  Ethane is expected to be the dominant photochemical product on Titan's
surface and, in the absence of a process that sequesters it from exposed
surface reservoirs, a major constituent of its lakes and seas. Absorption of
Cassini's 2.2 cm radar by Ligeia Mare however suggests that this north polar
sea is dominated by methane. In order to explain this apparent ethane
deficiency, we explore the possibility that Ligeia Mare is the visible part of
an alkanofer that interacted with an underlying clathrate layer and investigate
the influence of this interaction on an assumed initial ethane-methane mixture
in the liquid phase. We find that progressive liquid entrapment in clathrate
allows the surface liquid reservoir to become methane-dominated for any initial
ethane mole fraction below 0.75. If interactions between alkanofers and
clathrates are common on Titan, this should lead to the emergence of many
methane-dominated seas or lakes.
