Titan Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 157

spectral signature of the dune fields ('brown' in one widely-
used color mapping) is distinct from that of water ice
exposures. A final clue comes from the presence of a
spectral band associated with the sand seas (Clark et al.
2010). Although absorption of light by methane in Titan's
atmosphere only permits spectroscopy in a few isolated
windows, making much less of the wavelength range of the
instrument (1-5 microns) available for composition iden-
tification, a band at 5.05 microns does appear to be spatially
correlated with the 'brown' sand unit, and is suggestive of
aromatic organics like benzene (though the actual compo-
sition is likely more complicated than that).

exceed the saltation threshold of 0.5-1 m/s and, further-
more, that bidirectional winds are encountered over the
course of a Titan year, due to a seasonal change in the
hemisphere-to-hemisphere Hadley circulation. However,
this model (indeed, all models of Titan's general circula-
tion) predicts that the near-surface winds at low latitudes are
predominantly easterlies (i.e., blowing westwards), in con-
tradiction to the appearance of the dunes. Tokano (2008)
reports GCM experiments with introducing Xanadu as a
large positive relief feature (i.e., a hill) and changing its
albedo in an attempt to modify the winds, but was unable to
form low-latitude westerlies.
This apparent paradox (Wald 2009) may have been
resolved (Lorenz 2010) at least in part by studying the
windspeed history with finer time resolution. Tokano (2010)
found in a refined version of his model (incorporating a
crude topography, although this is not believed to be a
principal factor) that while the equatorial near-surface
winds were indeed easterlies, for a brief period around the
equinoxes, as the intertropical convergence zone (ITCZ)
crosses the equator, stronger vertical mixing leads to strong
westerlies. Even though these winds persist for only a small
fraction of the Titan year, if the saltation threshold is high
enough (i.e., [*1 m/s), the sand motion may reflect only
those stronger westerlies and not the weaker (albeit more
prevalent) easterlies and thus the dune pattern indicates a
west-to-east sand transport. Of course, one model showing
that a mechanism is possible does not prove that the
mechanism is responsible, and the problem remains an
active area of research.
Whilst the dunes almost exclusively indicate eastwards
sand transport, there are regional deviations of up to 45
(see Fig. 13.9 ). The pattern remains to be fully interpreted,
although one immediate impression is one of deviation
around the 2500 km continental-scale feature Xanadu—
much as dunes deviate around bright obstacles only a few
kilometers across. There may also be some convergence of
flow towards the center of large dune fields like Belet
which

13.4 Implications for Meteorology

The dunes represent an important set of constraints on
Titan's meteorology in three respects: their extent, their
form, and their orientation. They further give implications
for photochemistry, in that the dunes represent the largest
known reservoir of organic material (Lorenz et al. 2008) as
described above.
First, their distribution, confined to the tropics, defines
the latitudes equatorward of 30 N and S as having at least
sometimes the conditions required for dune formation
(namely available and transportable (i.e., dry) sediment, and
winds strong enough to move the material), whereas other
latitudes appear not to satisfy these requirements. Since the
sand source is not yet unambiguously identified, this con-
straint pertains to winds and humidity. With regard to
humidity, models had already (e.g., Rannou et al. 2006)
suggested low latitudes on Titan should eventually dry out
unless resupplied by a surface methane source.
Mitchell (2008) explored this question further and esti-
mate some 1-2 m of liquid methane per year could be
removed from low latitudes. He found that the latitudinal
extent of the dry region depends on the total methane
inventory, with between 7 and 20 m agreeing best with
observations (such as dune extent, and the humidity recor-
ded by Huygens).
Second, the predominance of the longitudinal (linear)
dune form requires a modestly-changing (typically bidi-
rectional) wind regime (e.g., Lancaster 1995). Sources of
such a variation include seasonal change (the usual reason
for this wind regime on Earth) and possibly the gravitational
tide in the atmosphere.
Finally, the dune orientation pattern represents an
important diagnostic on the tropospheric winds, for which
there are few clouds to act as tracers. Tokano (2008) has
explored the winds in a global circulation model (GCM)
and finds that indeed surface winds should not infrequently

might be a 'sea-breeze' effect of daytime solar

heating of the dark dunefield causing an updraft and

convergence.
A further interesting question is: why, given that
Titan's gravity is so different from Earth, the atmosphere
49 thicker, and the sand presumably made of rather dif-
ferent stuff, should the resultant dune forms resemble the
Earth's so much in morphology and size? The morphology
is, of course, a rather basic effect—the dune-forming pro-
cess is the same everywhere, and the morphology depends
on the sand supply and the variability of the wind vector as
discussed in Chaps. 6 and 9 . But why the size should be the
same in Belet and in the Namib is less obvious.