Recent developments about polarimetry as a tool to provide information about dust particles on asteroids, in cometary comae and in the zodiacal cloud
Résumé
Solar system small bodies may be covered by dust layers forming a regolith, or build up of free-floating dust particles in cometary comae, cometary dust tails and the interplanetary dust (or zodiacal) cloud. While a few in-situ and sample-return missions have provided unique information, remote observations of the linear polarization of solar light scattered by dust still provide major clues to the bulk properties of the dust particles. The polarization is a dimensionless ratio that does not depend upon the changing distances to the Sun and to the observer of the scattering object. Polarimetric phase curves in the visible and near infrared domains are smooth and typical of irregular scattering particles. Variation of the polarimetric properties with the wavelength of the observations presents mostly a linear dependence in the visible. Observations have already provided classifications of asteroids and cometary comae through dust polarimetric behaviour. Recent results about targets of on-going space missions to, e.g., asteroid (21) Lutetia and comets 103P/Hartley 2 or 67P/Churyumov-Gerasimenko will be discussed [Hadamcik et al. JQSRT 112, 1881, 2011; Hadamcik et al. A&A 517, A86, 2010]. Interpretation, in terms of physical properties, of the polarization data, stems from experimental and numerical simulations [Levasseur-Regourd et al. ASR 20, 1585, 1997]. Measurements on clouds of analogue particles in jet streams and in vials (under, e.g., microgravity conditions) and numerical simulations with clouds of irregular grains and porous aggregates of grains, have been developed. Comparison with cometary comae observations suggests that cometary dust particles are built of fluffy aggregates and of compact grains, with significant proportions of both rather transparent astronomical silicates and absorbing carbonaceous compounds [Hadamcik et al. JQSRT 100, 143, 2006; Lasue et al. JQSRT 100, 220, 2006]. As far as the interplanetary dust cloud is concerned, it is suggested that at least half of the interplanetary dust, between 0.7 and 1.5 AU from the Sun near the ecliptic, consists of fluffy absorbing carbonaceous material of cometary origin, which progressively evaporates as the dust particles spiral towards the Sun under Poynting-Robertson effect [Levasseur-Regourd et al., PSS 55 1010, 2007; Lasue et al., A&A 473, 641, 2007]. The significance of such studies to better understand the formation and evolution of small solar system bodies will be addressed.