Clasts on (101955) Bennu's boulders: Implications for their possible origin
Abstract
Since December 2018, NASA's OSIRIS-REx [1] spacecraft has been orbiting the Apollo-type near-Earth asteroid (101955) Bennu with the main aim of collecting a regolith sample to bring back to Earth. To identify the safest and most scientifically interesting sampling site [2], a complex multi-instrument analysis of Bennu's surface was performed, which showed that Bennu is a rubble-pile asteroid with impact craters of different sizes and ubiquitous boulders having different albedo, morphology, and dimension [3,4]. In particular, between February and April 2019, the Detailed Survey Baseball Diamond (BBD) mission subphase occurred. During this subphase, the OCAMS (OSIRIS-REX Camera Suite [5]) PolyCam imaged the entire surface of Bennu with a 2- to 6-cm/pixel spatial scale and with smaller phase angles (20 to 50°) than previous campaigns (generally 90°). Such illumination conditions, coupled with unprecedented imaging resolution, allowed the identification of different types of boulders, including one type that is characterised by a dark-toned, hummocky appearance [6]. The main goal of this work is to focus on different boulders of this type to identify the number, size, and shape of the visible clasts. By quantifying their spatial densities and size distribution, we aim to understand whether clast size ranges are similar among the studied boulders. This has important implications for their possible origin, whether as clasts exposed [7] by weathering processes occurring on the asteroid surface [8], or as particulate fallback material [7], potentially sourced from particle ejection events. Preliminary results show that the clast diameters range from 10 cm to 120 cm, with median values for individual boulders of 16 to 18 cm, independently of the size of the "hosting" boulder. Such similar median values of the identified clasts suggest that even if the studied boulders formed on different locations on Bennu's parent body, they are impact breccias that may have been brecciated at comparable depths. Acknowledgments: Maurizio Pajola, John Robert Brucato, Giovanni Poggiali and Elisabetta Dotto were supported for this research by the Italian Space Agency (ASI) under the ASI-INAF agreement no. 2017-37-H.0. Marco Delbo, Patrick Michel and Stephen R. Schwartz acknowledge support from the Centre National d'Études Spatiales, as well as the Academies of Excellence on Complex Systems and Space, Environment, Risk and Resilience of the Initiative d'EXcellence 'Joint, Excellent, and Dynamic Initiative' (IDEX JEDI) of the Université Côte d'Azur. This material is based upon work supported by NASA under Contracts NNM10AA11C and NNG12FD66C issued through the New Frontiers Program. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible. References: [1] Lauretta, D.S. et al. (2017) Space Sci. Rev. 212, 925-984. [2] Lauretta, D.S. et al. (2019) Nature, 568, 55-60. [3] Walsh, K.J. et al. (2019) Nature Geoscience, 12, 242-246. [4] DellaGiustina, D.N. et al. (2019) Nature Astronomy, 3, 341-351. [5] Rizk, B. et al. (2018) Space Sci. Rev. 214, 26. [6] Jawin, E.R. et al. (2020) LPSC Abstract #1201. [7] Rizk, B. et al. (2019) EPSC-DPS 2019 Abstract #105. [8] Molaro, J.L. et al. (2019) LPSC, Abstract #2132.
- Publication:
-
43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E.273P