  Small high-speed impact craters formed in rocks, ice, and other brittle
materials consist of an outer, broad shallow concentric region formed by
tensile fracture (spall), surrounding a smaller central "pit" crater of greater
depth. On the Earth, that "spall crater" morphology ceases to exist for craters
greater than a few meters in diameter. They are not commonly recognized for
craters in the solar system but might be an issue for cratering on the small
brittle asteroids. We consider the physics of the processes of shock-wave spall
cratering and formulate the scaling laws to apply those processes to the bodies
of the solar system. Our scaling is based upon analyses of shock-wave
propagation and tensile fracture mechanisms, including the important feature of
size-dependent tensile fracture, and the role of gravity in lofting spalled
material to form the outer parts of the spall craters. We consider the existing
scaling laws for cratering in the strength regime and derive the conditions for
which spall features will be present or absent. The conditions giving rise to
spall cratering are found to be a distinct subset of the 'strength' regime,
forming a new sub-regime of cratering. We find that this regime may be very
consequential for planetary cratering; in fact, it might dominate all cratering
on small rocky asteroids. That has important implications in the interpretation
of crater counts and the expected surface effects for rocky, 10-100 km objects.
