Surface densities and size-frequency distributions of meter-size boulders inside craters on (101955) Bennu

Meter-size boulders are ubiquitous on the surface of near-Earth asteroid (101955) Bennu, as revealed by the OSIRIS-REx image dataset. In this work, we study the surface densities and size-frequency distribution (SFD) of boulders located inside Bennu's craters. We focus on such features because they are homogeneous morphological units with distinct boundaries, and our aim is to quantify whether there are any surface/texture differences among them, as well as to determine the possible physical reasons behind that, such as different emplacement processes or localized surface degradation phenomena.

For this work, we use tens of images taken on December 1 and 2, 2018, by the PolyCam imager of the OSIRIS-REx Camera Suite (OCAMS). This dataset covers the asteroid with a spatial scale ranging between 0.42 and 0.32 meters/pixel, hence it is fully suitable to identify all boulders larger than 2-3 meters in size.

Our preliminary results based on the craters studied so far show that the obtained fits are generally comparable to the global SFD results (-2.9 ± 0.3), despite a wider fitting range. However, the surface densities per square meter vary with the sizes of the hosting craters, showing a general decrease as the crater diameters decrease.

Acknowledgments: M. Pajola, J. R: Brucato and E. Dotto were supported for this research by the Italian Space Agency (ASI) under the ASI-INAF agreement no. 2017-37-H.0. M. Delbo, P. Michel and S. 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. Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration. 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 the making the encounter with Bennu possible.