Is it possible to crack in the absence of tension?

The ongoing eruption of water vapor and ice on Enceladus and proposed past eruptions of liquid water on Europa have led to discussion about the feasibility of cracking a planetary ice shell. We use a boundary element method to model crack propagation in an elastic layer subjected to tension and hydrostatic compression. We consider the presence of a region at the base of the ice shell in which the far-field extensional stresses vanish due to viscoelastic relaxation, impeding the propagation of fractures towards the subsurface ocean. The maximum extent of fracture propagation can be limited by hydrostatic pressure or by the presence of the unstressed basal zone, depending on its thickness. Our simulation results indicate that Enceladus' ice shell may be cracked if it is capable of supporting ~1-3 MPa tension and is less than ~30 km thick. Europa's ice shell may only be cracked under 1-3 MPa tension if it is about 1 km thick.