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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.06.005 Byline: I.N. Belskaya (a)(b), S. Bagnulo (c), M.A. Barucci (b), K. Muinonen (d)(e), G.P. Tozzi (f), S. Fornasier (b)(g), L. Kolokolova (h) Keywords: Centaurs; Transneptunian objects; Polarimetry; Photometry Abstract: Results of the first polarimetric observations of Centaurs (5145) Pholus and (10199) Chariklo, and new observations of (2060) Chiron are presented together with the estimates of their absolute magnitudes. Observations were carried out at the 8m ESO Very Large Telescope in 2007-2008. They revealed noticeable negative polarization in the phase-angle range 0.5-4.4[degrees] with a minimum varying from -1% to -2.1% in the R band. All three objects show diverse polarization phase-angle behaviour, each distinctly different from that of transneptunian objects. We found evidence of surface heterogeneity for Chariklo while Chiron and Pholus appear to have rather homogeneous surfaces. Polarization phase behaviours of Chiron and Pholus are significantly different from any other Solar System bodies studied so far. A shift of negative polarization minima toward small phase angles seems to be a characteristic feature of polarization properties of Centaurs. Presence of a small amount of water frost on a dark surface is considered as one of the possible ways to explain these properties. Author Affiliation: (a) Institute of Astronomy, Kharkiv National University, 35 Sumska Str., 61022 Kharkiv, Ukraine (b) LESIA, Observatoire de Paris, 5, pl.Jules Janssen, 92195 Meudon cedex, France (c) Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, United Kingdom (d) Department of Physics, University of Helsinki, P.O. Box 64, Gustaf Hallstromin katu 2a, FI-00014, Finland (e) Finnish Geodetic Institute, P.O. Box 15, Geodeetinrinne 2, FI-02431 Masala, Finland (f) INAF - Oss. Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy (g) University of Denis Diderot - Paris VII, 2 Place Jussieu, 75251 Paris, France (h) University of Maryland, College Park, MD 20742, USA Article History: Received 5 April 2010; Revised 9 June 2010; Accepted 9 June 2010 Article Note: (footnote) [star] Based on observations made with ESO Very Large Telescope at the La Silla-Paranal Observatory under programme ID 178.C-0036 (PI: A. Barucci).

Byline: D.M. Harrington (a), K. Meech (a), L. Kolokolova (b), J.R. Kuhn (a), K. Whitman (a) Keywords: Polarimetry; Comet Tempel-1; Comets; Comets; composition; Impact processes Abstract: High resolution 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 3.67-m telescope on Haleakala, Maui. We observed atypical polarization spectra that changed significantly in the few hours after the impact. The polarization of scattered light as a function of wavelength is very sensitive to the size and composition (complex refractive index) of the scattering particles as well as the scattering geometry. As opposed to most observations of cometary dust, which show an increase in the linear polarization with the wavelength (at least in the visible domain and for phase angles greater than about 30, a red polarization spectrum) observations of 9P/Tempel 1 at a phase angle of 41[degrees] beginning 8 min after impact and centered at 6:30 UT 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 950 nm. This corresponds to a spectropolarimetric gradient, or slope, of -0.9% per 1000 A 40 min after impact, decreasing to a slope of -2.3% per 1000 A an hour and a half after impact. This is an atypical blue polarization slope, which became more blue 1 h after impact. The polarization values of 4 and 7% at 650 nm are typical for comets at this scattering angle, whereas the low polarization of 2 and 3% at 950 nm is not. We compare observations of Comet 9P/Tempel 1 to that of a typical comet, C/2004 Machholz, at a phase angle of 30[degrees] which showed a typical red slope, rising from 2% at 650 nm to 3% at 950 nm in two different observations (+1.0 and +0.9% per 1000 A). Author Affiliation: (a) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA (b) University of Maryland, College Park, MD 20742, USA Article History: Received 17 April 2006; Revised 21 August 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.06.017 Byline: J.E. Thomas-Osip (a), B.A.S. Gustafson (b), L. Kolokolova (c), Y.-L. Xu (d) Keywords: Experimental techniques; Radiative transfer; Titan Abstract: A combination of laboratory experiments, theoretical modeling, and spacecraft observations is employed to characterize the aerosols in the atmosphere of Titan. The scattering properties of model aerosols were measured using the Microwave Analog Light Scattering Facility at the University of Florida and complemented with theoretical modeling of single scattering characteristics and radiative transfer in Titan's atmosphere. This study compares these modeling results with photopolarimetric observations made over a range of phase angles by the Pioneer 11 and Voyagers 1 and 2 spacecraft. Important results of this work include a survey of the scattering properties of different particle morphologies and compositions necessary to accurately interpret these observations without introducing non-physical assumptions about the particles or requiring additional free parameters to the radiative transfer models. Previous studies use calculation methods which, due to computing memory and processing time requirements, a priori exclude much of the parameter space that the microwave analog laboratory is ideal for exploring. The goal of the present work, to directly constrain aerosol physical characteristics, is addressed by studying in a consistent manner how a variety of particle morphologies and refractive indices affect the polarization and intensity reflected by Titan's atmosphere. Based on comparisons of model results to spacecraft observations, many model morphologies are excluded from further consideration. The most plausible physical particle models suggest that a combination of Rayleigh-like single particles and aggregates that are larger than those previously suggested and investigated [West, R.A., Smith, P.H., 1991. Evidence for aggregate particles in the atmospheres of Titan and Jupiter. Icarus 90, 330-333; Rannou, P., Cabane, M., Botet, R., Chassefiere, E., 1997. A new interpretation of scattered light measurements at Titan's limb. J. Geophys. Res. 102, 10997-11013] provide the best fit to the existing data. Additional laboratory experiments and more refined modeling awaits the results of the new rich observational dataset from the Cassini/Huygens encounter with Titan. Author Affiliation: (a) Las Campanas Observatory, The Observatories of the Carnegie Institute of Washington, Colina El Pino, Casilla 601, La Serena, Chile (b) Department of Astronomy, University of Florida, Rm 211, Bryant Space Sciences Bldg, PO Box 112055, Gainesville, FL 32611-2055, USA (c) Department of Astronomy, University of Maryland, Rm 2337, Computer and Space Science Bldg, College Park, MD 20742, USA (d) C104, Lockheed Martin/GB Tech, 2400 NASA Parkway, Houston, TX 77058, USA Article History: Received 20 June 2002; Revised 2 May 2005

It is shown that the maximum value of the polarization of light scattered by either spherical or spheroidal polydisperse particles is mainly governed by the composition of grain material (complex refractive index), whereas its color is mainly determined by the grain size distribution. This conclusion is also valid for particles of a more general shape. Relations are obtained which make it possible to evaluate the properties of cometary grains from the color and polarization measured for the cometary continuum at large phase angles. These relations are especially suitable for carrying out a comparative analysis of cometary dust. From our own photometric and polarimetric data of five recent comets we predict their dust properties and compare them with the interpretation of infrared observations available in the literature.