Condition for the deflection of vertical cracks at dissimilar ice interfaces on Europa
The surface of Europa contains many quasi-circular morphologies called lenticulae. Although the formation mechanism of lenticulae is not understood, sill intrusion from the subsurface ocean is one promising hypothesis. However, it remains unclear how vertical cracks from the ocean deflect horizontally to allow sill intrusion in Europa. In this study, the critical stress intensity factor of Europan ice required for deflection was evaluated by considering crack theory at the interface between dissimilar materials and experimental results on ice. For deflection to occur at the interface between two dissimilar ices, the ratio of the critical stress intensity factor of the interface to that of the upper layer should be at most 0.45--0.5. This critical ratio may be attained if the interface is caused by brine-containing ice with a volume fraction of >30 ppt (3%) and pure (no-brine) ice. Thus, a region with a temperature equal to the eutectic point (e.g., an area of approximately 240 K in the convective layer) is a candidate for the region in which the deflection occurs.