QUOTE (Littlebit @ Jul 23 2007, 01:54 PM)
There is also the chromatographic soil effect: At a nuclear facility near Hanford, Washington, extremely low concentration radioactive wastes were dumped in an evaporative sludge pond. Over time (probably decades), elements were chromatographically separated in the clay, and a layer of high energy waste was concentrated naturally near the surface to an unnatrual level - almost self sustaining.
Likely nucleides for heating mantles (at least for Earth) are Uranium, Thorium, and Potassium-40.
According to Wikipedia (my copy of Cotton and Wilkenson's is back at the office), uranium likes being in oxidation states U(+4) or U(+6), with the most common form in nature (terrestrial conditions) being U3O8. "Both oxide forms are solids that have low solubility in water and are stable over a wide range of environmental conditions."
But at the bottom of an ammonia water ocean, what would be the preferred form?
Would it be a uranium hydroxide (U[OH]6), or would it be ligated to ammonium (NH4)xUy(OH)z? Could there be hot water percolating throughout a silicate core concentrating some bizarre uranium species at the silicate/water interface?
In Littlebit's Hanford scenario, the water percolates upward and evaporates. (Or does it percolate down, and concentrate the stuff at the "top of the column" - leaching away everything but the uranium)
On Charon, maybe the material percolates "upwards" but the uranium species (U3O8?) crashes out when it hits cooler water?
This just begs for a cool (and easy to do experiment with a scintillation counter) experimental model.
Just not in my lab....
(I wonder what the forms of other likely radionucleides are? WWTD [What Would Thorium Do?])
[Potassium(40) is easy: KOH would be the preferred form and it is extremely soluble in water - it would not be able to concentrate]