BBC News reports amazing new discovery:
Hubble finds 'best evidence' for Ganymede subsurface ocean.
Ganymede, 5,300km-wide moon of Jupiter and largest in the solar system, joins the short list for places you can find extraterrestrial liquid water: Mars (barely), Europa (of 2001 Space Odyssey fame), Enceladus (as much water as the planet Earth!), and now Ganymede, although you may have to get through 100 km of ice to get to the warm liquid water center surrounding a rocky mantle and core.
From the BBC:
Ganymede is just one of a large list of objects in the Solar System now thought to hide an ocean deep below the surface. These include the dwarf planets Pluto and Ceres; other Jupiter moons - Europa and Calisto; Saturn's moons Enceladus, Titan and Mimas; and possibly Neptune's moon, Triton.
What's super cool is that what we see in our own solar system is apparently a common pattern we'd expect to find throughout the universe: gas giants with ice moons that have a liquid water center, warmed by magnetic fields and tidal forces from the gas giant mother, and some from also radioactive cores (like our own planet).
That means that warm liquid water environments are probably very common, and many are way outside the so-called habitable zones we think that life can live in.
To think that there's way more liquid water in our solar system than on the earth is truly mind blowing, and also thwarts many a bad sci-fi plot where the aliens come to steal our water.
But this is truly important because it drastically widens the potential pool of habitable environments for live to thrive. We used to think (like a week ago or so) that only planets in the so-called "Goldilocks" or "habitable zones" were even worth looking at.
The presence of a sub-surface ocean would heighten interest in Ganymede as a potentially habitable world. [...] The idea that a sub-surface ocean exists on Ganymede is exciting because wherever you have liquid water, you have one of the main ingredients for life.
And what kind of life would that be, your entire reality and world being below miles and miles of ice? Think of the limitations of living under that ice sheet: You've never seen a star or the sun or the massive gas giant you orbit, and none of your ecology's energy comes from the sun. It's boiling near the core and freezing at the "surface" and there's several hundred miles of warm liquid water to slosh around in between, stretching thousands of miles around the largest moon in the solar system. Even a hyper-advanced civilization that might develop under the ice would not discover the stars perhaps... ever? Nor would they know or possess "fire," at least how we think of it.
Another implication of living under the ice, everything would have to be very, very dark, unless there's some sort of bioluminesce. The sense of sight would seem to be completely superfluous--there's no light-producing heavenly bodies that are visible, unless the rocks glow from radiation. The constant aurora of Ganymede (which is really what "proved" there had to be a liquid water ocean) would never be visible under all that ice.
Some day intrepid explorers from Earth (most likely robotic) will drill through the ice of the numerous ice moons with a liquid center in our own solar system... and perhaps meet something drilling up from below. Exciting times!
Update:
Some awesome discussions below in the comments! Here's some additional links:
NASA confirms there's an ocean on Jupiter's moon Ganymede: 'Almost everywhere we look there's water.'
And from Scientific American, Have we got Solar System Habitability Backwards? h/t Athenian
I think at this point only the most contrived of explanations could avoid concluding that most of the liquid water in our solar system resides beyond the Earth – conceivably 15 to 16 times or more the volume of all terrestrial surface water.