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While it has not been confirmed, Callisto (Jupiter IV) could potentially have a small subsurface ocean below a thick ~100 km ice layer, according to Wikipedia. A mission to explore that ocean might require the presence of surface support for data relay to remain in place for a long time, which makes the lower radiation environment there more attractive than that of Europa (Jupiter II).

However, the substantially greater thickness of the ice (~100 km vs 10 to 30 km) is a disadvantage.

Within this context, I'd like to just ask if the thickness of the ice below a deep depression or crater is less than average, or if the ice will end up about the same thickness because the heat transport problem is still the same.

I'm asking this because Callisto does have some topography, for example the Valhalla basin.

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  • $\begingroup$ I didn't down vote, but I can guess that it might have happened because the question contains a lot of extraneous story-telling. You'd like to ask if the thickness of the ice below a deep depression or crater is less than average, or if the ice will end up about the same thickness because the heat transport problem is still the same. Asking like that might result in your question being better received. $\endgroup$ – uhoh May 14 '18 at 23:00
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    $\begingroup$ If you'd like, I can make a suggested edit and you can roll-back if you don't like it. $\endgroup$ – uhoh May 14 '18 at 23:01
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    $\begingroup$ Sure, suggest an edit $\endgroup$ – JavaScriptCoder May 14 '18 at 23:02
  • $\begingroup$ Take a look. It's a suggestion, you can easily roll-back (I'll show how if you don't know) or you can re-edit and add your own preferred wording. $\endgroup$ – uhoh May 14 '18 at 23:18
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In short: not as much as you might think. All moons with water ice shells over liquid water oceans, and even those with no liquid water oceans but warmer and thus less rigid ice beneath a cold ice crust, modify craters after their initial formation via isostatic rebound.

Imagine what would happen if you suddenly excavated a crater-shaped hole in a liquid water ocean: the surrounding water would rush into the void and fill it up, until the water surface in the former void came to the same gravitational potential as the surrounding water. Ice does the same thing, only much, much slower, and this is isostatic rebound. Unlike the liquid water the really cold ice does have some strength, along with, maybe, a decrease in the porosity of the newly-formed ice in and around the crater, making it denser. That combination can stop the rebound just short of returning to a flat surface, and you get mild impact-generated depressions like Valhalla.

Research on crater formation at Callisto has been going on sporadically for decades. Recently Bjonnes, Johnson, & Silber found that at Ganymede, as crater diameter increases, at ~20-30 km diameter the depth bottoms out at ~1 km. Larger craters are no deeper. Slumping immediately after formation, and isostatic rebound later, smooth things to the ~1-km level. Callisto's cratering behavior will be similar.

If the crater's interior is completely isostatically compensated, i.e. it behaves as if it and everything around it are floating on whatever is beneath, then the depressed surface elevation will be echoed by a raised base at the bottom of the ice layer. It's similar to the iceberg phenomenon: if a cylindrical iceberg extends 100 m above the sea surface, then it will extend about 700 m below the surface.If it extends only 50 m above the sea surface, then it extends only 350 m below the surface. For Earth-like pressures, that ~1:7 ratio holds (US Coast Guard's number, for sea water; in pure water it's more like 1:9). Water and ice densities change a bit with the huge pressures encountered on these thick-water-layer moons, but if there is full isostatic compensation, a 1-km depression on the surface might mirror a 5-10 km rise in the ice shell bottom below it.

That's still small change compared to the ice shell thickness, which has been estimated at anywhere from 100 km to 150 km, though at planetary science meetings I've heard interior modelers say it could be even thicker.

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    $\begingroup$ Great answer! We're lucky to have a planetary scientist here. A bit more about hydrostatic equilibrium here. $\endgroup$ – uhoh May 15 '18 at 4:14

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