Hydrocarbons Lakes on Titan

Introduction: Titan has a massive atmosphere with a pressure at surface level of ~1.5 bars. In the atmosphere the molecular nitrogen N2 is the dominant constituent (~94 percent) and the methane CH4 is the second most abundant component (~5 percent) [1]. The atmosphere of Titan is photochemically active with an efficient production of ethane, ethylene, acetylene and propane as minor components. At the present photolysis rate, methane mass in the atmosphere is photodissociated in ~4∙10 years [2]. During the Voyager mission, the hypothesis was advanced that large areas of Titan’s surface would be covered by liquid hydrocarbons [2,3]. Since Cassini-Huygens indicates that such oceans are not present, the atmospheric methane abundance likely results from a recent outgassing episode from the interior [4]. If methane outgassing activity only supplies methane to the atmosphere, then we expect that the outgassing episode is 10 years ago. On other hand, a large volcanic outburst of methane (for example ~10 years ago) could saturate the atmosphere. This would have led to formation of some small lakes. This reservoir would release methane into the atmosphere and allow atmospheric methane to exist to the present time. The Descent Imager Spectral Radiometer (DISR) Experiment on the Huygens Probe does not show evidence of exposure of liquid at the landing site [5], but liquids are present just below the surface [1] and the surface science package accelerometry data admits either wet or dry surface materials [6]. Otherwise during the descent, DISR has shown networks of channels and valleys-like features that strongly suggest fluvial erosion processes of liquids (likely methane and ethane with dissolved nitrogen [7]). Cassini remote sensing (Radar and Cassini Imaging) highlight that network channels are common features on the surface [8,9]. The Cassini narrow-angle camera during the Titan flyby on the June 6, 2005 detected a dark area in the South Polar Region with smooth boundaries (~230 km per 70 km) that could be a dry lakebed or a lake of liquid hydrocarbons [8]. Clouds with two distinct morphologies have been detected on Titan. Near the South Pole are observed large storms. Clouds as spots are detected between -37° and -44° latitude and between 0°90° and 90°40° longitude [10]. These clouds suggest a correlation with surface processes [10]. In this paper we address two questions: (i) Are the observations of atmospheric methane relative humidity and thunderstorms/cloud frequency consistent with a “desert planet” containing only tiny fractional lake coverage? (ii) If any are observed, are hydrocarbon lakes stable on the surface of Titan? Stability of hydrocarbons lakes: We determine the evaporation rate E [kg m s] of liquid methane and of a mixture of methane, ethane and nitrogen on the surface of Titan with the bulk aerodynamic method [11,12], using direct measurements of pressure, ground wind velocity, temperature-, air densityand humidityprofiles by Huygens instrumentations. The evaporation rate is given by [11,12]   r air u q q K E   *  (1)