Unusual polarimetric properties of (101955) Bennu: similarities with F-class asteroids and cometary bodies

TL;DR

This study presents polarimetric observations of asteroid Bennu, revealing its unusual F-class properties and cometary features, with implications for its surface composition and the OSIRIS-REx mission.

Contribution

First polarimetric characterization of Bennu over a wide phase angle range, linking its properties to F-class and cometary bodies.

Findings

Bennu exhibits polarimetric properties similar to F-class asteroids.

Estimated geometric albedo of Bennu is 0.059.

Bennu shows features associated with cometary bodies.

Abstract

We have obtained polarimetric measurements of asteroid (101955) Bennu, a presumably primitive near-Earth object (NEO) that is the target of NASA's sample return mission OSIRIS-REx. During our observing campaign, Bennu was visible from Earth under a wide range of illumination conditions, with phase-angle in the range 16 to 57 degrees. Together with (3200) Phaethon and (152679) 1998 KU2, observed very recently, Bennu is the only existing example of a primitive NEO observed in polarimetric mode over a wide interval of phase angles. Based on our polarimetric data, we propose that Bennu belongs to the unusual F taxonomic class defined in the 80s. According to previous works, the F-class includes objects with cometary features. This fact can be of great importance for the interpretation of the results of the exploration of this object by OSIRIS-REx. From polarimetry we also derive an estimate of the geometric albedo of Bennu: 0.059 +/- 0.003.