  Spectropolarimetry of the Deep Impact target, comet 9P/ Tempel 1, was
performed during the impact event on July 4th, 2005 with the HiVIS
Spectropolarimeter and the AEOS 3.67m telescope on Haleakala, Maui. We observed
atypical polarization spectra that changed significantly in the few hours after
the impact. The polarization is sensitive to the geometry, size and composition
of the scattering particles. Our first measurement, beginning 8 minutes after
impact and centered at 6:30UT, showed a polarization of 4% at 650 nm falling to
3% at 950 nm. The next observation, centered an hour later, showed a
polarization of 7% at 650 nm falling to 2% at 950nm. This corresponds to a
spectropolarimetric gradient, or slope, of 0.9% per 1000 Angstroms 40 minutes
after impact, decreasing to a slope of -2.3% per 1000 Angstroms 75 minutes
after impact. Both are atypical blue polarization slopes. The polarization
values of 4% and 7% at 650nm are typical for comets at this scattering angle,
whereas the low polarization of 2% and 3% at 950nm is not. This, combined with
the IR spectroscopy performed by a number of observers during the event,
suggests an increase in size, number, and crystallinity of the individual
silicate particles (monomers) that are a constituant of the dust particles
(aggregates) in the ejecta.
