Recursive plasma wake formation on the Moon and its effect on polar volatiles

Solar wind plasma flows almost horizontally in near-terminator regions of the Moon. One question is how does this plasma flow around topographic features like near-terminator mountains and polar craters? We present kinetic simulations of plasma inflow into a complicated polar crater having a second “child” crater within the “parent” crater (i.e., a 'crater-in-a-crater' model). We find that the effect on the inflowing ions of this second 'child' feature is a function of its position downstream relative to the primary crater wall: close to the wall where the solar wind is occulted there is little ion inflow and thus not a significant perturbation from the child obstruction. However, beyond about two crater wall heights downstream, there is substantial ion flow and the child obstruction will create a second ambipolar wake region similar to that created by the first obstruction. We also present a general analytical formalism that explains the most critical properties of the inflow perturbations. Finally, we consider the effect of the ion inflow into polar craters with 1% water 'frost' mixed within the top micron layers of the regolith. We find that in order to maintain equilibrium, a persistent frost most likely has to be maintained by a dynamic water source of ∼109 molecules m−2 s−1 in polar craters to offset solar wind ion inflow sputtering losses.