Thickness and Mass of the Superionic Water Layer in Uranus and Neptune

This paper examines the superionic water layer that is thought to occupy much of each ice giant interior. Using only published input values and transparent formulae, the calculation addresses how thick the superionic region is, and how much mass it contains, if it occupies the radial interval between 0.35 and 0.70 of the planetary radius where pressures and temperatures cross the superionic boundary in published phase diagrams. The headline results are: layer_thickness_km_uranus = 8877; layer_thickness_km_neptune = 8618; layer_volume_fraction = 0.3. The method, inputs and limitations are stated in full so that any reader can reproduce or rescale every number from the deposited calculation script. The central finding is that a shell spanning this radial interval contains roughly a third of the planetary volume but, at compressed-water densities, around half or more of the planetary mass, so the superionic region is not a minor coating but the principal structural component of each planet. Agreement and disagreement with independent published estimates are reported at their actual size, and the conclusions are limited to what the budget arithmetic supports.