Post by Thomas Cornet, Olivier Bourgeois, Stephane Le Mouelic et al.,
Laboratoire de Planétologie et Géodynamique de Nantes, , Université de Nantes, UMR 6112, CNRS, Nantes, France.
Titan, Saturn’s major moon, possesses hydrocarbon lakes and seas in the polar regions [Stofan et al., 2007, Hayes et al., 2008]. Among these, Ontario Lacus (72°S, 180°E, Image 1) is the largest in the south (235 km-long, 75 km-wide). So far it is interpreted as a liquid-covered lake in Titan’s southern hemisphere because of its dark appearance in Cassini image data [Barnes et al., 2009; Turtle et al., 2009; Hayes et al., 2010; Wall et al. 2010], the identification of liquid ethane in its interior [Brown et al., 2008] and the smoothness of its surface [Wye et al., 2009].
The detailed geomorphological study of Ontario Lacus and its surroundings (Image 2), however revealed the presence of channels inside the southern part of the interpreted liquid-covered depression, seen with multiple sensors at the same location between 2007 and 2010 [Cornet et al., 2012]. This seems to indicate that the southern part of the depression floor was actually not fully liquid-covered at the time of the observations, and was most probably an exposed flat-floor with a substratum saturated in liquids.
Given the extremely flat topographic settings of the region (Image 2) [Wye et al, 2009; Wall et al., 2010], and the presence of channels and small lakes in the flat alluvial plain in which Ontario Lacus lies [Wall et al., 2010; Cornet et al., 2012], an “alkanofer”, (an analog to the groundwater aquifer on Earth) would be located close to the topographic surface, thus inundating topographic lows inside Ontario Lacus’ depression and the surrounding plain.
Titan’s climate is characterized by sporadic torrential hydrocarbon rainstorms [Turtle et al., 2011], with expected yearly-averaged evaporation rates greater than yearly-averaged precipitation rates [Mitri et al., 2007]. This balance between evaporation and precipitation is very similar to that in arid/semi-arid areas on Earth, such as in the Owambo Basin in Namibia (Image 1) [Mendelsohn et al., 2002].
In this flat and wide sedimentary basin (Image 3), flat-floored depressions called “pans” develop by dissolution of a surface soluble calcretes layer and the vertical motion of the groundwater table [Miller et al., 2010; Bowen and Johnson, 2012]. The largest pan of the Owambo Basin, namely the Etosha Pan (18°S, 16°E, Image 1), is closely similar in size and shape to Ontario Lacus.
Since solid hydrocarbon compounds that form in Titan’s atmosphere fall onto the surface they can accumulate up to several tens of meters in thickness over geological timescales [Malaska et al., 2011]. These compounds are potentially soluble in liquid methane and ethane [Cordier et al., 2009; Malaska et al., 2011] and suggest that a surface soluble layer could therefore have formed on Titan’s surface. Based on the analogy with the Etosha Pan and on thermochemical models, one possible mechanism to form Ontario Lacus’ depression could be the dissolution of a surface soluble layer on Titan, due to vertical motions of the “alkanofer”.
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