Spectropolarimetry of the Deep Impact comet 9P/Tempel 1

David M. Harrington
Karen J. Meech
NASA Astrobiology Institute
Institute for Astronomy
University of Hawaii
dmh@ifa.hawaii.edu

Ludmilla Kolokolova
Astronomy Department
University of Maryland

Jeff R. Kuhn
Kathryn Whitman
Institute for Astronomy
University of Hawaii

     High resolution spectropolarimetry of the Deep Impact target, comet 9P/Tempel 1, was performed during the impact on July 4, 2005.  Using the HiVIS Spectropolarimeter and the AEOS 3.67m telescope on Haleakala, Maui, we observed atypical polarization spectra that changed significantly in the hours after the impact.  The polarization of scattered light as a function of wavelength is very sensitive to the properties of constituent particles (monomers) of  aggregate particles which make up the coma dust: to the monomer size,  abundance, and composition. These measurements compliment the other observations because polarization is sensitive to the monomer properties, whereas many other, observations (e.g. thermal infrared) are more sensitive to the properties of the aggregate as a whole.  Thus polarization measurements can constrain the properties and evolution of the impact ejecta and dust comae. The polarization 40 minutes after impact (6:30 UT) was 4% at 650 nm falling to 3% at 950 nm, whereas it was 7% falling to 2% an hour later (-0.9% per 1000 Angstroms  to -2.3% per 1000 Angstroms).  This is an atypical blue slope, which became more blue 75 minutes after impact.  These observations, combined with many other observations performed at many observatories as part of the mission, indicate that there was an abundance of silicate-rich dust released from the impact, with relatively larger sizes and a higher degree crystallinity.