  When a liquid drop strikes a deep pool of a second liquid, an impact crater
opens while the liquid of the drop decelerates and spreads on the surface of
the crater. If the density of the drop is larger than the surrounding, the
interface between the drop liquid layer and its surrounding becomes unstable,
producing mushroom-shaped plumes growing radially outward. We interpret this
instability as a spherical Rayleigh-Taylor instability associated with the
deceleration of the interface between the drop and its surrounding, which
significantly exceeds the ambient vertical gravity. We investigate
experimentally how changing the density contrast and the impact Froude number
affects the instability and the resulting mixing layer. Using backlighting and
planar laser-induced fluorescence methods, the position of the air-liquid
interface, the thickness of the mixing layer, and an estimate of the
instability wavelength are obtained. First, the evolution of the mean crater
radius is derived from an energy conservation model. The observed mixing layer
dynamics is then explained by a model involving a competition between the
geometrical expansion of the crater tending to decrease its thickness, and
mixing produced by the Rayleigh-Taylor instability tending to increase its
thickness. The estimated instability wavelength is finally compared to an
approximate linear stability analysis of a radially accelerated fluid sphere
into a less dense fluid. Mixing properties of this impact-related instability
have geophysical implications regarding the differentiation of terrestrial
planets, in particular by estimating the mass of magma ocean silicates that
equilibrates with the metal core of the impacting planetesimals.
