Macroporosity and Grain Density of Rubble Pile Asteroid (101955) Bennu

Rubble pile asteroids such as (101955) Bennu, the target of NASA’s OSIRIS-REx mission, have large bulk porosities, which are believed to result from void spaces in between the constituent boulders (macroporosity) as well as void spaces within the boulders themselves (microporosity). Instead of estimating both quantities based on comparison with analog meteorites, we use semi-empirical models for the macroporosity of multi-component mixtures (Zou+ 2011, Grott+ 2020) to determine Bennu's macroporosity based on the observed size-frequency distribution of boulders on the surface; global bulk porosity of Bennu was determined from the observed gravity field and shape. The employed size-frequency distribution covers the size range from 0.2 m to 95 m and was obtained by merging local and global particle counting sets. We find that Bennu's macroporosity can be significantly smaller than usually assumed (e.g., (Britt 2002, 2011), as the observed size-frequency distribution allows for an efficient packing of boulders, resulting in a macroporosity of 15± 3 %.. Bennu's high bulk porosity must therefore be a direct consequence of very large boulder microporosity. Furthermore, using estimates of the two main types of boulder microporosity of 52% and 31% as derived from remote-sensing thermal inertia measurements (Rozitis+ 2020), the average grain density in boulders is 2794±250 kg m³, similar to values obtained for CM, CI, and the Tagish lake meteorites. Bennu's bulk porosity corresponding to the above values is 57 %. Bennu’s macroporosity is very similar to that of Ryugu (Grott+ 2020), which we present for comparison.