  Formation of a splash crown and a cavity following the impact of a sphere on
a body of liquid is a classical problem. In the related problem of a droplet
splashing on a flat surface, it has been established that the properties of the
surrounding gas can influence the splashing threshold. At lower impact speeds,
this is due mainly to the influence of gas kinetic effects, since the height of
the gas lubrication film which is displaced during dynamic wetting is often
comparable to the mean free path of the gas. At higher Weber and Reynolds
numbers, on the other hand, inertial effects dominate and the density of the
gas becomes important in determining whether a splash occurs. In this article,
sphere impacts on a liquid body are investigated in a rarefied atmosphere using
high-speed photography. It is found that the threshold entry speed for cavity
formation is influenced by the density of the surrounding gas, whereas changing
the mean free path of the gas has no effect. We attribute this phenomenon to
the gas slowing the sealing of the thin crown sheet behind the sphere. This
assertion is supported with experimental measurements of the liquid sheet
thickness. In the range of parameters considered, the splash crown influences
the movement of the contact line, an effect not previously observed.
