Geomorphological significance of Ontario Lacus on Titan: Integrated interpretation of Cassini VIMS, ISS and RADAR data and comparison with the Etosha Pan (Namibia)

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Ontario Lacus is the largest lake of the whole southern hemisphere of
Titan, Saturn's major moon. It has been imaged twice by each of the
Cassini imaging systems (Imaging Science Subsystem (ISS) in 2004 and
2005, Visual and Infrared Mapping Spectrometer (VIMS) in 2007 and 2009
and RADAR in 2009 and 2010). We compile a geomorphological map and
derive a ``hydrogeological{''} interpretation of Ontario Lacus, based on
a joint analysis of ISS. VIMS and RADAR SAR datasets, along with the T49
altimetric profile acquired in December 2008. The morphologies observed
on Ontario Lacus are compared to landforms of a semi-arid terrestrial
analog, which resembles Titan's lakes: the Etosha Pan, located in the
Owambo Basin (Namibia). The Etosha Pan is a flat-floored depression
formed by dissolution, under semi-arid conditions, of a surface
evaporitic layer (calcretes) controlled by groundwater vertical motions.
We infer that Ontario Lacus is an extremely flat and shallow depression
lying in an alluvial plain surrounded by small mountain ranges under
climatic conditions similar to those of terrestrial semi-arid regions.
Channels are seen in the southern part of Ontario Lacus in VIMS and
RADAR data, acquired at a 2-years time interval. Their constancy in
location with time implies that the southern portion of the depression
is probably not fully covered by a liquid layer at the time of the
observations, and that they most probably run on the floor of the
depression. A shallow layer of surface liquids, corresponding to the
darkest portions of the RADAR images, would thus cover about 53\% of the
surface area of the depression, of which almost 70\% is located in its
northern part. These liquid-covered parts of the depression, where
liquid ethane was previously identified, are interpreted as topographic
lows where the ``alkanofer{''} raises above the depression floor. The
rest of the depression, and mostly its southern part, is interpreted as
a flat and smooth exposed floor, likely composed of a thick and
liquid-saturated coating of photon-absorbing materials in the infrared.
This hypothesis could explain its dark appearance both in the infrared
and radar data and the persistence of channels seen on the depression
floor over the time. Shorelines are observed on the border of Ontario
Lacus suggesting past high-stand levels of the alkanofer table. The
analogy with the Etosha Pan suggests that Ontario Lacus' depression
developed at the expense of a soluble layer covering the region.
Dissolution of this layer would be controlled by vertical motions of the
alkanofer table over the time. During flooding events, liquid
hydrocarbons covering the depression floor would dissolve the surface
layer, increasing progressively the diameter of the depression on
geological timescales. During drought episodes, liquid hydrocarbons of
the underground alkanofer would evaporate, leading to crystallization of
``evaporites{''} in the pores and at the surface of the substratum, and
to the formation of the regional soluble layer. The presence of specific
landforms (lunette dunes or evaporites) is compatible with such
evaporitic regional settings. Alternatively, but not exclusively, the
surface soluble layer might have formed by accumulation on the ground of
soluble compounds formed in the atmosphere. (C) 2012 Elsevier Inc. All
rights reserved.