HIDDEN OCEANS: THE NEW VIEWS OF WATER IN THE OUTER SOLAR SYSTEM SATELLITES

One of the most remarkable discoveries of the Galileo mission is the evidence at Europa, Callisto and possibly Ganymede for an electromagnetic induction response to the time varying Jovian field. This response requires a conducting layer within but near the surface of the satellite, and the only plausible interpretation is a salty water ocean more than several kilometers in thickness. In Europa, the ice layer may be only ~20km thick and is extensively flexed by Jupiter's tide, so that there is also geological evidence for the ocean. In Ganymede, Callisto, and plausibly Titan, the ocean will be near the minimal freezing point of water (2 kilobar pressure, at around 200 km beneath the surface) bounded above by ice I and bounded blow by high pressure phases of ice. In these cases, the surface may not exhibit evidence of the ocean's presence. In these ice-rich bodies, radioactivity alone is sufficient to drive the temperature up to the melting point, and this heat source has not declined sufficiently to allow freezing. Presence of other constituents, especially ammonia, can act as antifreeze preventing ocean freezing. In smaller Triton and Pluto, an ocean is marginally possible in the water-ammonia system because complete freezing requires that the temperature drop below 173 K. Too little is known about the conditions necessary for life to argue convincingly one way or the other about biology in these outer solar system oceans.