Methane Inventory and Cold-Trap Depletion in Ice-Giant Atmospheres

This paper examines the methane inventory of the ice giants and its removal by the tropopause cold trap. Using only published input values and transparent formulae, the calculation addresses how much methane each hydrogen-helium envelope contains at its measured deep abundance, and how strongly the cold tropopause depletes methane in the stratosphere above it. The headline results are: deep_methane_mole_fraction_uranus = 0.023; deep_methane_mole_fraction_neptune = 0.015; methane_mass_fraction_uranus = 0.141. 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 the deep inventories are enormous by terrestrial standards, hundreds to about a thousand Earth-ocean masses of methane, yet the gas that reaches the stratosphere and drives photochemistry is controlled by a temperature difference of only a few kelvin at the cold trap. Agreement and disagreement with independent published estimates are reported at their actual size, and the conclusions are limited to what the budget arithmetic supports.