Polarimetry of comets in the near-IR

We present near-IR (2.2 mu m) imaging polarimetry for seven recent comets. This data, and previous measurements of comets C/1995 O1 (Hale-Bopp) and 1P/Halley, yield a trend in polarization versus phase angle that is typically 1 - 2% higher in the near-IR than in the visual. We also find that comets divide into two classes at phase angles greater than 30 degrees based on their polarization. These classes are associated with the presence or absence of mid-IR silicate emission. Using previously published comet Hale-Bopp polarization data as an example, we show the polarization at 40 degrees phase angle varies by a factor of 1.6 from 0.3 to 2.2 mu m. This weak wavelength dependence indicates that the polarization is not strongly sensitive to grain size by a wavelength of 2.2 mu m suggesting the grains/aggregates must be at least 1 mu m in size.