Water World with Ice Core.

[marvo]

Here's a question that came up in last nights game session. I am sure someone here has the answer.

On a world with 100% hydro, what happens if it has no solid core, i.e. it's one huge globule of water in space. My group has 2 opinions. 1) at some enormous depth the water becomes so cold it becomes an icy core, or 2) the increasing pressure prevents this from happening since ice is less dense than water, the pressure just increases and infact the temperature would eventually increase.
If option 1 were true, it would be interesting if an iceberg sized chunk broke off the core and then shot upward for a few miles to the surface.

 

[Malenfant]

Both, actually

The increasing pressure with depth would actually cause the water to change phase into high pressure ice (which is actually denser than water). If it's all water and no rock, then you don't have any radiogenic heating going on to warm it up, though you'd have circulation transferring some heat from the surface down to the depths.

See:
http://www.lsbu.ac.uk/water/phase.html
http://skua.gps.caltech.edu/hermann/ice.fig2.html

(1 atm = 100,000 Pascals)

It is possible, at ridiculously high pressures (more than you'd find in an earth-sized planet), to get forms of ice that are stable well above 100 degrees C!

It wouldn't be very possible to get such a world though since they'd accrete rock as well as ice so they'd always have a rocky core. Though some of the moons of saturn are so small that they're mostly ice.

 

[marvo]

Thanks for the info and the links. Some time ago I remember reading about the other crystaline states of ice. At that time I think it only went up to Ice IV. I never realized that the temperature it can exist at goes up with increasing pressure.

I think I will go with the rocky core surrounded by some high pressure ice at a very great depth.

 

[Malenfant]

That, basically would be kinda like what the interiors of Ganymede and Callisto (Jupiter's two largest moons) are like.

Ganymede has an iron sulphide core, surrounded by a rocky mantle that makes a rocky body about the size of our own moon. Then on top of that is probably a thick Ice VI layer, an even thicker liquid salty water layer, and a 100-200 km thick ice I shell on top.

Callisto doesn't appear to have a significant separate iron/rock core - the centre is a small undifferentiated rocky/metal core, and above it is an ice/rock mixture that goes out for most of the satellite's radius, with less and less rock as you go out. Then above that might be a clean Ice III layer, then a thin liquid saltwater layer, then a thick Ice I shell at the surface like Ganymede's.

 

[The Hinterworlds Rambler]

Wasn't it also postulated somewhere that Mid range Gas giants, Like Neptune or Uranus have a layer of super pressurized, 8000 degree (f) water in them?

 

[Straybow]

8000°F (4800°K)? I don't think so… Nah, couldn't be.

 

[TKalbfus]

An all water world would also have twice the diameter of Earth if it were to have 1-g on its watery surface. The same phenomenon that pulls gas giants towards its star would also pull in all the ice balls like Europa, Pluto, and Titan inward too. Some of these iceballs would get pulled inwards and left in the star's life zone and would proceed to melt becoming a water world. Water worlds of this sort are non-terrestrial and non-gas giants and can only exist where water can exist as a liquid. Unlike the water worlds of traveller there is no chance of having any land. A water world would have to have an atmosphere above its watery surface, if it didn't, its watery surface would boil away until it released enough dissolved gases to prevent it from oiling any further. If nothing else the atmosphere would be made out of low temperature water vapor. Life might exist on a water world. Too bad the Traveller planetary generation system doesn't account for them. Their should be a chance of having a water giant, that is a planet that's mostly water rather than just a terrestrial world that's completely covered with ocean, the latter is mostly rock and Iron like the Earth is, while the former is mostly water.

 

[Malenfant]

Originally posted by Straybow:
8000°F (4800°K)? I don't think so… Nah, couldn't be.

Actually, that may be about right...

http://www.space.com/reference/brit/neptune/climate.html

The temperature is about 7000 K at the rocky core. And remember liquid water at stupidly high pressures and temperatures is a supercritical fluid, so it's kinda like a gas and liquid at the same time...

You can be sure of lots of weirdness when you talk about high pressure H2O

 

[Malenfant]

More info on what Tom is saying can be found here:
http://arxiv.org/abs/astro-ph/0308159

(the paper is the PDF download there)

Very interesting stuff.

I'm working on an expanded, more realistic CT-style atmosphere generation system that includes these worlds. Though if it sees the light of day it'll be as a JTAS article...