  Planetary impact events eject large volumes of surface material. Crater
excavation processes are difficult to study, and in particular the details of
individual ejecta fragments are not well understood. A related, enduring issue
in planetary mapping is whether a given crater resulted from a primary impact
(asteroid or comet) or instead is a secondary crater created by an ejecta
fragment. With mapping and statistical analyses of six lunar secondary crater
fields (including Orientale, Copernicus, and Kepler) we provide three new
constraints on these issues: 1) estimation of the maximum secondary crater size
as a function of distance from a primary crater on the Moon, 2) estimation of
the size and velocity of ejecta fragments that formed these secondaries, and 3)
estimation of the fragment size ejected at escape velocity. Through this
analysis, we confirmed and extended a suspected scale-dependent trend in ejecta
size-velocity distributions. Maximum ejecta fragment sizes fall off much more
steeply with increasing ejection velocity for larger primary impacts (compared
to smaller primary impacts). Specifically, we characterize the maximum ejecta
sizes for a given ejection velocity with a power law, and find the velocity
exponent varies between approximately -0.3 and -3 for the range of primary
craters investigated here (0.83-660 km in diameter). Data for the jovian moons
Europa and Ganymede confirm similar trends for icy surfaces. This result is not
predicted by analytical theories of formation of Grady-Kipp fragments or spalls
during impacts, and suggests that further modeling investigations are warranted
to explain this scale-dependent effect.
