I was born in a water moon. Some people, especially its inhabitants, called it a planet, but as it was only a little over two hundred kilometres in diameter “moon’ seems the more accurate term. The moon was made entirely of water, by which I mean it was a globe that not only had no land, but no rock either, a sphere with no solid core at all, just liquid water, all the way down to the very centre of the globe.

If it had been much bigger the moon would have had a core of ice, for water, though supposedly incompressible, is not entirely so, and will change under extremes of pressure to become ice. (If you are used to living on a planet where ice floats on the surface of water, this seems odd and even wrong, but nevertheless it is the case.) This moon was not quite of a size for an ice core to form, and therefore one could, if one was sufficiently hardy, and adequately proof against the water pressure, make one’s way down, through the increasing weight of water above, to the very centre of the moon. Where a strange thing happened.

For here, at the very centre of this watery globe, there seemed to be no gravity. There was colossal pressure, certainly, pressing in from every side, but one was in effect weightless (on the outside of a planet, moon or other body, watery or not, one is always being pulled towards its centre; once at its centre one is being pulled equally in all directions), and indeed the pressure around one was, for the same reason, not quite as great as one might have expected it to be, given the mass of water that the moon was made up from. This was, of course,

-Iain M. Banks, The Algebraist

It simply ends right there.

These are the dying words of an Artificial Intelligence being hunted by a death-squad of alien super soldiers.

But this got me wondering, is an object like a "water moon" physically possible? Perhaps enough comets could accumulate around a gas giant during the formation of a star system, and be kept in a liquid state via friction from the mother planet's tidal forces (such appears to be the case with Europa).

Your thoughts?

2313027 Such a world is not physically possible. The moon would be of insufficient size to hold an atmosphere, and without the atmosphere to protect it, the water would immediately vaporize and be blown away by it's sun's solar wind.

2313065
Maybe, but perhaps the moon was orbiting extremely close to a Jupiter-like gas giant, one with a very powerful magnetosphere.

Granted, the tidal forces on the moon would be intense (perhaps rendering it into the shape of a raindrop), and the surface of the moon would be quite radioactive, but water is a very good insulator against radiation.

2313027
Wouldn't the surface of this moon just evaporate into space?

2313027 I don't think it could happen. Surely one of the defining characteristics of water and gas is that they are free to float around, because the bonds tying each molecule together are weak. As such, it's not possible to have a "core" of water because the liquid would not be able to form a stable center of gravity, as said center would shift constantly because of the properties of liquid. So the moon would just fall apart at the first opportunity.

Though if there's one thing we've learnt from astronomy, it's that weird things can happen in space. I guess one way this could (sort of) work is this:

1. A moon forms with a core/mantle of solid ice.
2. Accretion on top of the moon (maybe from another moon) forms a thick and firm crust.
3. Moon travels close to a planet to allow convection.
4. Core and mantle melt, surface stays intact.

So you might have a short-lived water moon on this, though again because of gravity it'll just spin off and crash...but then again, unpredictable moons do exist in a stable-ish orbit for millions of years. Saturn's moon Hyperion for example.

2313027
I'm pretty sure the surface would freeze over. The closest I can find to what the quote describes is Saturn's moon, Enceladus. Tidal forces create cracks in the surface, but can't keep it all liquid.

Edit: The idea of a solid ice core is interesting, though. It would likely be an entirely different form of ice than what we are used to (likely Ice II).

Fun fact: There are at least 15 different crystalline forms of ice that can be made by varying the temperature and pressure, and there is also an amorphous solid form of ice that is thought to be common in the interstallar medium.

2313204 And now we know why it's called science fiction.

2313181 As atmospheric pressure drops, the boiling point of water decreases. At zero pressure, any liquid water would immediately boil away.

So basically, the only way for a "water moon" to exist is by putting a giant force field around it? Right?

Aw darn...reality strikes again...

Well, for it to be called a "moon", it needs to be a satellite- an object that orbits a planet. Technically, under the IAU's definition, the "water moon" would be classified as a planet.

I think the heat buildup from all of that pressure might cause boiling...

And also, any solid matter denser than water that ended up on such a moon would sink to the center. Before long it would have a solid core of meteor fragments and perhaps sunken settlements.