Spectropolarimetry of the Deep Impact Target Comet 9P/Tempel 1 with HiVIS

Abstract

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.