AN ESTIMATE OF THE CHEMICAL COMPOSITION OF TITAN's LAKES

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 aboard the Huygens probe and recent photochemical models based on the vertical temperature profile derived by the Huygens Atmospheric Structure Instrument. Thermodynamic equilibrium is assumed between the atmosphere and the lakes, which are also considered nonideal solutions. We find that the main constituents of the lakes are ethane (C2H6) (similar to 76%-79%), propane (C3H8) (similar to 7%-8%), methane (CH4) (similar to 5%-10%), hydrogen cyanide (HCN) (similar to 2%-3%), butene (C4H8) (similar to 1%), butane (C4H10) (similar to 1%), and acetylene (C2H2) (similar to 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.