  We use numerical modeling to investigate the combined effects of impact
velocity and acoustic fluidization on lunar craters in the simple-to-complex
transition regime. To investigate the full scope of the problem, we employed
the two widely adopted Block-Model of acoustic fluidization scaling assumptions
(scaling block size by impactor size and scaling by coupling parameter) and
compared their outcomes. Impactor size and velocity were varied, such that
large/slow and small/fast impactors would produce craters of the same diameter
within a suite of simulations, ranging in diameter from 10-26 km, which
straddles the simple-to-complex crater transition on Moon. Our study suggests
that the transition from simple to complex structures is highly sensitive to
the choice of the time decay and viscosity constants in the Block-Model of
acoustic fluidization. Moreover, the combination of impactor size and velocity
plays a greater role than previously thought in the morphology of craters in
the simple-to-complex size range. We propose that scaling of block size by
impactor size is an appropriate choice for modeling simple-to-complex craters
on planetary surfaces, including both varying and constant impact velocities,
as the modeling results are more consistent with the observed morphology of
lunar craters. This scaling suggests that the simple-to-complex transition
occurs at a larger crater size, if higher impact velocities are considered, and
is consistent with the observation that the simple-to-complex transition occurs
at larger sizes on Mercury than Mars.

Earth and Planetary Astrophysics