Extremely strong polarization of an active asteroid (3200) Phaethon

Takashi Ito,Makoto Watanabe,Ryo Ishimaru,Tomoko Arai,Yoonsoo P Bach,Hiroyuki Naito,Kiyoshi Kuramoto,Masataka Imai,Yuna G Kwon,Masateru Ishiguro,Tomohiko Sekiguchi,Masanori Kobayashi

doi:10.1038/s41467-018-04727-2

Abstract

The near-Earth asteroid (3200) Phaethon is the parent body of the Geminid meteor stream. Phaethon is also an active asteroid with a very blue spectrum. We conducted polarimetric observations of this asteroid over a wide range of solar phase angles α during its close approach to the Earth in autumn 2016. Our observation revealed that Phaethon exhibits extremely large linear polarization: P = 50.0 ± 1.1% at α = 106.5°, and its maximum is even larger. The strong polarization implies that Phaethon’s geometric albedo is lower than the current estimate obtained through radiometric observation. This possibility stems from the potential uncertainty in Phaethon’s absolute magnitude. An alternative possibility is that relatively large grains (~300 μm in diameter, presumably due to extensive heating near its perihelion) dominate this asteroid’s surface. In addition, the asteroid’s surface porosity, if it is substantially large, can also be an effective cause of this polarization.

Highlights

The near-Earth asteroid (3200) Phaethon is the parent body of the Geminid meteor stream
Polarimetric observation that directly measures the status of light scattering, which strongly depends on the surface texture, is complementary to and sometimes superior to spectroscopic observation in terms of studies of the small solar system bodies
Polarimetric measurement of the small bodies at a large solar phase angle is technically difficult, because the observational opportunities are limited to some near-Earth asteroids that get inside Earth’s orbits, and because the observation should be conducted at small solar elongation angles

Summary

Introduction

The near-Earth asteroid (3200) Phaethon is the parent body of the Geminid meteor stream. The strong polarization implies that Phaethon’s geometric albedo is lower than the current estimate obtained through radiometric observation. This possibility stems from the potential uncertainty in Phaethon’s absolute magnitude. Polarimetric studies of airless bodies are generally useful for understanding their surface physical properties, geometric albedo and grain size. Polarimetric measurement of the small bodies at a large solar phase angle is technically difficult, because the observational opportunities are limited to some near-Earth asteroids that get inside Earth’s orbits, and because the observation should be conducted at small solar elongation angles. As polarimetric measurement of the small bodies over a wide range of α better reveals their surface material property, its implementation is always desirable whenever it is feasible

Methods

Results

Conclusion