  When a liquid slams into a solid, the intermediate gas is squeezed out at a
speed that diverges when approaching the moment of impact. Although there is
mounting experimental evidence that instabilities form on the liquid interface
during such an event, understanding of the nature of these instabilities is
limited. This study therefore addresses the stability of a liquid-gas interface
subject to a diverging flow in the gas phase. We perform a linear modal
stability analysis of the surface to obtain an amplitude equation that is
subsequently analysed in detail and applied to two cases of interest for impact
problems, namely, the parallel impact of a wave onto a vertical wall, and the
impact of a horizontal plate onto a liquid surface. In both cases we find that
long wavelengths are stabilised considerably in comparison to what may be
expected based upon classical knowledge of the stability of interfaces subject
to a constant gas flow. In the former case, this leads to the prediction of a
marginally stable wavelength that is completely absent in the classical
analysis. For the latter we find much resemblance to the classical case, with
the connotation that the instability is suppressed for smaller disk sizes. The
study ends with a discussion of the influence of gas viscosity and gas
compressibility on the respective stability diagrams.
