Implications of High Polarization Degree for the Surface State of Ryugu

TL;DR

This study measures Ryugu's surface polarization, revealing it has a high polarization degree indicating large, submillimeter-sized grains, which informs understanding of its surface structure and scattering properties.

Contribution

It provides the first detailed polarimetric analysis of Ryugu's surface, linking polarization degree to grain size and surface structure, and offers insights into its surface composition.

Findings

Ryugu's polarization degree reaches 53% at 100° phase angle.

Surface covered with large, submillimeter-sized grains.

Surface scattering properties suggest a surface with large grains.

Abstract

The asteroid exploration project "Hayabusa2" has successfully returned samples from the asteroid (162173) Ryugu. In this study, we measured the linear polarization degrees of Ryugu using four ground-based telescopes from 2020 September 27 to December 25, covering a wide-phase angle (Sun-target-observer's angle) range from 28$^\circ$ to 104$^\circ$. We found that the polarization degree of Ryugu reached 53$\%$ around a phase angle of 100$^\circ$, the highest value among all asteroids and comets thus far reported. The high polarization degree of Ryugu can be attributed to the scattering properties of its surface layers, in particular the relatively small contribution of multiply-scattered light. Our polarimetric results indicate that Ryugu's surface is covered with large grains. On the basis of a comparison with polarimetric measurements of pulverized meteorites, we can infer the presence of submillimeter-sized grains on the surface layer of Ryugu. We also conjecture that this size boundary represents the grains that compose the aggregate. It is likely that a very brittle structure has been lost in the recovered samples, although they may hold a record of its evolution. Our data will be invaluable for future experiments aimed at reproducing the surface structure of Ryugu.