Direct light scattering measurements on irregularly shaped particles and aggregates are likely to provide the missing link to interpret the light scattering observations. Elaborate techniques are required, in order of obtaining results representative of low-density dust clouds, and of avoiding multiple scattering on gravity-packed layers. Microwaves facilities, as well as polar nephelometers, are now fully operational. These latter instruments can be used under microgravity conditions to avoid any sedimentation of the dust particles .
Microwave analogue measurements scale the light scattering up to centimetre-sized particles (see e.g. [27,28,29]). The University of Florida facility allows 'measurements to be performed at two wavelengths, on artificially constructed complicated particles of various indices. The polarisation at a wavelength equivalent to the red is usually lower than at a wavelength equivalent to the blue, except for some absorbing polydisperse aggregates, that could be representative of cometary dust ,
Systematic nephelometer type measurements have been carried out at LAS in Marseilles, at two wavelengths with irregular compact particles levitating in a gas flow . Smooth polarisation curves, with a near 90° maximum, are obtained for some dielectric particles.
Two nephelometer type instruments, operating at various wavelengths in the visible and near infrared, have recently been designed in France to allow measurements on low-density dust clouds of natural and industrial particles, of Moon and Mars dust analogues, as well as of powdered dust of meteoritic origin. The PROGRA2 experiment, developed at Aeronomie and at LPCE, has been used in the laboratory and in reduced gravity conditions, on board airplanes devoted to parabolic flight campaigns [32,33]. The CODAG-LSU experiment, developed at Aeronomie, has been used in reduced gravity conditions, mainly for calibration purposes, and in microgravity conditions, during an ESA rocket flight .
Polarisation phase curves have already been retrieved for numerous samples with PROGRA2. The polarisation at a given wavelength (greater than about 30°) usually decreases with the wavelength for compact particles, while, on the opposite, it seems to increase with the wavelength for highly porous aggregates of submicron-sized particles [35,36]. Such aggregates have an important scattering efficiency, and only a small fraction of them drastically changes the scattering properties of the sample.
Calibration brightness and polarisation phase curves obtained with CODAG-LSU have emphasized the slight discrepancies that may occur between computational models and laboratory measurements, due to the fact that experimental micron-sized spheres are neither perfectly spherical nor smooth. Monitoring of the phase curves during the ESA MASER-8 rocket flight has demonstrated the possibility of obtaining naturally formed aggregates through ballistic aggregation processes, under microgravity conditions [25,37],
It should indeed be possible to accurately measure the scattering properties of well-documented naturally formed aggregates and regoliths under long duration microgravity conditions. It is expected that the ICAPS (Interactions in Cosmic and atmospheric particles Systems) European project will, on board the International Space Station, provide the information still needed to interpret the scattering properties of the interplanetary dust cloud in terms of physical properties.
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