Hydrated Salt Minerals on Europa, Ganymede and Other Objects: Presence, Origin and Behavior

Hydrated salt minerals on the surface of Europe and Ganymede are reported from the Galileo mission. This discovery is important for it is related to the original composition and evolution of these satellites and probably is related to the presence and nature of liquid water deposits below the surface and their ability to sustain life. Thermal evolution models predict salts and suggest circulation of water through warm/hot materials on or below the ocean floor. Further, salt deposits are present in primitive meteorites, suggesting they are the result of a common process(es) and that they may be found on other outer solar system objects such as Saturn's satellites and Ceres, to be explored by the Cassini and DAWN missions, respectively. The evidence for hydrated salts is the distorted water absorptions present in the IR reflectance spectra. The physical location of these absorptions in the disrupted regions of Europa (lineaments and chaotic terrain) indicates an association with endogenic processes and the putative ocean below. These materials and their spectra provide chemical constraints on the nature of the ocean, especially if a detailed analysis could be obtained from orbit, a lander or returned samples. The presence of these salts on Ganymede is less evident than for Europa and suggests older deposits, perhaps related to a different evolution timescale. The behavior of hydrated salt minerals on the surfaces of these objects has been explored recently in the laboratory. The results indicate that some are more stable to thermal and radiative disruption than is water ice. They would survive for the age of the solar system and probably develop lag deposits due to preferential loss of water ice. Chemical models confirm these results. Finally, injection of brines into an Europa-like surface environment produces materials even more hydrated and with more Europa-like spectral characteristics than for more crystalline materials usually found under ambient terrestrial laboratory conditions. These salts provide direct evidence for liquid water below the surface that was/is hospitable to life.