Astron. Astrophys. 360, 777-788 (2000)

1. Introduction

Understanding the formation of the solar system has been a main goal for astrophysicists and geophysicists for many years. To study in detail processes of planet formation in the very early stage of the solar system, investigation of old solar system material plays a key role. The study of such material is accelerating lately with the development of new space programs such us the NASA Stardust mission and the European Space Agency (ESA) Rosetta mission. The Stardust NASA space mission started with the launch of a spacecraft in February 1999. It will fly close to the comet Wild2 and bring cometary material back to Earth. ESA will start the Rosetta mission in 2003 which will visit the comet Wirtanen. This mission will concentrate on the investigation of cometary matter from the surface and the coma. The orbiter will carry a lander to the nucleus and deploy it on the surface.

The majority of meteorites are fragments from previously formed so-called planetesimals, situated in the then developing solar system in the region between Mars and the giant planets. The most primitive meteorites, carbonaceous chondrites, have an overall chemical composition similar to that of the Sun and were never severely metamorphosed or chemically altered during the history of the solar system (see e.g. Beatty & Chaikin 1990). These meteorites represent unique material for the investigation of the solar system as a whole.

Irregular dust particles play an important role in the study of comets and asteroids. The spectacular display of a bright comet is mostly caused by a cloud of micrometer-sized dust particles. This dust originates from a central source of ices and dust (the nucleus). Upon sublimation of the ices, the dust is entrained in the gas stream leaving the nucleus. From the similarity of the solar spectrum compared to that of a dusty comet, it was recognized that the light from a comet is mostly scattered sunlight. Measurements of the brightness and polarization of this light can give information on the nature of these dust particles. Differences in the polarization data of different comets have been noticed through both remote and polarization observations (e.g. Chernova et al. 1993; Levasseur-Regourd et al. 1996, 1999a; Kiselev 1999). These differences in polarization suggest differences in the physical properties of cometary dust particles.

Scattering properties of spherical particles can be calculated from Mie theory (Mie 1908), but the scattering properties of nonspherical particles can differ dramatically from those of volume- or surface- equivalent spheres. On the other hand, an exact solution for the scattering of light by nonspherical dust particles, covering all sizes and shapes that occur in nature does not exist. We refer to the book by Mishchenko et al. (2000), for a detailed description of the advantages and constraints of the often used numerical codes for light scattering by nonspherical particles. Therefore, an experimental study of the scattering behavior of irregular dust particles that are candidates for cometary material is of main importance in order to interpret space- and ground-based observations. Only a small number of results of laboratory measurements have been published. Microwave measurements require manufacturing centimeter-sized scattering objects with the desired shape and refractive index, studying the scattered microwave beam for this object, and transforming the results to other wavelengths by keeping the ratio size/wavelength fixed (Gustafson 2000). A disadvantage of such measurements is that they can be performed only for one particle size, shape and orientation at a time. Another approach is to work at visible wavelengths by letting a beam of laser light be scattered by a single particle (Weiss-Wrana 1983) or an ensemble of particles falling through the beam. The latter method has been used by several authors to establish the angular distribution of all elements of the scattering matrix (e.g. Holland & Gagne 1970; Perry et al. 1978; Kuik et al. 1991; Volten et al. 2000) or only the phase function and the degree of linear polarization for incident unpolarized light as a function of the scattering angle (Jaggard et al. 1981; West et al. 1997). Polarization phase curves for clouds of dust particles under microgravity conditions have been obtained recently (Worms et al. 1999).

Since mid- and far-infrared spectra have provided strong evidence for the presence of crystalline Mg-rich olivine in comets (Campins & Ryan 1989; Hanner et al. 1994; Colangeli et al. 1995; Kolokolova & Jockers 1997; Brucato et al. 1999), we have measured the whole scattering matrix as a function of the scattering angle of randomly oriented particles of a natural Mg-rich olivine sample and a ground piece of Allende meteorite as cometary analogues. The measurements have been carried out with lasers at two different wavelengths, 442 and 633 nm.

Laboratory measurements can be used in two different ways; 1) directly for interpreting optical observations of brightness and polarization of astronomical objects with dust, such as comets and asteroids, or 2) indirectly, by fitting the measured values to results of calculations by varying the size and shape (e.g. spheroids, cylinders, Gaussian random shapes) of the particles until a good fit has been obtained. The resulting particle properties, i.e. size and shape, can be used for computations of radiative properties at an arbitrary wavelength in the visible and infrared part of the spectrum for which the refractive index is known. The laboratory measurements presented in this work have been used in a direct way, by comparing them with observational data of comets and asteroids obtained by other authors (e.g. Dollfus 1989; Levasseur-Regourd et al. 1996, 1999a). In this way, we have derived some physical characteristics of dust particles in comets and asteroids.

A review of the theory involved in these experiments and a description of the experimental setup used to measure the scattering matrix elements are presented in Sect. 2. In Sect. 3, the samples are characterized. Results of our experiments are presented and discussed in Sect. 4. Conclusions are given in Sect. 5.

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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