Abstract
Rubble pile asteroids such as (162173) Ryugu 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). In general, both macroporosity and
microporosity are estimated based on comparisons between the asteroid
bulk density and both the bulk and grain density of meteorite analogues,
and relatively large macroporosities are usually obtained. Here we use
semi-empirical models for the macroporosity of multi-component mixtures
to determine Ryugu’s macroporosity based on the observed size-frequency
distribution of boulders on the surface. We find that Ryugu’s
macroporosity can be significantly smaller than usually assumed, as the
observed size-frequency distribution allows for an efficient packing of
boulders, resulting in a macroporosity of $16 \pm
3$~\%. Therefore, Ryugu’s high bulk
porosity must be a direct consequence of a very large boulder
microporosity. Furthermore, using estimates of boulder microporosity
around 48~\% as derived from in-situ
measurements, the average grain density in boulders is $2751
\pm 236$ kg m$^{-3}$, similar to values obtained
for CM, CI, and the Tagish lake meteorites. Ryugu’s bulk porosity
corresponding to the above values is
57~\%.