Comparison With Other Dust Particles

3.1. Collected samples

Direct laboratory analysis of dust particles collected in the near-Earth environment (IDPs) indicates that most of the particles collected in the Earth environment are roughly ellipsoidal aggregates, in a 1 p.m to 1 mm size range, of complex smaller grains (see e.g. [16]). Although the particles collected at 1 AU in the ecliptic represent a biased sample, and have suffered some atmospheric heating, their shapes agree well with the shapes suggested by the polarisation phase curves.

Under close inspection, the IDPs are found to be aggregates of mostly black material (including carbon) with occasional clear (silicate) grains and reflective (sulphide) grains.

Their low albedos [17] are attributed both to their porous structure and to the presence of strongly absorbing material. The density of the 10 micrometer-sized particles is a 0.3 to 6.2 g cm"3 range, with an average at 2.0 g cm"3 [18], but larger particles might have densities

3.2. Cometary and asteroidal dust

The interplanetary dust complex is continually replenished by dust particles from comets and asteroids. Besides, some dust particles originating from the Kuiper belt and from the local interstellar cloud progressively reach the asteroid belt and later the inner solar system. Comets are a major source of dust, with heated nuclei releasing dust particles, including large ones that build up meteor streams. An other significant source comes from the asteroid belt, where impacts and collisions generate numerous fragments with a very wide size distribution. The local interplanetary dust bulk albedo at 0° and 1 AU in the ecliptic actually has a value, of the order of (0.15 ± 0.08), which seems intermediate between that of comets and of asteroids.

As already mentioned, the shape of the phase curve of interplanetary dust is quite comparable to that of cometary dust or asteroidal regolith. Different classes of asteroids and of comets can be pointed out through the differences noticed in the values of the inversion angle, of the slope at inversion, and of the maximum in polarisation (e.g. [15,19]). Three cometary classes, mainly corresponding to different sizes distributions, have typically been defined, corresponding to (i) comets with a near 90° maximum of 0.10 to 0.15, (ii) comets with a higher maximum of 0.25 to 0.30, and (iii) comet Hale-Bopp, whose polarisation at a given phase angle is always the highest (see e.g. Hanner, this volume).

The anti-correlation noticed, for interplanetary dust, between polarisation and albedo, is also noticed for asteroids, but does not seem to be observed for comets. From a comparison between comets Halley and Hale-Bopp, and possibly between various cometary regions, a higher polarisation actually seems to be correlated to a higher albedo [20].

It may be of interest to notice that the polarisation at a given phase angle (greater than about 30°) decreases with increasing wavelength for asteroidal regoliths. On the opposite, it increases with the wavelength for cometary dust, which exhibit a clear trend towards higher polarisations at longer wavelengths. For the well-documented comet Hale-Bopp, the polarisation at a given phase angle clearly increases with the wavelength, with possibly a maximum near 1600 nm [21], However, from observations of comets Hale-Bopp and Halley, the trend seems to be reversed for distances to the nucleus smaller than about 2000 km [22,23], suggesting that the physical properties of fresh dust particles are not the same than those of older particles, which have already suffered evaporation or fragmentation.

3.3. Virtual particles

The constraints imposed by the various sets of results, including the discrepancies between the polarisation-albedo dependence and the polarisation-wavelength dependence, should give clues to the physical properties of the different sets of scattering particles. It is natural to compare the values observed to those derived from light scattering computations, and it makes sense to perform computations with realistic irregular particles, which are neither spheroidal nor cylindrical.

Irregular compact particles have actually been used for light scattering computations, as well as aggregates, which could be fractal particles build up of smaller elementary spheres or gaussian solids (see e.g. [24,25]). Significant results have already been obtained, which suggest the existence of both fluffy aggregates of absorbing material and compact particles in the interplanetary dust cloud. The negative branch in the polarisation phase curves could be a clue to the existence of absorbing material or of dust aggregates, in which some multiple scattering takes place. However, working with realistic "virtual" particles represents a difficult task, since the computational times are usually long, and since questions about the validity range and the unicity of the solutions may be raised.

Was this article helpful?

0 0

Post a comment