February 19, 2011 | David F. Coppedge

Titan’s Methane Lakes Shallow, Dynamic

Strange things are happening on Titan, Saturn’s largest moon: lakes are appearing and disappearing.  This can only mean that the lakes are shallow and the liquid hydrocarbons in them are moving around.
    Lakes were discovered a few years ago in the northern regions of the Mercury-size moon.  They consist predominantly of methane (CH4) and ethane (C2H6).  Another large lake called Ontario Lacus (Lake Ontario, because of its similarity to Earth’s counterpart) was discovered near the south pole.  Then, in Oct 2004, new dark areas appeared in Arrakis Planitia near the south after a presumed cloudburst of liquid methane (picture); the lakes in this area have also shrunk considerably in 44 months between observations.
    A new paper in Icarus1 presented observations in visible light, infrared and radar covering the period 2004-2009.  They indicate that Ontario has been shrinking rapidly between 2005 and 2009 (picture).  The southwest shoreline has retreated by 9-11 kilometers (5.5 to 7 miles).2  Though estimates are difficult due to the distance and resolution of some measurements, the authors’ best guess is that “The observed retreat represents a decrease in area of ~500 km2 over almost 4 years.
    Estimating volume loss is more difficult.  While impossible to calculate Ontario’s volume loss directly, they estimated how much Arrakis gained and lost as a proxy.  Based on estimates of methane-carrying capacity of the 2004 cloud system (about a million square kilometers), the cloudburst must have dropped 2.4 to 14 cm of methane rain into the Arrakis basin (upper limit 4.2 m).  This yields estimates that between 24 to 140 km3 of liquid was lost at Arrakis in 4 years from a combination of evaporation and infiltration; probably similar amounts at Ontario.
    There are clues that the lake bottoms might be impermeable.  The northern lakes are in Titan spring and have not shrunk between observations.  Earlier estimates expected one meter of seepage into the interior per year.  The rapid shrinkage at Arrakis and Ontario over a timescale of several months “strongly suggests either a shallow impermeable layer or that the local methane table lies close to the surface.”  It will be interesting to watch what the “methane cycle” does to the southern and northern lakes as the seasons change and more sunlight hits the north.
    One other interesting observation was that the exposed lake bottom is not dark, as might be expected from sedimentation of hydrocarbons.  Either wave action cleansed the bottom as the shoreline retreated, or any sediments are light colored.  The authors favor the latter, saying that “bright organic condensates may be deposited within the lakes and exposed as the liquid level drops (Barnes et al., 2009).”  This view is strengthened by the fact that the Cassini orbiter’s cameras saw numerous dark features in the south in 2004-2005, but light material as Ontario retreated.  It is not possible to know from albedo (reflected brightness) alone the composition of the bright sediments.

1.  E.P. Turtle, J.E. Perry, A.G. Hayes, A.S. McEwen, “Shoreline Retreat at Titan�s Ontario Lacus and Arrakis Planitia from Cassini Imaging Science Subsystem Observations” (accepted manuscript; final pending), Icarus, Feb 2011, S0019-1035(11)00054-6, DOI: 10.1016/j.icarus.2011.02.005.
2.  Highest resolution was possible at the southern parts of the lake; more uncertainty exists at the northern boundaries.  Radar altimetry suggests that the southwest shore has a gradual slope, while the eastern shore is steep.

The study of Titan is a work in progress, so any conclusions drawn at this time are subject to revision as more data come in.  We can, however, step back and consider what planetary scientists expected to find and what they have found so far.  In the decades after the Voyager visits (1981), when scientists realized an irreversible erosion of atmospheric methane was precipitating hydrocarbons onto the surface (especially ethane, which has no way to get back into the atmosphere), scientists expected to find, over the course of 4.5 billion years, an accumulation of half a kilometer or more of liquid ethane in a global ocean.  That was a clear prediction that has been spectacularly falsified by Cassini observations (see list of previous articles).  In fact, the Huygens probe was designed to float on that ocean that failed to materialize.
    Instead, we found Titan to have paltry accumulations of liquid in scattered lakes near the poles, while the equatorial regions are largely covered in icy sand dunes.  Now we are learning that the polar lakes are probably shallow, could have impermeable bottoms, and move around so rapidly that they don’t deposit sediment on the lake floors (or else they deposit bright sediments).  But if the sediments are bright, which would be surprising in itself, is there enough sediment to account for 4.5 billion years of deposition?  In addition, Titan, the largest moon with the greatest gravitational attraction, has few craters (three to five) after all that time.
    You have to ask yourself whether it is credible these processes have been going on for billions of years.  Did 4.5 billion years ever exist?  Is it a fiction?  In order to save the blessed timescale so precious to planetary scientists (because Darwin depends on it), all kinds of evidence-free theory-rescue devices are being rigged: maybe the ethane seeped into the interior where no one can find it; maybe the interior has a methane reservoir that erupts through cryovolcanoes, replenishing the atmosphere; maybe this, maybe that.  If scientists stuck to the observations and drew reasonable conclusions from data alone, they would have to conclude that there are severe upper limits on how long Titan has been acting this way.  Let facts be submitted to a candid world.

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