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.}

The thermal conductivity of granular planetary regolith is strongly dependent on the porosity, or packing density, of the regolith particles. However, existing models for regolith thermal conductivity predict different dependencies on porosity. Here, we use a full-field model of planetary regolith to study the relationship between regolith radiative thermal conductivity, porosity, and the particle non-isothermality. The model approximates regolith as regular and random packings of spherical particles in a 3D finite element mesh framework. Our model results, which are in good agreement with previous numerical and experimental datasets, show that random packings have a consistently higher radiative thermal conductivity than ordered packings. From our random packing results, we present a new empirical model relating regolith thermal conductivity, porosity, temperature, particle size, and the thermal conductivity of individual particles. This model shows that regolith particle size predictions from thermal inertia are largely independent of assumptions of regolith porosity, except for when the non-isothermality effect is large, as is the case when the regolith is particularly coarse and/or is composed of low thermal conductivity material.