John B Stone wrote:
Do we have any idea what the effect of volcanoes would be on such a deep ice crust? Are volcanoes more or less likely with all that pressure? Is there scope for mega circulation currents from ice bottom to ocean top over geological hot spots?
Interesting question, and one I pondered during PhD (in the context of Ganymede, which has a similar structure to the panthalassics but is much smaller)
. The thing about pressure is that (a) high pressure will keep any gases in the magma dissolved in it - which basically means it can't be an explosive eruption - and (b) it makes it that much harder for the rising magma to force the rock open and force its way to the rock surface because the stresses have to be a LOT higher.
TBH I'm not sure what the combined effect of that is - most likely I think it means that you'd have fewer eruptions across the rocky body, but if they do break through they'd be localised fissure eruptions that just spread out over the rocky surface. Though you'd have ice right on top of it, so that could melt the ice at the base... though the weird thing about ice X is that even at 1000K it won't melt! Basically, I have no clue what would happen under the conditions we're talking about here. In practical terms nobody would be able to get down there to take a look though. At best I'd say that maybe you could get convection happening in the ice layer, but that's about it.
As an aside, the ocean is probably going to be warm throughout - I think the only reason that the bottom of Earth's oceans is so cold (a uniform 4°C) is because of all the polar water that's sinking to the bottom and not warming up. If the planet is warm though - as is likely to be the case here - then the bottom of the ocean could be pretty warm too (in practical terms, a colder ocean floor would allow for a slightly thicker ice VI layer, but that's about it. The ice VI itself can be stable up to about 80°C though at the pressure's we're talking about).
Considering the massive amount of water needed for Panthalassic worlds compared to inner planets in our solar system - I assume all this water in our system has ended up in Gas Giants?
Yeah, the idea is that all the water is coming from ice-rich planetesimals that would usually have been incorporated into gas giants. In most cases these worlds would have formed at the inner edge of the Outer Zone and then migrated inwards a bit, so they're very stunted gas giants - if they'd remained in the outer system then they most likely would have become GGs themselves.
I am trying to visualize what surface installations would look like on this thing or if there would be any surface features like floating vegetation mats or "forests". Maybe floating towers - a variation on the proposed floating wind turbines.
There'd only be floating vegetation if something like that evolved there or was imported, and there's no solid surface at all (unless it happens to be cold enough at the poles for ice to form). And with the nearest solid surface about 80-100 km below the sea surface, you'd need a pretty long anchor
Presumably all non-organic building materials would have to be brought from off world? Even scavenging for meteor debris on the sea floor looks too hard.
Yeah, minerals that couldn't somehow be extracted from the seawater would be hard to find (and I'm not even sure it'd be THAT salty... there'd be no runoff from continents adding to the water, and nothing from a rocky seafloor... so the only input would be the original saltiness of the volatiles that formed the ocean, and any subsequent impacts. Meteors and many small asteroids would be largely disintegrated in the thick atmosphere too, and any bigger asteroids would just vaporize the ocean at the impact site the same as they would if they were hitting land. But still, it'd be a way for minerals to get into the system, and some bits may settle to the sea floor. So if the technology allows for at least robotic mining subs to trawl the seafloor looking for meteorite fragments (similar to how people find them on the ice of Antarctica - they stand out somewhat from the ice!) then it could be possible - but they'd have to withstand pressures of 10,000-15,000 atmospheres (and the 200 km round-trip too).
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