Experimental Phase Function and Degree of Linear Polarization Curves of Millimeter-sized Cosmic Dust Analogs

Olga Muñoz ,C Tubiana,E Frattin,I Bustamante,A C Levasseur-Regourd,J Markannen,D Guirado,H Sierks,L Kolokolova,A C Caballero,C Güttler,M Fulle,A Rotundi,F Moreno,S Ivanovski,José Luís Martínez Ramos ,Vincenzo Della Corte ,Teresa Jardiel ,Dominique Bockelée‐Morvan ,Владимир Захаров ,Ivano Bertini ,Jürgen Blum ,Fernando Vargas-Martı́n ,Juan Carlos Gómez‐Martín ,Sihane Merouane

doi:10.3847/1538-4365/ab6851

Abstract

Abstract We present laboratory measurements of the phase functions and degree of linear polarization (DLP) curves of a selection of millimeter-sized cosmic dust analog particles. The set includes particles with similar sizes but diverse internal structure (compact and porous) and absorbing properties. The measured phase functions are found to be in all cases very different from those of micron-sized particles. They show a monotonic decrease with increasing phase angle from the back- to the side-scattering region, reaching a minimum at large phase angles before a steep increase of the forward peak. This is in stark contrast to the phase functions of micron-sized particles, which are rather flat at low and intermediate phase angles. The maximum of the DLP for millimeter-sized compact particles is shifted toward larger phase angles (∼130°) compared to that of micron-sized particles (∼90°). Porosity plays an important role in the measured DLP curves: the maximum significantly decreases for increasing porosity as a result of multiple scattering within the particle. Large porous particles with highly absorbing inclusions can reproduce both the OSIRIS/Rosetta phase functions and ground-based DLP observations of comet 67P/Churyumov–Gerasimenko.

Highlights

The angular dependence of the brightness and degree of linear polarization (DLP) of stellar light scattered by cosmic dust clouds results primarily from the size, shape, and composition of the dust
The measurements are performed at 520 nm, spanning the phase angle range from 10° to 170°
It is interesting to note that the slope of the phase function in the 150°−10° angle range is larger for the two absorbing millimeter-sized compact particles with soft or moderate surface roughness, i.e., charcoal and MgFeAlSi particles

Summary

Introduction

The angular dependence of the brightness and degree of linear polarization (DLP) of stellar light scattered by cosmic dust clouds results primarily from the size, shape, and composition of the dust. These physical properties may be retrieved from the analysis of photopolarimetric observations of the light scattered by clouds in different environments, such as circumstellar regions around young and evolved stars (Canovas et al 2015; Milli et al 2017; Ren et al 2019), or planetary (McLean et al 2017) and cometary atmospheres (Bertini et al 2017; Rosenbush et al 2017).

Methods

Findings

Discussion

Conclusion