  Several physical mechanisms are involved in excavating granular materials
beneath a vertical jet of gas. These occur, for example, beneath the exhaust
plume of a rocket landing on the soil of the Moon or Mars. A series of
experiments and simulations have been performed to provide a detailed view of
the complex gas/soil interactions. Measurements have also been taken from the
Apollo lunar landing videos and from photographs of the resulting terrain, and
these help to demonstrate how the interactions extrapolate into the lunar
environment. It is important to understand these processes at a fundamental
level to support the on-going design of higher-fidelity numerical simulations
and larger-scale experiments. These are needed to enable future lunar
exploration wherein multiple hardware assets will be placed on the Moon within
short distances of one another. The high-velocity spray of soil from landing
spacecraft must be accurately predicted and controlled lest it erosively damage
the surrounding hardware.
