  The compression property of regolith reflects the strength and porosity of
the regolith layer on small bodies and their variations in the layer that
largely influence the collisional and thermal evolution of the bodies. We
conducted compression experiments and investigated the relationship between the
porosity and the compression using fluffy granular samples. We focused on a
low-pressure and high-porosity regime. We used tens of {\mu}m-sized irregular
and spherical powders as analogs of porous regolith. The initial porosity of
the samples ranged from 0.80 to 0.53. The uniaxial pressure applied to the
samples lays in the range from 30 to 4x10^5 Pa. The porosity of the samples
remained at their initial values below a threshold pressure and then decreased
when the pressure exceeded the threshold. We defined this uniaxial pressure at
the threshold as "yield strength". The yield strength increased as the initial
porosity of a sample decreased. The yield strengths of samples consisting of
irregular particles did not significantly depend on their size distributions
when the samples had the same initial porosity. We compared the results of our
experiments with a previously proposed theoretical model. We calculated the
average interparticle force acting on contact points of constituent particles
under the uniaxial pressure of yield strength using the theoretical model and
compared it with theoretically estimated forces required to roll or slide the
particles. The calculated interparticle force was larger than the rolling
friction force and smaller than the sliding friction force. The yield strength
of regolith may be constrained by these forces. Our results may be useful for
planetary scientists to estimate the depth above which the porosity of a
regolith layer is almost equal to that of the regolith surface and to interpret
the compression property of an asteroid surface obtained by a lander.
