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4.2. Comparison with cometary observations

A nice agreement is found between the phase curves obtained for mixtures of fluffy particles and for those obtained for comets [1] with: a negative branch at phase angles smaller than 20°, a maximum value of polarization for a phase angle of about 100° with values smaller than 50 % when the grains size is larger than 50 nm, an increase of Pmax with wavelength.

To better disentangle the physical properties of cometary dust, a comparison with other diagnostics is necessary. Infrared silicate features indicate the presence of submicron grains [14], but they also could be observed with high porosity (> 95 %) aggregates [15]. Interplanetary dust particles (IDPs) collected in the stratosphere, with pyroxene, olivine and high carbon contents, could have similar composition to cometary dust. The particles have porous structures of fine-grained heterogeneous aggregates [16]. The best fits by a dynamical model of the in-situ measured fluxes by Giotto/DIDSY and Giotto/OPE are obtained for densities of the order of 100 kg m"3 [17,18],

Mixtures of fluffy particles have physical properties similar to those expected for cometary particles with high porosity (95 to 99)% and submicron constituent grains. The polarimetric response of such mixtures would thus plead in favor of the presence, in cometary comae, of dust particles with similar physical properties.

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