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

Abstract

Asteroid (3200) Phaethon is a Near-Earth Apollo asteroid with an unusual orbit that brings it closer to the Sun than any other known asteroid. Its last close approach to the Earth was in mid-December 2017 and the next one will be on October 2026. Previous rotationally time-resolved spectroscopy of Phaethon showed that its spectral slope is slightly bluish, in agreement with its B/F taxonomic classification, but at some rotational phases, it changes to slightly reddish. Motivated by this result we performed time-resolved imaging polarimetry of Phaethon during its recent close approach to the Earth. Phaethon has a spin period of 3.604 hours and we found a variation of the linear polarisation with rotation. This seems to be a rare case in which such variation is unambiguously found, also a consequence of its fairly large amplitude. Combining this new information with the brightness and colour variation, as well as previously reported results from Arecibo radar observations, we conclude that there is no variation of the mineralogy across the surface of Phaeton. However, the observed change in the linear polarisation may be related to differences in the thickness of the surface regolith in different areas or local topographic features.

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

“…Devog èle et al ( 2018) conducted independent polarimetric observations in 2017 and noticed that Phaethon's polarimetric inversion angle, α 0 (the phase angle when the polarization degree is zero) was within the range of typical asteroids but beyond the range of F-type asteroids and cometary nuclei, therefore supporting the idea of asteroidal origin. Borisov et al ( 2018 ) utilised a set of data in Devog èle et al ( 2018) and further found that the rotational variation in the polarization degree was probably caused by local heterogeneity. Shinnaka et al ( 2018 ) and Okazaki et al ( 2020 ) pointed out that the polarization degree of Phaethon in 2017 was different from that in 2016 at larger phase angles ( α > 60…”

Rotationally Resolved Spectroscopic Characterization of Near-Earth Object (3200) Phaethon

“…Based on the best available shape and rotation information fromHanus et al (2016), our observations spanned almost the entire surface of the asteroid. The large reported near-UV absorption variation thus cannot be explained by spatial variation -unless the northern rotational pole of Phaethon is significantly different from the rest of the asteroid spectroscopically.Interestingly,Borisov et al (2018) also recently inferred that the northern pole may have different polarimetric properties. Furthermore, Taylor et al (2018) note a radar-dark spot near one of the rotational poles as well.…”

(85989) 1999 JD6: a first Barbarian asteroid detected by polarimetry in the NEA population

“…As the data specify the rotation phase, we used the light-curve data acquired eight days before our observation (Vaduvescu et al 2017). Several large asteroids and the 5.8 km sized asteroid Phaethon have reported rotational variations of the degree of polarization smaller than 0.3% (Degewij et al 1979;Cellino et al 2016b;Nakayama et al 2000;Takahashi et al 2004Takahashi et al , 2009Belskaya et al 2010;Borisov et al 2018). However, our measurements are not accurate enough to assess this threshold.…”