Rapid hydrofracture of icy moon shells: insights from glaciology

Europa's surface features many regions of complex topography termed 'chaos terrains'. One set of hypotheses for chaos terrain formation require upward migration of liquid water from perched water bodies formed by convection and tidal heating within the icy shell. However, consideration of the behaviour of terrestrial ice sheets suggests the rapid downwards, not upwards, movement of water from perched water bodies, initiated through hydrofracture and without a propagation limit provided there is a sufficient volume of water to fill the resulting vertical fracture. I suggest that drainage of perched water bodies to the subsurface ocean through hydrofracture is a likely phenomenon at Europa and other icy moons. This may at first appear to rule out perched water bodies in the formation of chaos areas, but the violence of such drainage events and loss of mechanical support could lead to the collapse of the surface shell and a temporary surface-ocean connection. Alternatively, hydrofracture provides a mechanism for entire ice-shell fracture if a surface impact melts enough near-surface ice.