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Many planets out there will be water worlds. Those generally come in two flavors: either the oceans are so deep that the water turns into high-pressure ices like ice-six, -seven, -ten and -eleven or they aren't, leading to worlds with rocky seafloors.

On a world with Terran gravity, ice-six would from at a depth of ca. 63 km. The oceans of Ganymede and Titan probably have these kinds of icy seafloors. So since there are ice-free and endlessly icy seafloors, I was wondering about the borderline case. Let's say a planet with global oceans with a depth of 63 +/- 6 kilometers. I'd imagine that this would result in rocky highlands and seamounts and ice-plains in the lowlands and trenches.

However, I was wondering how the boundary between ice-six and water would actually look like. Would it be a sharp distinction, like a smooth ice-plain or rather uneven and like the underwater side of ice-bergs? Or would there be a slushy transition zone with poorly defined borders? Or something totally different?

Additionally, should I think of ice-six just as dense ice or is seeing it as rock made out of water a more accurate description?

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    $\begingroup$ I'd like to note that I've heard that ice VII is more likely to be the dominant form of ice on the bottoms of ocean planets (see e.g. iopscience.iop.org/article/10.1086/509800/pdf), along with some ice X, which might change the answer to the question slightly. $\endgroup$ – HDE 226868 May 7 '20 at 14:50
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The water phase diagram can help you understand how the interface will behave.

water phase diagram

The transition from liquid to solid is sharp, by changing the pression like you would have because you are going lower or higher, you would go from the liquid to the solid phase (you would move along a vertical line in the chart). Local temperature variation (horizontal in the chart) might affect the physical state, too.

But it wouldn't be anything much different than the interface between ice and water you can see in any glass of drinks.

By the way, the submerged surface of icebergs is uneven because of its interaction with water currents, the same way a wall exposed to sand-carrying wind is unevenly eroded. But on a small scale the interface is equally sharp as it is in your glass of iced tea.

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  • $\begingroup$ Thanks, so to generalize, a sharp line in a phase diagram means a sharp phase transition, and no line like for example behind the critical point means that the transition is gradual? So I would have iceberg- underside style landscapes and ice-six can be seen as rock made out of water? $\endgroup$ – TheDyingOfLight May 7 '20 at 13:42
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    $\begingroup$ I'm not sure this is accurate. There's going to a small intermediate pressure region where water transitions into a crystalline structure - you're claiming it's a near-instantaneous jump. And it might be, but there's going to be a small transition region, and I think that's what the OP is asking about. How small - well, ideally an answer would address that. $\endgroup$ – HDE 226868 May 7 '20 at 14:06

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