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, { we confirm that}
Ryugu’s high bulk porosity is a direct consequence of a very large
boulder microporosity. Furthermore, using estimates of boulder
microporosity of around { 50~\%} as
derived from in-situ measurements, the average grain density in boulders
is { $2848 \pm 152$ kg m$^{-3}$, similar to
values obtained for CM and the Tagish lake meteorites}. Ryugu’s bulk
porosity corresponding to the above values is {
58~\%.} { Thus, the macroporosity of
rubble pile asteroids may have been systematically overestimated in the
past.}