December 4, 2009
APPLIED PHYSICS LABORATORY
"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.