New data about Ceres has just been released. Raymond et al. (2020) show the existence of gravity anomalies interpreted as low density pockets of brine under the crust. They show how impacts can trigger eruption of this brine onto the surface.
I understand how a low density layer can remain trapped under a higher density layer by lack of pores and/or fractures to rise through by buoyancy. However, I cannot figure how it got trapped there in the first place? For subsurface oceans under an icy crust (like Europa), I can imagine a simple freezing mechanism, but Ceres has a rocky crust.
Wikipedia's Ocean World - Formation goes:
Since water is highly soluble in magma, a large fraction of the planet's water content will initially be trapped in the mantle. As the planet cools and the mantle begins to solidify from the bottom up, large amounts of water (between 60% and 99% of the total amount in the mantle) are exsolved to form a steam atmosphere, which may eventually condense to form an ocean.
But it doesn't really help as it applies to surface oceans only. Neither does Extraterrestrial oceans on that particular matter.
How do subsurface oceans form under a rocky crust?