  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.
