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

Abstract

AbstractIn this work, we present the experimental phase function and degree of linear polarization of twosets of samples consisting of forsterite and spinel particles. The size distributions of the studiedsamples span over a broad range in the scattering size parameter domain. This work is part ofan ongoing experimental project devoted to understand photopolarimetric observations of asteroidsand comets. In particular, we study the effect of the size on the scattering matrix elements, findinga strong dependence of characteristic parameters, e.g. maximum of polarization and inversion angle,on particles size.IntroductionPolarimetric observations of dust clouds are a powerful tool in planetary science. They allow usinvestigating the nature and properties of solar system bodies and planetary systems in differentstages of evolution, e.g. asteroids, comets, and protoplanetary disks. For example, they can beused as a reference to refine the taxonomic classification of asteroids [1, 2] or they can help in thediscrimination 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/aluminummineral, is a characteristic component of the unusual class of presumably ancient Barbarian asteroidsas well as an important component of Calcium Aluminium rich Inclusions (CAI) found in primitivemeteorites [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 thesematerials, we need to experimentally characterize their photopolarimetric curves.MeasurementsWe analyze six samples of olivine and spinel with different sizes. The samples denoted as Pebbleconsist of millimeter-sized grains and lie in the geometrical optics regime. Further, two sizedistributions consisting of particles smaller than 30 and 100 micrometers are produced out of theolivine and spinel bulk samples. The measurements have been performed at the IAA Cosmic DustLaboratory (CODULAB), Granada, Spain [4]. The instrument allows to measure the scatteringmatrix of a cloud of particles and can be set also to retrieve the scattering matrix of single mm-sizedparticles [5]. The measurements have been obtained at 520 nm for the mm-sized grains and at 514nm for the micron-sized samples. The scattering angle covers the range from 3° to 177°.ResultsFigures 1 and 2 show the phase function and degree of linear polarization (DLP) respectively ofolivine and of spinel samples.The phase function curves show a strong dependence on particle size. We see that the micron-sizedsamples have lower values with a rather flat trend at side- and back-scattering regions and a strongincrease in the forward direction. In contrast, the pebbles show u-shaped phase functions. The slopeof 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 anegative branch at low phase angles. Spinel Pebble shows the higher maximum of polarization. Thethree spinel samples show a well-defined negative polarization branch with an inversion angle locatedaround 28° regardless of the particle size. It is interesting to note in the case of the olivine samplesthe inversion angle is highly dependent on particle size. The high inversion angle of Barbarianasteroids polarization curves could be related to the presence of spinel in the form of millimetergrains of regolith.Figure 1: Phase function curves (left) and degree of linear polarization (right) for the three olivinesamples.Figure 2: Phase function curves (left) and degree of linear polarization (right) for the three spinelsamples. References[1] Belskaya I.N. et al., Refining the asteroid taxonomy by polarimetric observations. ICARUS, Vol.284, pp. 30-42, 2017.[2] López-Sisterna C. et al., Polarimetric survey of main-belt asteroids. VII. New results for 82main-belt objects. A&A, Vol. 626, A42, 2019.[3] Cellino A. et al., Unusual polarimetric properties of (101955) Bennu: similarities with F-classasteroids and cometary bodies. MNRAS, Vol.481, pp.L49-L53, 2018.[4] Muñoz O. et al., Experimental determination of scattering matrices of dust particles at visiblewavelengths: The IAA light scattering apparatus. JQSRT, Vol. 111, 187 196, 2009.[5] Muñoz O. et al., Experimental Phase Function and Degree of Linear Polarization Curves ofMillimeter sized Cosmic Dust Analogs ApJSS, Vol. 247, pp.19, 2020.[6] Cellino A. et al., A successful search for hidden Barbarians in the Watsonia asteroid family.MNRAS, Vol. 439, L75-L79, 2014.[7] Devogéle M. et al., New polarimetric and spectroscopic evidence of anomalous enrichment inspinel-bearing calcium-aluminium-rich inclusions among L-type asteroids. ICARUS, Vol. 304,pp. 31-57. 2018.