Scientific Colloquium
December 4, 2009

"Lakes of Hydrocarbons and Seas of Sand on Titan"

Titan's surface is covered in organics.  Cassini has found that about 20% of the surface, all at low latitudes, is blanketed by fields of massive sand dunes, while  the polar regions contain hundreds of lakes of liquid ethane and methane.  Ethane is produced from the photolysis of methane high in Titan's atmosphere. Cassini  has discovered, unexpectedly, that higher molecular weight materials including benzene, anthracene and organics up to 2000 amu and beyond are produced in abundance at high altitude.
These latter solids appear to form the bulk of the dunes, which can be ~200m high and hundreds of km long, forming the sand seas of Belet and Shangri-La.  The ~ 3km spacing of dunes appears consistent with a recent theory that dune spacing is capped by the height of the planetary boundary layer. 
While winds evidently sculpt the sands, recent Cassini measurements show that the lakes are literally flat as millponds.  Empirical wind-wave relationships for Earth need adjustment for the Titan environment; the lack of waves may be due to the quiescent season (about to change) of observations so far, or because the seas are viscous and thus dissipative.
Interestingly, Titan's north pole has many more lakes (and two seas, Kraken and Ligeia Mare, ~1000 and ~500km across) than the south.  A leading possibility, and one that may account for the absence of impact craters at the poles, is that asymmetry in the seasons pumps volatiles to the north in the present epoch. Astronomical change, like the Croll-Milankovich cycles that drive the Earth's glacial periods and cause climate change on Mars, will flip this seasonal forcing with a period of ~50,000 years.   This kind of climate/landscape coupling underscores Titan as having behaviors more fitting to a terrestrial planet than an icy moon.

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