Experimental phase function and degree of linear polarization curves of millimeter-sized cosmic dust analogs

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.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. © 2020 The Author(s).We are indebted to Rocío Márquez from the Scientific Instrumentation Center of the University of Granada for providing the SEM images. This work has been partially funded by the Spanish Plan Nacional de Astronomía y Astrofísica LEONIDAS project RTI2018-095330-B-100 and the Centro de Excelencia Severo Ochoa Program under grant SEV-2017-0709.Peer reviewe