Gemini-N mid-IR observations of the dust properties of the ejecta excavated from Comet 9P/Tempel 1 during Deep Impact

Abstract

We present mid-infrared spectra and images from the Gemini-N (+MICHELLE) observational campaign of Comet 9P/Tempel 1 before, during, and after its encounter with Deep Impact. We use our thermal grain model to probe the 10 µm properties of the dust grains in the coma of the comet. Before impact (3 July 2005 UT), and more than 24 h after impact (5, 16, and 28 July 2005 UT), the comet dust grains were composed mostly of amorphous olivine, and were relatively large (peak of the grain size distribution a p = 0.7-1.5 µm). For the night of impact, we extract spectra by centering on the nucleus, and offset 1 from the nucleus in the direction of the impact ejecta plume. We find small dust grains (∼0.2 µm) of a diverse mineralogy (amorphous olivine, amorphous pyroxene, amorphous carbon, and crystalline olivine) populating the ejecta. The submicron sized dust grains move faster than the other, larger grains ( 0.7 µm), with amorphous olivine and amorphous carbon traveling together, and amorphous pyroxene and crystalline olivine dispersing at a similar rate. Deriving a velocity law from a time-of-flight analysis, we find that the material traveled with a velocity law scaled by v 0.2 µm = 220 ± 20 m s −1 and with a power of p = 0.5. This velocity power-law requires a sustained release of grains for the duration of 45-60 min after impact. Since the mineral species are traveling at different speeds, and there was a sustained release of grains due to a possible "gas-plume," we conclude that the different minerals did not originate from grain aggregates destroyed by the impact, but instead arise from an inhomogeneous nucleus.

Comet 9P/Tempel 1: Interpretation with theDeep ImpactResults

“…Furusho et al (2007) supported this interpretation by measuring slightly higher linear polarizations for the high-velocity ejecta compared with the low-velocity ejecta. Harker et al (2007) also supported these interpretations from their infrared observations. These conclusions (hereafter KFH model) completely contradict our conventional images of comet nuclei based on the standard model.…”

Cometary dust: the diversity of primitive refractory grains

“…IR spectral features in cometary comae are best modelled by submicrometre solid grains or by micrometre size and larger porous aggregates of submicrometre subgrains. Five compositions suffice to well-fit comet IR spectra: amorphous Mg–Fe silicates (amorphous olivine-like and amorphous pyroxene-like compositions), amorphous carbon and Mg-rich crystalline silicates [17,146–149]. The IR crystalline silicate resonances are best matched with forsterite and ortho-enstatite [32,150].…”

“…Anhydrous CP IDPs with Mg-rich crystals are good analogues to modelled comet particles [17,20,146,154,156]. Modelled comet particles require moderate porosities of 65–80% [41,157], with increasing porosities for larger particles [41].…”

“…From the perspective IR spectroscopy of comet dust, UCAMMs have materials most similar to the comae compositions of observed comets. IR spectra are well fitted with a minimal number of components that include Mg–Fe amorphous silicates, amorphous carbon 28 and Mg-rich crystalline olivine (forsterite) [17,20,33,41,53,144,146,148,150]. These comet dust compositions are deduced from thermal models of the IR emission of fine-grained dust (§8).…”

Low abundance, strong features: window-dressing crystalline forsterite in the disk wall of HD 100546

“…The surface of shorter period comets might be subject to amorphization by solar irradiation, but ejecta from the inside comet 9P/Tempel released during the Deep Impact experiment show that these comets can also have a high crystallinity of 20-40% (Harker et al 2007). …”