  We compare low velocity impacts that ricochet with the same impact velocity
and impact angle into granular media with similar bulk density, porosity and
friction coefficient but different mean grain size. The ratio of projectile
diameter to mean grain length ranges from 4 in our coarsest medium to 50 in our
finest sand. Using high speed video and fluorescent markers, we measure the
ratio of pre- to post-impact horizontal and vertical velocity components, which
we refer to as coefficients of restitution, and the angle of deflection caused
by the impact in the horizontal plane. Coefficients of restitution are
sensitive to mean grain size with the ratio associated with the horizontal
velocity component about twice as large for our coarsest gravel as that for our
finest sand. This implies that coefficients for hydro-static-like, drag-like
and lift-like forces, used in empirical force laws, are sensitive to mean grain
size. The coefficient that is most strongly sensitive to grain size is the lift
coefficient which decreases by a factor of 3 between our coarsest and finest
media. The deflection angles are largest in the coarser media and their size
approximately depends on grain size to the 3/2 power. This scaling is matched
with a model where momentum transfer takes place via collisions with individual
grains. The dependence of impact mechanics on substrate size distribution
should be considered in future models for populations of objects that impact
granular asteroid surfaces.
