Spall Craters in the Solar System

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.