Cornet C/2012 S1 (ISON) coma composition at ∼4 au from HST observations

We analyze the first color and polarization images of Comet ISON (C/2012 S1) taken during two measurement campaigns of the Hubble Space Telescope (HST) on UTC 2013 April 10 and May 8, when the phase angles of Comet ISON were alpha approximate to 13.7 degrees and 12.2 degrees, respectively. We model the particles in the coma using highly irregular agglomerated debris particles. Even though the observations were made over a small range of phase angle, the data still place significant constraints on the material properties of the cometary coma. The different photo-polarimetric responses are indicative of spatial chemical heterogeneity of coma in Comet ISON. For instance, at small projected distances to the nucleus (< 500 km), our modeling suggests the cometary particles are composed predominantly of small, highly absorbing particles, such as amorphous carbon and/or organics material heavily irradiated with UV radiation; whereas, at longer projected distances (> 1000 km), the refractive index of the particles is consistent with organic matter slightly processed with UV radiation, tholins, Mg-Fe silicates, and/or Mg-rich silicates contaminated with similar to 10% (by volume) amorphous carbon. The modeling suggests low relative abundances of particles with low material absorption in the visible, i.e., Im(m) <= 0.02. Such particles were detected unambiguously in other comets in the vicinity of nucleus through very strong negative polarization near backscattering (P approximate to -6%) and very low positive polarization (P approximate to 3-5%) at side scattering. These materials were previously attributed to Mg-rich silicates forming a refractory surface layer on the surface of cometary nuclei (Zubko et al., 2012). The absence of such particles in Comet ISON could imply an absence of such a layer on its nucleus. (C) 2015 Elsevier Ltd. All rights reserved.