Conclusions

The scattering properties of the interplanetary dust are quite well known, at least in the ecliptic plane. The polarisation phase curves are smooth, with a slight negative branch in the backscattering region, a near 15° inversion and a wide positive branch with a maximum at about 90°. For a fixed phase angle (greater than 30°), the local polarisation increases with increasing solar distance, while the local albedo decreases with solar distance. The polarisation seems to slightly decrease with increasing wavelength, at least in the near infrared domain, but the wavelength dependence of the local polarisation is not yet be known. Comparisons with other observations (IDPs, cometary dust, asteroidal regolith), together with the results of numerical simulations (irregular compact particles, aggregates), and those of laboratory measurements (microwave analogue technique, polar nephelometers), lead to the same conclusions: the interplanetary dust cloud is built of a mixture of irregular compact particles and of fluffy aggregates, mainly of asteroidal and cometary origin, which are likely to suffer some weathering and to evolve with time.

More constraints about the significance of some key parameters (characteristics of the polarisation phase curves, polarisation-wavelength dependence, polarisation-albedo dependence), and thus about the nature of the interplanetary dust, should be provided in a near future through new observations (including out-of-ecliptic observations), together with the development of sophisticated numerical and laboratory measurements under microgravity conditions.

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