Light scattering and the nature of interplanetary dust

Observations of the solar light scattered by interplanetary dust particles provide information about their physical properties. It is clear, from polarisation observations, that the nature of the dust particles is not the same everywhere, and that it depends upon their distance to the Sun. It is quite likely, from the shapes of the polarisation phase curves, that the dust cloud is built up of irregularly shaped compact particles or aggregates, the average size of which is greater than a few micrometers. Laboratory measurements represent a most promising approach to derive physical properties of the dust from some key parameters, such as the minimum, inversion and maximum regions in the phase curves, as well as the polarisation-wavelength dependence or the polarisation-albedo dependence. From comparisons with other dust populations, with light scattering computations, and with laboratory measurements, it may already be assumed that the interplanetary dust is built up of both compact particles (possibly of asteroidal origin) and of absorbing fluffy aggregates (possibly of cometary origin). Changes in the properties of the dust most likely result from the relative location of these sources, and from the evolution with time of the fragile dust particles.