Numerical Modelling of Impact Cratering on the Moon and Icy

There is growing evidence of a brine ocean beneath the ice crust of Europa. The possibility of this ocean hosting the development of simple life depends on the interaction of the ocean with the satellite’s surface where oxidants and organic compounds are believed to be formed by the breakdown of H2O and CO2 ices [1]. Transfer of these organics is considered possible if the ice crust is thin, making the determination of Europa’s crustal thickness an important exobiological issue. For planetary bodies on which geophysical profiling is not viable, one of the most powerful means of investigating the interior is the study of impact craters as they offer direct probes of a body’s subsurface. Providing that the underlying process is understood, craters allow crustal structure to be inferred on the basis of their morphology. Simple craters on the Moon and icy satellites display similar morphology, implying a comparable near-surface rheology; and cratering models used for the terrestrial planets have been successfully applied to icy bodies [2, 3]. The differences of complex craters on the icy satellites and the Moon are probably the result of the relative weakness of ice. Crater morphologies seen only on the icy satellites are believed to also be affected by the presence of subsurface oceans [4]. This work describes numerical simulations of cratering on the Moon and icy satellites with the specific aim of inferring the thickness of Europa’s ice crust Acoustic Fluidization: This work used the iSALE hydrocode, a multi-material, multirheology extension of the SALE hydrocode [5], to simulate impact crater formation in its entirety within an acoustically fluidized target. Acoustic fluidization [6] involves the weakening and fluid flow of a target (planet or satellite) when subject to strong vibrations. These vibrations are transmitted as sound waves via rock to rock contacts acting to locally increase or decrease the overburden pressure. This initiates sporadic, localised slips that allow the material to act fluidly on a macroscopic scale. A simple mathematical approximation of acoustic fluidization, known as the block model [7] has been implemented in iSALE in which the amount and longevity of acoustic fluidization is controlled by two parameters: the kinematic viscosity of the fluidized region, η, and the decay time of the block vibrations, τ [8]. Using a simple mechanical argument, Ivanov and Artemieva [9] suggested that both τ and η are directly proportional to the characteristic size B of blocks that comprise the sub-crater material. The exact equations they derive are of the form: