  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. This supports the spectrum-based argument that
IDP's may be the best analog material available on Earth and suggests that
high-density, well-lithified objects such as chondrules and chondrule-bearing
chondrites may be rare on Ryugu. Implications of this result for the origin of
chondrules, a long-standing problem in cosmochemistry, are discussed. We
propose that chondrules and most chondrites formed together in rare
lithification events, which occurred during the accretion of chondritic
envelopes to large, differentiated planetesimals at a time when they were still
hot from $^{26}$Al decay.
