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

David E. Harker, Charles E. Woodward, Diane H. Wooden, R. Scott Fisher, Chad A. Trujillo

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40 Scopus citations

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 ap = 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 v0.2 μ m = 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.

Original languageEnglish (US)
Pages (from-to)432-453
Number of pages22
JournalIcarus
Volume190
Issue number2
DOIs
StatePublished - Oct 2007

Bibliographical note

Funding Information:
D.E.H. and C.E.W. acknowledge partial support for this work from NSF Grant AST-0205814. D.E.H., D.H.W. and C.E.W. also acknowledge partial support for this work from NASA Planetary Astronomy Grant RTOP 344-32-21-04. The authors also would like to thank the Gemini Observatory staff for their support in conducting these challenging observations, and to the Associate Director, J.R. Roy for facilitating this program. Finally, the authors would like to thank Dr. Sugita and the entire Subaru Deep Impact team for a constructive collaborative effort on the night of impact, and the referees who helped improve this manuscript.

Keywords

  • Comet Tempel-1
  • Infrared observations

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