Experimental scattering matrices of levitated single millimeter-sized dust pebbles at visible wavelengths

This work investigates the photopolarimetric behavior of millimeter-sized levitated dust pebbles. A secondary goal is to bridge the gap in photopolarimetric knowledge between micron-sized dust clouds and centimeter-scale pebbles. We studied a set of nine millimeter-sized volcanic ash pebbles from a natural palagonite bulk. The pebbles were characterized by their surface colors, sizes, and surface features. The pebbles were levitated using an acoustic levitator to isolate them during the experiments. Measurements of the scattering matrices were performed at scattering angles ranging from 3^̧irc to 177^̧irc at wavelengths of 488 and 640 at the Cosmic Dust Laboratory at the Instituto de Astrofísica de Andalucía. nm nm We compare our measured scattering matrices for the set of palagonite pebbles with those of micron-sized dust clouds of similar composition, revealing clear differences between the two size regimes. The main differences emerge from the phase function and the degree of linear polarization (DLP) curves. The pebbles exhibit a U-shaped phase function with minima of between 16^̧irc and 33^̧irc, whereas clouds of micron-sized dust particles show a nearly flat behavior at side- and back-scattering regions. The DLP curves of the pebbles exhibit maxima of between 48^̧irc and 66^̧irc, whereas micron-sized powders tend to have them close to or beyond 90^̧irc. The negative polarization branch (NPB) of the pebbles closely resembles that of micron-sized dust particles with effective radii lying at the transition between the resonance and geometric optics regimes at visible wavelengths. This indicates that the use of the NPB features as a proxy for the maximum of the DLP curves must be taken with caution. Additionally, we evaluate the range of applicability of the linear-exponential model to fit the NPB. The retrieved key polarimetric parameters are clearly affected by the pebbles' chemical composition. They also enable a preliminary comparison of the pebbles to the asteroid taxonomy due to the strong similarities between their DLP curves.