Extremely strong polarization of an active asteroid (3200) Phaethon

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.

Polarimetric properties of the near-Sun asteroid (155140) 2005 UD in comparison with other asteroids and meteoritic samples

“…We analysed the MSI polarimetric data in the same manner as Ishiguro et al ( 2017 ) and Ito et al ( 2018 ). The outline consists of (i) preprocessing using bias and dome-flat data, (ii) cosmic ray rejection, (iii) masking field stars near the target asteroid, (iv) source flux extraction from ordinary and extraordinary regions on MSI images using the aperture photometry algorithm while a v oiding the masked regions for the field stars, and (v) deri v ation of the Stokes parameters ( I , Q , and U ), the linear polarization degree ( P ), and the position angle of polarization ( θ P ).…”

“…Polarimetric studies on Phaethon were recently conducted, and different research groups published a series of papers. First, Ito et al ( 2018 ) noticed through their 2016 observations that Phaethon exhibited a large polarization degree of up to ∼ 50 % at the largest phase angle (Sun-asteroid-observer angle) of their observation ( α = 106.5…”

“…• ) and conjectured that Phaethon might have large-scale surficial inhomogeneity. We conducted the polarimetric observation using the same instruments as Ito et al ( 2018 ) employed for Phaethon observation, which provides a reliable comparison between these two NSAs. Moreo v er, we re-analysed polarimetric data acquired through observations in Devog èle et al ( 2020), including a set of unpublished data at a large phase angle.…”

What mechanisms dominate the activity of Geminid Parent (3200) Phaethon?

“…The recent polarization measurements of Ito et al (2018) suggest that grains of ∼150 μm radius may characterize Phaethon's surface. Considering that the grains on the very surface may be easier to fracture due to space weathering or thermal stress, whereas grains in the subsurface dust mantle may aggregate and grow by sintering, we expect the average grain size on the surface to be smaller than the mean grain size in the subsurface dust mantle, if the formed dust mantle is enough thick (>1 m for example).…”

Rotational variation of the linear polarization of the asteroid (3200) Phaethon as evidence for inhomogeneity in its surface properties

“…On the other hand, the fact that we didn't see any variation of the colour of the object with rotation suggests that there might be no variation of the mineralogy as a function of longitude. Ito et al (2018) discuss that high polarisation, for a fixed albedo, should be caused by large particles (d∼360 µm). This is also supported by the blue slope of its spectrum, which usually is interpreted also in terms of larger grain sizes (Clark et al 2010).…”