS observation also reveals that small (< 1 μm) amorphous pyroxene and carbon-rich grains flew away in the forefront of the ejecta plume before our spectroscopic observation, leaving behind relatively large submicron (0.8–1 μm) crystalline particles that we subsequently observed. The submicron grains within the subsurface layer of comet 9P/Tempel’s nucleus have a higher crystalline-to-amorphous silicate ratio than that in the comae of other JF comets, and even that of Oort cloud comets. Near the comet nucleus 3.5 hours after the impact the crystalline-to-amorphous ratio is similar to that of Hale-Bopp coma dust at 0.97 AU.