Inversion angle of phase-polarization curve of near-Earth asteroid (3200) Phaethon

TL;DR

This study analyzes the polarization phase curve of near-Earth asteroid Phaethon over a wide phase angle range, revealing its surface properties, high polarization maximum, and possible surface heterogeneity.

Contribution

It provides the first detailed polarization measurements of Phaethon over a broad phase angle range, classifies its surface as B-type, and suggests surface heterogeneity based on polarimetric variations.

Findings

P_max exceeds 42.4%, much higher than moderate albedo asteroids.

Classified Phaethon as B-type in polarimetric taxonomy.

Detected surface heterogeneity with different scattering properties near the orbital pole.

Abstract

The linear polarization degree (referred to the scattering plane, P_r) as a function of the solar phase angle, {\alpha}, of solar system objects is a good diagnostic to understand the scattering properties of their surface materials. We report Pr of Phaethon over a wide range of {\alpha} from 19.1 deg to 114.3 deg in order to better understanding properties of its surface materials. The derived phase-polarization curve shows that the maximum of P_r, P_max, is >42.4% at {\alpha} >114.3 deg, a value significantly larger than those of the moderate albedo asteroids (P_max ~9%). The phase-polarization curve classifies Phaethon as B-type in the polarimetric taxonomy, being compatible with the spectral property. We compute the geometric albedo, p_v, of 0.14 +/- 0.04 independently by using an empirical slope-albedo relation, and the derived p_v is consistent with previous results determined from mid-infrared spectra and thermophysical modeling. We could not find a fit to the period in our polarimetric data in the range from 0 up to 7.208 hr (e.g., less than twice the rotational period) and found significant differences between our P_r during the 2017 approach to the Earth and that of the 2016. These results imply that Phaethon has a region with different properties for light scattering near its orbital pole.