ICES AND OCEANS IN THE OUTER SOLAR SYSTEM

Since the early 1970's, planetary scientists have used theoretical and observational arguments to deliberate the existence of surface and interior oceans on and within the outer solar system's icy planetary bodies. A combination of radiogenic and tidal heating could allow oceans to persist over long time scales, especially if antifreeze (e.g. ammonia) is present to depress the freezing temperature. Ice convection (where it occurs) tends to freeze an interior ocean, but there is a complex interplay among ice rheology, ice shell thickness, tidal heating, and heat transport mechanisms. Consistent with imaging evidence, Galileo magnetometer observations suggest oceans within Jupiter's three icy Galilean satellites. The oceans of Callisto and Ganymede are sandwiched between >100 km thick Ice I above and denser ice polymorphs below. In contrast, Europa's ocean is capped by a relatively thin and tidally heated ice shell (with geological evidence affirming recent surface-ocean exchange), and the ocean is probably in direct contact with an underlying rocky mantle (facilitating direct deposition of any hydrothermal chemical energy). These factors make Europa a key target in the search for life in our solar system. Liquid hydrocarbon seas are theoretically possible on Titan's surface, but Cassini radar images reveal dark sand seas instead. Gully systems argue for hydrocarbon rain at Titan, possibly concealed as ground-fluid reservoirs. Titan plausibly contains an interior water-ammonia ocean. The Cassini spacecraft has discovered geyser-like activity at tiny Enceladus, where water vapor rises from prominent fractures in the warm and tectonically deformed south polar terrain. Enceladus may have deep-seated and localized tidal heating, and perhaps a south polar sea. Triton is too cold for a once-hypothesized liquid nitrogen surface ocean, but the moon's icy interior probably melted during the high-eccentricity phase of its capture by Neptune. Triton's past tidal and current radiogenic heating combine to permit a volatile-rich interior ocean today. Pluto's hypothesized Charon-forming cataclysmic impact is ancient, so it is uncertain whether subsurface water remains today. The interior oceans of icy bodies may be the most common habitats for life in the universe, and we would be remiss not to explore them.