^<br />Ditto for the seismograph! Many a night have I dreamed of setting three of those bad boys on the surface, and finally figuring out its subsurface structure. Seismometers for Io come a close second, in my wish book. <img src="/images/icons/laugh.gif" /><br /><br />Back to the original topic: <br />I think the consensus is that Ganymede's liquid water would be considerably more difficult to reach than Europa's; indeed, it's probably more of a mantle than a true subsurface ocean. A stickler for life on Europa has been the fact that a Europan ocean would be less favorable for life if there were no influx of materials from the surface, like peroxides created by radiolysis, though this is probably controversial. Ganymede's ocean has probably been completely cut off from the surface for billions of years.<br /><br />That said, I think Ganymede may actually become more favorable for life than Europa, due to its higher gravity, once our Sun swells into a giant and gives the outer Solar System a few million years of hothouse conditions. <img src="/images/icons/wink.gif" />