How deep can the ocean get?

[Tom Mazanec]

IIRC, at some pressure water turns into various ices, even at high temperatures. So how deep can a water ocean at 1G get before it 's bottom "freezes"?

[primummobile]

There is a lot of information here about water phase transitions: http://www.lsbu.ac.uk/water/phase.html

I don't know all the variables of all the different forms of water-ice (I believe there are around 15 or 16 different types of solid water) but I highly doubt any of them can form just from pressure. Water has a fairly unique property: it expands when it undergoes a liquid-to-solid phase transition. Ice melts if you compress it. That's also why ice floats on water. If it didn't, the oceans would have probably frozen solid long ago.

[primummobile]

Here's some information on how high-density (more dense than liquid water) ice can form: http://en.wikipedia.org/wiki/Amorphous_ice

It seems to require temperatures that do not occur naturally on Earth.

[Grashtel]

There is a lot of information here about water phase transitions: http://www.lsbu.ac.uk/water/phase.html

I don't know all the variables of all the different forms of water-ice (I believe there are around 15 or 16 different types of solid water) but I highly doubt any of them can form just from pressure. Water has a fairly unique property: it expands when it undergoes a liquid-to-solid phase transition. Ice melts if you compress it. That's also why ice floats on water. If it didn't, the oceans would have probably frozen solid long ago.

Um, you didn't actually read that page you linked to did you? The phase diagram on it shows several forms of ice (ice-VI and ice-VII are the ones relevant to this discussion) that can form from pressure alone. Also if you follow the link to the page about phases of ice most of them have a density higher than liquid water, though fortunately the ones that have lower density than liquid water are also the only ones that can form at the pressures found naturally on Earth's surface.

[Ivan Viehoff]

Consider for example the structure of the icy moons of outer planets. Those that have a liquid water layer have it beneath the solid water layer. This liquid layer lies above a rocky layer, see for example Europa. It would be curious if there were an intermediate liquid layer sandwiched between two solid layers, and I do not believe that this is generally proposed.

Consider for example the structure of Uranus. Although its mantle is described as being "ice", it is in fact liquid, all the way down to the small rocky core, so far as we can tell. The same is true of Neptune.

It appears that in icy planetary bodies in general solid layers of water in general lie on top of liquid layers, not the other way around.

Though in fact earth has a minor counterexample to this. In some locations, solid methane hydrate lies on the ocean floor. So water can become solid at the base of an ocean, in a particular combination with other substances, for example methane. Such exceptions may well also exist in the other icy bodies, but are unlikely, so far as we can tell, to be major components of their structure.

[primummobile]

Um, you didn't actually read that page you linked to did you? The phase diagram on it shows several forms of ice (ice-VI and ice-VII are the ones relevant to this discussion) that can form from pressure alone. Also if you follow the link to the page about phases of ice most of them have a density higher than liquid water, though fortunately the ones that have lower density than liquid water are also the only ones that can form at the pressures found naturally on Earth's surface.

Um, yeah I actually did. I also said I didn't know all the variables, and that it didn't look to me like it could occur on Earth. (read the next post)

[Grashtel]

Um, yeah I actually did. I also said I didn't know all the variables, and that it didn't look to me like it could occur on Earth. (read the next post)

Do you know how to read a phase diagram? The vertical axis represents pressure (on a logarithmic scale) and the horizontal axis represents temperature. Reading off of it (or rather the zoomed in bit just below it) finds that at a temperature of 4 degrees centigrade ice-six (which is 30% denser than water) will form at roughly 700 Mpa. The pressure at the bottom on the Challenger Deep is 108 MPa so its not a factor for Earth but the OP is asking about maximum possible depth not existing ocean depths, though that can be used to work out that assuming that the temperature remains constant and water remains incompressible (decidedly unlikely IMO) the maximum ocean depth before the bottom freezes out into pressure ice in 1g is in the order of 70km.

[primummobile]

Do you know how to read a phase diagram? The vertical axis represents pressure (on a logarithmic scale) and the horizontal axis represents temperature. Reading off of it (or rather the zoomed in bit just below it) finds that at a temperature of 4 degrees centigrade ice-six (which is 30% denser than water) will form at roughly 700 Mpa. The pressure at the bottom on the Challenger Deep is 108 MPa so its not a factor for Earth but the OP is asking about maximum possible depth not existing ocean depths, though that can be used to work out that assuming that the temperature remains constant and water remains incompressible (decidedly unlikely IMO) the maximum ocean depth before the bottom freezes out into pressure ice in 1g is in the order of 70km.

Yes, I know how to read a phase diagram. I wouldn't have posted a link to a phase diagram for water if I didn't know how to read it. As should be obvious from my posts, I was assuming the poster was asking about how deep an ocean would need to be on Earth in order for the bottom to freeze. Whether or not I assumed wrong is not relevent so please don't be snarky about it. I very clearly pointed out that I was referring to Earth conditions. I left it for someone else to do the calculations, but I can do them if you wish for a hypothetical liquid water ocean on a hypothetical planet. I'm not sure that an ocean on Earth could be deep enough to allow for a layer of ice to form at the bottom. But since I don't know the exact mechanics of how oceans form, I said that I don't know all the variables.

Assuming consistent temperature, pressure increases roughly 101 KPa (actually 101.3 KPa) for every 10m of depth. So to get 700 MPa of pressure you would need to have a depth of 69102m which is 69km. Pretty close to what you got. Reading the chart wasn't the problem. It was the question of how deep it is possible for an ocean on Earth to be. Since I didn't know that, I was just trying to be helpful by linking to some information that I do understand for the OP. That's it.

EDIT: I forgot to take into account the atmosphere, so it would actually only require slightly less depth if it was on Earth.

[HenrikOlsen]

You'd also need to know the temperature at depth, which is another unknown.

[grapes]

You'd also need to know the temperature at depth, which is another unknown.

Since the bottom would have the densest water, it'd be around 3-4 degrees, but that would depend again on pressure and composition.

[Tom Mazanec]

Of course, I know that an ocean depth on Earth almost certainly cannot reach 70 km, since the crust would "flow" into the trench. That was not the effect I was asking about, but what depth the pressure would solidify the water.
Interesting that we get ~15% of the way there in the ocean's deepest real depth. My intuitive guess would have been something like 1%. My intuition is not always (or even often) right.