Europa, the smallest of the Galilean satellites, is characterized by a young water-ice surface showing a rich structure of cracks, ridges and chaotic terrain. Below the ice shell a subsurface water-ocean might exist, which in Europa’s case would be in direct contact with the silicate mantle. This direct contact is important from an astrobiological perspective, as living organisms might have developed at places where a hot part of the mantle contacts the ocean. As a consequence, many scientists consider Europa as the planetary body with the largest probability to harbor life within our solar system.
It is therefore of utmost importance to find out whether a subsurface ocean exists underneath the ice shell. One plausible method to determine the existence of an ocean is based on the measurement of the radial deformation induced by the eccentricity-tide acting on Europa. If an ocean is present in the interior, the radial deformation at the surface will be one to two orders of magnitude larger than in the case that an ocean is absent. Such a large difference in deformation can be detected from measurements performed by a dedicated orbiter. As a result, a mission to the Jovian system would be required.
An alternative method to determine whether there is a subsurface ocean underneath the ice shell is based on establishing a connection between the shape of the lineaments observed on the surface and the acting stress fields. Stresses at the surface might be induced by several different mechanisms, from which only the two most important will be analyzed in this research: the eccentricity-tide acting on Europa and non-synchronous rotation (NSR) of the ice shell. The first mechanism, i.e. the eccentricity-tide, induces a highly variable stress field that explains the formation of cycloidal features even without taking into account NSR stresses. The second mechanism, i.e. NSR, induces a nearly static stress field that explains the formation of slightly-curved or global lineaments. As both types of lineaments exist on Europa’s surface, the strength of NSR stresses should have changed throughout the geological history of Europa. Such a change can be driven by a variable rate of non-synchronous rotation, which can be the result of thickness variations in the ice shell.
One important result obtained in this research is that tensile stresses at the surface of models without a subsurface ocean are too small to originate a crack at the surface if NSR is not taken into account. If NSR stresses are added to the modeling of the stress field, tensile stresses only become large enough to break ice when the stress field is practically static. As a result, the existence of cycloidal features strongly suggests the existence of a subsurface ocean underneath the ice shell of Europa.