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Fig. 13.13 Possible stabilized or 'fossil' dunes, at a latitude of some
53N. Not only does their orientation deviate from the typical, but
these features (well north of the sand seas) have a generally radar-
brighter appearance than their equatorial counterparts, suggesting a
somewhat different surface texture. Image NASA/JPL/Cassini Radar
Team/J. Radebaugh
Fig. 13.12 Radar measurements of the interdune area fraction in
dune-covered areas as a function of latitude by Le Gall et al. (2011).
Work by others has substantiated this trend, which is presumably a
result of varying sand mobility due to the asymmetry of Titan's
seasons in the present epoch
construct the dunes. Since more heavily-eroded areas have
since been found, it may be that this calculation has to be
revisited. The observed impact crater distribution—origi-
nally thought to be a likely source for sand-sized material (it
is believed to be the dominant sand source on Venus)—is
unable to provide the required volume, unless some other
process has broken down larger ejecta.
Thus, at present, a photochemical origin appears to be
favored: observations (see next paragraph) favor an organic
composition, and estimates (Lorenz et al. 2006, 2008, 2010)
of organic production in the atmosphere seem to be con-
sistent with the dune volume, suggesting that the sand
formed from what were once haze particles. Conversion of
\1 micron haze particles into 250 micron sand grains could
perhaps occur by sintering over long timescales (although
some details need to be worked out, like why the process
would stop…), or perhaps, more likely, it may involve
cycles of wetting and drying in Titan's lakes. The latter
scenario would require that the sand move from the lakes at
the poles to the equatorial regions where the dunes are
found. The sand source remains an outstanding problem in
Titan science.
As for its composition, there are a few constraints. The
microwave properties of the sand seas (Le Gall et al. 2011,
2012) support a porous, organic composition. Water ice
(once suspected to be the dominant bedrock on Titan, but
apparently largely covered by an organic veneer in most
places) appears to be ruled out. The material is dark at
visible
materials such as impact ejecta or fluvial sediments, or by
agglomerating finer material such as the atmospheric haze.
Lorenz et al. (2008) have estimated the total sand vol-
ume, noting that about 40 % of the low-latitude half of
Titan (i.e., about 20 % of the total) appears covered in
dunes, and used radarclinometric, radiometric, and simi-
larity arguments to estimate the average depth to be
between 200,000 and 800,000 km 3 of material, corre-
sponding to a thickness of several meters over the whole
planet. That estimate has been revised with the benefit of
further radar coverage and more careful analysis by Le Gall
et al. (2011) who find 12.5 % of the surface (i.e., a total of
*10 millions km 2 ) covered in dunes with an estimated
total volume of 250,000 km 3 , although this of course relies
on the assumed dune height.
Neish et al. (2010) used radarclinometry to study the
height H of a variety of dunes and validated the accuracy of
this method with the relatively low Cassini resolution (see
Fig. 18.24 ) against SAR images and topographic data on the
Namib desert on Earth. They found that the dunes were fit
with a relationship of the form H = cD n , where D is the
spacing, and c and n are constants. They suggest that the
values for Titan (n = 0.78 for the large Belet dunefield) can
be interepreted to imply the dunes are active, noting that n
* 0.54 at the margins of the Namib, but n = 1.72 at the
center.
An initial estimate of the volume in river channels
(Lorenz et al. 2006) was that there is not enough volume in
the channels to account for the volume of sand needed to
and
near-infrared
wavelengths,
and
the
infrared
 
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