bdm wrote:The same goes with a hypothetical methane ocean on an ice world. Liquid methane has a density about half that of ice, so that configuration is stable.
I don't think that example would actually be possible though - the methane would have to be liquid due to the high pressure rather than low temperature (or the water would freeze). From what I recall of the phase diagrams of water and methane I don't think it's possible for their liquid to ovrelap (I could be wrong, but it doesn't sound likely).
Also, we don't have oceans on the surface solely because it's less dense that rock (though I guess that's a factor) - we have them there largely because it's a volatile material that tends to be outgassed from a planet's interior, and because it can be deposited easily from outside via comets and icy objects. And it can be rained out in large amounts from the atmosphere onto the surface.
However, if we're talking about lead then what you talk about regarding the density would come into play. It wouldn't be a stable situation, the lead would want to seep down into the crust (if the crust was made of any reasonable natural material - ie silicates) and towards the core.
Ocean basins are not watertight bowls, destined to remain watertight for all eternity. They can get splits in them which can let the oceans drain out. There are splits in Earth's oceans, at the midocean ridges. Because the water is lighter, it cannot escape.
Um, no. You can't 'drain' oceans like that, it doesn't work that way. First there's nowhere for them to drain to. Second, any cracks that open in the seafloor usually have stuff coming up them (eg lava, like the Mid ocean ridges). But most importantly, seawater does actually percolate through cracks in the seafloor anyway - that's how hydrothermal vents form. The water seeps into the rock via cracks (it can get pretty far down too), gets heated up by magma and then gets sent back up laden with minerals from its journey through the hot rock - that makes the 'black smokers' you see at underwater vents. But it's not possible to "drain" the oceans by those cracks, unless for some reason the water isn't sent back up (and a combination of pressure from below and low density of the steam will send it back up anyway).
Now imagine a heavy lead ocean on a lighter rocky crust. If there's a split in the crust, the lead will force its way out the bottom due to its density. The rock under the oceans would be soft, so the lead can force its way out. Even if there was no split, the density differential between lead and rock will eventually make one, rather like an upside-down lava lamp.
It's true that there will eventually be a density overturn like you describe. Dense stuff does not like to be on top of less dense stuff - the less dense stuff is buoyant.
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