Extremely strong polarization of an active asteroid (3200) Phaethon

Area of Science:

• Planetary Science
• Asteroid Research
• Observational Astronomy

Background:

• Near-Earth asteroid (3200) Phaethon is the parent body of the Geminid meteor stream.
• Phaethon is characterized as an active asteroid with a distinct blue spectral signature.

Purpose of the Study:

• To conduct polarimetric observations of asteroid (3200) Phaethon during its close approach to Earth.
• To analyze the relationship between solar phase angle and Phaethon's polarization properties.
• To investigate the implications of observed polarization on Phaethon's surface characteristics and albedo.

Main Methods:

• Utilized polarimetric observations over a broad range of solar phase angles (α).
• Conducted observations during the asteroid's close approach to Earth in autumn 2016.
• Analyzed the degree of linear polarization (P) as a function of solar phase angle.

Main Results:

• Observed exceptionally high linear polarization for Phaethon, reaching P = 50.0 ± 1.1% at α = 106.5°.
• The maximum polarization values observed were even greater.
• The results indicate a significant deviation from expected polarization patterns for typical asteroids.

Conclusions:

• The extreme polarization suggests Phaethon's geometric albedo may be lower than previously estimated from radiometric data.
• Alternatively, Phaethon's surface might be dominated by large grains (approx. 300 μm) possibly formed by perihelion heating.
• High surface porosity is also proposed as a significant factor contributing to the observed strong polarization.