The scattering properties of cometary dust

Abstract

The dust particles in different comets exhibit generally similar scattering properties. Polarization is characterized by a small negative branch at scattering angles θ>160° and a maximum of 15–25% near 90° in red light. The material is, on average, very dark, with geometric albedo of 3–5% at visual to near-infrared wavelengths for comets within $\text{2.5}\text{AU}$ of the sun. The scattering properties are consistent with a mixture of silicate and carbonaceous material, as measured by the Halley space probes. In comparing comets, there is a correlation of stronger polarization, redder polarimetric color, higher albedo, stronger infrared silicate emission, and higher infrared color temperature. Aggregate particles having constituent grain size parameters X⩾1.5 show promise in matching the observed scattering properties. An enhancement in the abundance of small silicate grains with X∼1.5–2 at $\text{0.5}\text{μm}$ wavelength may cause the higher polarization, higher albedo, and stronger silicate emission in comets such as Hale-Bopp.