On the conditions for fluidisation in cometary mantles

In our current understanding, active cometary nuclei comprise a volatile-depleted outer crust covering a mixture of dust and ices. During each perihelion passage the thermal wave penetrates the crust and sublimates a portion of these ices, which then escape the nucleus, dragging with them dust particles that replenish the coma and dust tail. The flux of released gases is likely to vary as a complex function of solar distance, nucleus structure, spin rate, etc. It has been previously hypothesised that at some point a fluidised state could occur, in which the gas drag is approximately equal to the weight of overlying dust and ice grains. This state is well understood and used in industrial processes where extensive mixing of the gas and solid components is desired. The literature on fluidisation under reduced gravity and pressure conditions is here reviewed and published relations used to predict the conditions under which fluidisation could occur in the near-surface of a cometary nucleus.The general trend is that the minimum fluidisation velocity decreases with reducing gravity but increases with reducing pressure. However, true fluidisation is no longer possible in free molecular flow, when the gas viscosity is meaningless. As a result, fluidisation is unlikely to be driven by H2O sublimation below a dust mantle, where pressures are too low to allow such flow. Requisite pressures could, however, be achieved at the crystalline/amorphous phase change boundary, which could support fluidisation if the overlying material is granular.