Properties of Accreted Ices on Icy Ocean Worlds as Fingerprints of Sub-Ice Oceans

The formation and evolution of ice crusts on the ocean worlds of the outer solar system are an important element in understanding their potential to support life. The properties of these ice crusts in part govern the feasibility of surface-ice-ocean exchange—thought to be critical to supporting potential habitats within the sub-ice oceans.

Although the genesis of ice crusts on these ocean worlds is poorly constrained, in the case of Europa and Enceladus, there has been compelling evidence that the subsurface oceans are not isolated from the overlying ice crusts. Processes occurring at the ice-ocean interfaces, such as accretion, ablation, drainage, and fracture, may contribute to the dynamic and youthful surfaces of these moons. Whereas ablation at the ice-ocean interface and drainage of brines through the ice shell can serve as mechanisms to introduce surface-derived impurities into the ocean, only accretion permits the incorporation of young oceanic material into the ice shell. Unlike plume activity, which serves as a direct conduit from underlying water bodies to the surface, accretion allows for gradual modulation of the bulk ice crust properties. These accreted ices represent fingerprints from the ocean below.

The processes that mold and shape the ice-ocean interfaces of these alien worlds are not unlike those found in Earth's polar regions (from a pressure, temperature and, possibly, impurity concentration perspective). Studies of the properties of Arctic and Antarctic accreted ices have yielded improved understanding of the fractionation of impurities and the migration of brines. In this work we present two classes of accreted ice analogs (marine ice and sea ice) and describe where these ices may serve as appropriate analogs on icy ocean worlds. We use these analogs to constrain properties of ices for Europa and Enceladus, consistent with published ice shell evolution and exchange hypotheses, and discuss how these fingerprints may be observed by future missions.