  Hundreds of radar-dark patches interpreted as lakes have been discovered in
the north and south polar regions of Titan. We have estimated the composition
of these lakes by using the direct abundance measurements from the Gas
Chromatograph Mass Spectrometer (GCMS) aboard the Huygens probe and recent
photochemical models based on the vertical temperature profile derived by the
Huygens Atmospheric Structure Instrument (HASI). Thermodynamic equilibrium is
assumed between the atmosphere and the lakes, which are also considered as
nonideal solutions. We find that the main constituents of the lakes are ethane
(C2H6) (~76-79%), propane (C3H8) (~7-8%), methane (CH4) (~5-10%), hydrogen
cyanide (HCN) (~2-3%), butene (C4H8) (~1%), butane (C4H10) (~1%) and acetylene
(C2H2) (~1%). The calculated composition of lakes is then substantially
different from what has been expected from models elaborated prior to the
exploration of Titan by the Cassini-Huygens spacecraft.
