  Whether volatiles can be entrapped in a background matrix composing planetary
envelopes and be dragged via convection to the surface is a key question in
understanding atmospheric fluxes, cycles and composition. In this paper we
consider super-Earths with an extensive water mantle (i.e. water planets), and
the possibility of entrapment of methane in their extensive water ice
envelopes. We adopt the theory developed by van der Waals & Platteeuw (1959)
for modelling solid solutions, often used for modelling clathrate hydrates, and
modify it in order to estimate the thermodynamic stability field of a new
phase, called methane filled ice Ih. We find that in comparison to water ice
VII the filled ice Ih structure may be stable not only at the high pressures
but also at the high temperatures expected at the core-water mantle transition
boundary of water planets.
