moons, and the origins of terrestrial water tends to lead to all sorts of visions of nice moist worlds and warm tropical beaches. Or perhaps that's just me. It feels like it's been a long winter. This kind of bias is extremely persistent. Even when we talk about the potential for sub-surface oceans on moons like Europa or Ganymede it can be very hard to overcome the sense that these are secondary, and second-class, environments. The truth is actually rather surprising.
For quite some time planetary scientists have studied the possible interior environments of a wide range of solar system bodies. Much can be done with purely theoretical models that seek to determine the appropriate hydrostatic balance between an object's own gravity and its internal pressure forces - be they from gaseous, liquid, or solid states of matter. Thermal energy from formation, and critically from radiogenic heating (radioactive decay of natural isotopes), all play a role. Throw in a few actual datapoints, measurements of places like Europa or Titan, and these models get much better calibrated. The intriguing thing is that one can play around with compositions and the internal layering of material in a planet-like body to find the best looking fit. As a consequence the nature and extent of any subsurface zones of liquid water can be estimated.
Asking for liquid water is a bit like asking for 'a coffee' in Starbucks. Is that a demi-latte-mocha-skim-sweet-n-low, or hot water with caffeine in it? It's extremely unlikely for water anywhere in a planetary body to be pure. Water is a fabulous polar solvent, and can absorb astonishing quantities of other things. Throw in ammonia by the bucket load and while you'd not want it in that cappuccino you still have liquid water that might be microbial ambrosia. Adding solutes can also dramatically lower water's freezing point. Stuffing in 30% Ammonia by weight can get a freezing point below 200 Kelvin (-100 F). This opens up many avenues.
Allowing for a number of variables it is possible to evaluate the likely size of subsurface water oceans in our solar system. The numbers start to get interesting. Estimates vary but here's a sampling: Europa, 2x Earths ocean volume, Ganymede and Callisto each about 1/2 Earth's ocean volume, Titan possibly 10x Earth's ocean volume, Triton 2x Earth's ocean volume. None of these numbers are particularly optimistic or pessimistic, but from these bodies alone there could readily be 10 to 16 times more liquid water slurping around off-Earth than on it.
Things get even funkier when we start to consider what might be going on beneath the surface of Trans-Neptunian Objects - those distant cold objects of which Pluto is the prototype. Factoring in estimates of their number, their history of formation, and radiogenic heating then some claims suggest that these distant dark worlds could harbor more liquid water than all the rest of the solar system. Part of the trick is that a thick layer of tens of kilometers of frozen water, methane, nitrogen and so on actually provides great insulation against thermal loss to the vacuum of space.
Some caution is advised though. Clearly many, if not all, of these environments may operate with far less energy flux - thermal or chemical - than Earth-bound oceanic systems. Some might well drop below the mean levels required to sustain any kind of deep dark biosphere. But if we insist that liquid water, polluted or otherwise, is a key ingredient for life then there is far more real estate in the solar exurbs than in our neighborhood.