  The porosity structure of a granular body is an important characteristic that
affects evolutionary changes in the body. We conducted compression experiments
using fluffy granular samples with various particle sizes, shapes, and
compositions. We approximated the pressure-filling factor relationship of each
sample with a power law (a modified polytropic relationship). We also fit our
previous data and literature data for fluffy granular samples using a power-law
equation. The fitting with a power-law form was as good as that achieved with
the equations used for powders in previous studies. The polytropic indices
obtained in the current study ranged from ~0.01 to ~0.3 and tended to decrease
with increasing particle size for samples of similar porosities. We calculated
the radial porosity structure and bulk porosity of granular bodies with various
radii using the Lane-Emden equation. The results provide the initial, most
porous structures of accreted primordial bodies, or re-accumulated rubble-pile
bodies consisting of particles that have compression properties similar to
those of the assumed granular materials. A range of porosity structures is
allowed for a body of given size and macroporosity, depending on the
compression properties of the constituent granular material.
