The Macroporosity of Rubble Pile Asteroid Ryugu and Implications for the Origin of Chondrules

Understand

We use the known surface boulder-size distribution of the C-type rubble pile asteroid Ryugu (NEA 162173) to determine its macroporosity, assuming it is a homogeneous granular aggregate.

• We show that the volume-frequency distribution of its boulders, cobbles and pebbles, is well represented by a lognormal function with $\sigma = 2.4 \pm 0.1$ and $\mu = 0.2 \pm 0.05$.
• Application of linear-mixture packing theory yields a value for the macroporosity of $\phi = 0.14 \pm 0.04$.
• Given its low bulk density of 1.19 gm cm$^{-3}$, this implies an average density for Ryugu's rocks of $1.38 \pm 0.07$ gm cm$^{-3}$ throughout its volume, consistent with a recent determination for surface boulders based on their thermal properties.