Hubble Space Telescope observations of Jupiter’s geologically active, icy moon Europa have shown the moon ejecting plumes of liquid water, but linking the plumes to specific surface features has remained elusive. These plumes are of great interest due to the presence of an ocean underneath Europa’s ice shell, as they may represent a way to sample and study this ocean. However, a study led by Gregor Steinbrügge of Stanford University and Joana Voigt of the University of Arizona suggests that these plumes may not be derived from the deep ocean, but instead are caused by migrating pockets of brine in the shallow subsurface.
Brine migration occurs when small pockets of salty liquid water begin to freeze. As the ice forms, salt is rejected from the solid and concentrates in the liquid, making the remaining liquid even saltier and reducing its freezing point. If this brine pocket has a thermal gradient — that is, one portion of the pocket is warmer than the other — then this reduced melting point can cause the warmer side to melt, allowing the pocket to migrate over time. On Europa, such a thermal gradient can be created by a meteor impact, which would create a large amount of melted water. As this water refreezes from the outside of the crater inward, brine migration would cause the remaining water to become more and more concentrated in salts. Eventually, the expansion forces from the ice freezing around the brine pocket would create enough overpressure to trigger a plume eruption.
The authors applied their hypothesis to Manannán crater on Europa and found geologic evidence consistent with the presence of an overpressurized liquid reservoir beneath the center of the crater. The crater features a central depression that could have resulted when the ice over the reservoir collapsed down into space after an eruption. It also features linear “spider” fractures radiating out from the center, which form when a rigid surface is subjected to pressure from below. The authors believe that the arrival of the Europa Clipper spacecraft will provide more evidence to support their theory and that study of these features could help constrain the salinity of Europa’s ice shell. READ MORE