Experimental phase function and degree of linear polarization of mm-sized and micron-sized olivine and spinel particles.

Abstract
In this work, we present the experimental phase function and degree of linear polarization of two
sets of samples consisting of forsterite and spinel particles. The size distributions of the studied
samples span over a broad range in the scattering size parameter domain. This work is part of
an ongoing experimental project devoted to understand photopolarimetric observations of asteroids
and comets. In particular, we study the effect of the size on the scattering matrix elements, finding
a strong dependence of characteristic parameters, e.g. maximum of polarization and inversion angle,
on particles size.

Introduction
Polarimetric observations of dust clouds are a powerful tool in planetary science. They allow us
investigating the nature and properties of solar system bodies and planetary systems in different
stages of evolution, e.g. asteroids, comets, and protoplanetary disks. For example, they can be
used as a reference to refine the taxonomic classification of asteroids [1, 2] or they can help in the
discrimination of objects with cometary origin [3]. Some dust materials, like olivine and spinel,
are remarkably interesting for the investigation and characterization of solar system small bodies.
In particular, olivine is an extensively diffuse silicate mineral and spinel, a magnesium/aluminum
mineral, is a characteristic component of the unusual class of presumably ancient Barbarian asteroids
as well as an important component of Calcium Aluminium rich Inclusions (CAI) found in primitive
meteorites [6, 7]. Physical and optical properties of the dust, such as their refractive index, size,
composition, and structure define their ability to scatter the light. Therefore, in order to study these
materials, we need to experimentally characterize their photopolarimetric curves.

Measurements
We analyze six samples of olivine and spinel with different sizes. The samples denoted as Pebble
consist of millimeter-sized grains and lie in the geometrical optics regime. Further, two size
distributions consisting of particles smaller than 30 and 100 micrometers are produced out of the
olivine and spinel bulk samples. The measurements have been performed at the IAA Cosmic Dust
Laboratory (CODULAB), Granada, Spain [4]. The instrument allows to measure the scattering
matrix of a cloud of particles and can be set also to retrieve the scattering matrix of single mm-sized
particles [5]. The measurements have been obtained at 520 nm for the mm-sized grains and at 514
nm for the micron-sized samples. The scattering angle covers the range from 3° to 177°.

Results
Figures 1 and 2 show the phase function and degree of linear polarization (DLP) respectively of
olivine and of spinel samples.
The phase function curves show a strong dependence on particle size. We see that the micron-sized
samples have lower values with a rather flat trend at side- and back-scattering regions and a strong
increase in the forward direction. In contrast, the pebbles show u-shaped phase functions. The slope
of the phase function at side- and back-scattering regions is stronger in the case of the spinel.
The DLP curves also show a dependence on the size. They have the typical bell shape with a
negative branch at low phase angles. Spinel Pebble shows the higher maximum of polarization. The
three spinel samples show a well-defined negative polarization branch with an inversion angle located
around 28° regardless of the particle size. It is interesting to note in the case of the olivine samples
the inversion angle is highly dependent on particle size. The high inversion angle of Barbarian
asteroids polarization curves could be related to the presence of spinel in the form of millimeter
grains of regolith.

Figure 1: Phase function curves (left) and degree of linear polarization (right) for the three olivine
samples.

Figure 2: Phase function curves (left) and degree of linear polarization (right) for the three spinel
samples.

 

References
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