  Europa's surface features many regions of complex topography termed 'chaos
terrains'. One set of hypotheses for chaos terrain formation requires upward
migration of liquid water from perched water bodies within the icy shell formed
by convection and tidal heating. However, consideration of the behaviour of
terrestrial ice sheets suggests the upwards movement of water from perched
water bodies is uncommon. Instead, rapid downwards hydrofracture - unbounded
given a sufficient supply of water - can occur in relatively low tensile stress
states given a sufficiently deep initial fracture due to the negative relative
buoyancy of water. I suggest that downwards, not upwards, fracture may be more
reasonable for perched water bodies but show that full hydrofracture is
unlikely if the perched water body is located beneath a mechanically strong icy
lid. However, full hydrofracture is possible in the event of lid collapse over
a perched water body and likely in the event of a meteor impact that generates
sufficient meltwater and a tensile shock. This provides a possible mechanism
for the transfer of biologically important nutrients to the subsurface ocean
and the formation of chaos terrains.
