Polarimetric observations of comet Levy 1990c and of other comets: Some clues to the evolution of cometary dust

The evolution with the phase angle alpha of the polarization degree P of light scattered by comet Halley's dust is well documented. No significant discrepancy is found between Halley and Levy polarization curves near the inversion point. From all available cometary observations, we have derived polarimetric synthetic curves. Typically, a set of about 200 data points in the red wavelengths range exhibits a minimum for (alpha approximately equals 10.3 degrees, P approximately equals 1.8 percent) and an inversion point for (alpha approximately equals 22.4 degrees, P = 0 percent), with a slop of about 0.27 percent per degree. A significant spreading of some data (comets Austin 1982VI, Austin 1989c1, West 1976VI) is found at large phase angles. The analysis of our polarimetric maps of Levy reveals that the inner coma is heterogeneous. The increase of the inversion angle value with increasing distance from the photometric center is suspected to be due to the evolution with time of grains ejected from the nucleus. A fan like structure could be produced by a jet of grains freshly ejected

The strange polarimetric behavior of Asteroid (234) Barbara

We have discovered that the Asteroid (234) Barbara exhibits very anomalous polarimetric properties. The phase-polarization curve of this asteroid is unique and is not matched by any other known atmosphereless body of our Solar System. Although a few preliminary conjectures can be made, for the moment the reasons of the peculiar polarimetric properties of this asteroid remain essentially unknown.Fil: Cellino, Alberto. Istituto Nazionale di Astrofisica; ItaliaFil: Belskaya, I. N.. Kharkov National University; UcraniaFil: Bendjoya, P. H.. Université Côte D'azur; FranciaFil: Di Martino, M.. Istituto Nazionale di Astrofisica; ItaliaFil: Gil Hutton, Ricardo Alfredo. Felix Aguilar Observatory And Yale Southern Observatory; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Muinonen, K.. University of Helsinki; FinlandiaFil: Tedesco, E. F.. University of New Hampshire; Estados Unido

Polarization of Light Scattered by Cometary Dust Particles: Observations and Tentative Interpretations

International audienceAnalysis of the polarization of light scattered by cometary particles reveals similarities amongst the phase curves, together with some clear differences: i) comets with a strong silicate emission feature present a high maximum in polarization, ii) the polarization is always slightly lower than the average in inner comae and stronger in jet-like structures. These results are in excellent agreement with the Greenberg model of dust particles built up of fluffy aggregates of much smaller grains. Also, they suggest the existence of different regions of formation, and of different stages of evolution for the scattering particles inside a given cometary coma

Approaching interplanetary dust physical properties from light scattering and thermal observations and simulations

Physical properties of the interplanetary dust such as the local polarization and temperature, together with their heliocentric variations, may be derived from scattered and emitted light observations. With a realistic size distribution typical of the interplanetary dust, a very good fit of the polarization phase curve near 1 AU is obtained for a mixture of silicate and more absorbing organic material (approx 40 % in mass). The decrease of P_{90 deg} values with the solar distance is interpreted as a decrease of the organic percentage towards the Sun. Finally the contribution of dust particles of cometary origin is at least 20% at 1AU

Laboratory simulations of asteroidal surfaces by polarization measurements and comparison to remote observations

Sunlight scattered by asteroidal surfaces is partially linearly polarized. The polarization depends on the physical properties of the particles (size, refractive index, structure), and varies with the geometry of observations (phase angle) and wavelength. The polarization phase curves of asteroids with a bell-shaped positive branch and a shallow negative branch are typical of irregular particles. The polarization maxima are observable only for near-Earth objects, while observations of main belt asteroids are limited to relatively small phase angles, mainly in the negative branch. The polarimetric phase curve parameters allow a classification into various types [1], which are similar to the spectral classification. They also provide information on, e.g. the bulk surface albedo by the slope at inversion and a two parameters empirical function [2]. Observations of NEA objects are expected in the near future to better characterize their surface properties [3]. The determination of the parent bodies of most classes of asteroids still remains a puzzling question. It is thus of interest to compare the properties of some meteorites to their potential progenitors. Powdered meteorites in different size distributions deposited on a surface are used as regolith analogues of their parent body. Such work is in progress with the PROGRA2 experiment (http://www.icare.univ-lille1.fr). First, we present the instrumental set-up for the laboratory work on surfaces and explain how we use also microgravity measurements on clouds of the particles to infer the size influence on the polarimetric phase curves. Second, we present some results for meteorites and finally we compare the phase curves parameters to the observational results of asteroids to tentatively infer some physical properties of the regoliths [4]. We acknowledge CNES and ESA support for the microgravity flights and experimental work References [1] A. Penttilä, K. Lumme, E. Hadamcik and A.C. Levasseur-Regourd. Statistical analysis of asteroidal and cometary polarization phase curves. A&A 432 (2005) [2] A. Cellino, R. Gil-Hutton, E.F. Tedesco, M. DiMartino and A. Brunini. Polarimetric observations of small asteroids: preliminary results. Icarus 138 (1999) [3] A.C. Levasseur-Regourd et al., Small bodies studied by polarimetry, to select appropriate targets for sample-return missions to primitive near-Earth asteroids. Submitted to MPAS (2012). [4] E. Hadamcik, A.C. Levasseur-Regourd, J.-B. Renard, J. Lasue, A.K. Sen. Polarimetric observations and laboratory simulations of asteroidal surfaces: the case of 21 Lutetia. JQSRT 112 (2011

Y a-t-il de l'eau partout dans l'Univers ? Les porteuses d'eau

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