Titan Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 158

Fig. 13.14 Artistic impression (by Mike Carroll) of Titan's dunes,
with an airplane landing on the flanks. In reality, the rings will be
invisibly edge-on, and scattering in Titan's atmosphere will not allow
Saturn to be seen as sharply, but such artistic license is usually

employed to highlight Titan's exotic setting. If one could see the rings,
they would be oriented more vertically, since Saturn is fixed over
Titan's equator, and the dune fields are found only at latitudes less
than 30. Image courtesy Michael Carroll

The answer appears to be (Lorenz et al. 2010) that dunes
grow up to a limiting height (and thus spacing) which is
determined by the thickness of the planetary boundary
layer, the layer of air that is well-mixed near the surface.
The top of this layer acts, according to the theory of
Andreotti et al. (2009) as a 'capping surface', much like the
free surface of a liquid layer and resists upward deforma-
tion. Thus the airflow diverted by a dune becomes com-
pressed as the dune grows up to the PBL top, and the
resultant increased shear stress prevents further sand accu-
mulation, and so growth stops. The geometry of the com-
pressed streamlines and the duneform is such that the dune
spacing roughly equals the PBL thickness, and the maxi-
mum height is about a factor of 12 smaller.
In the coastal regions of the Namib, the thermal inertia of
the sea prevents the air warming strongly, and the PBL can
grow over the course of a day to only about 300 m. On the
other hand, some tens to hundreds of kilometers inland, the
dry ground can heat up much more and the PBL can grow to
*2 km. Indeed, the dunes at the coast are generally smal-
ler, and the dunes inland can grow to *150 m high and
*2 km apart. The dunes on Titan are well distant from the
long-lived polar seas (though transient lakes may exist in
Titan's overall dry tropics, as on Earth) and so there is little

variation of thermal inertia, and thus little variation of
boundary layer height. The Huygens probe descent profile
suggested a PBL thickness of *3 km (although an earlier
assessment suggested *300 m, this appears to have been
the new growing PBL during the morning of the Huygens
descent, distracting attention from the remains of the fully-
grown 2 km PBL from the previous day). Recent modeling
has explored the behavior of Titan's PBL (Charnay et al.
2012).

13.5 Future Exploration of Titan

Cassini has revealed Titan to be a world of great geomor-
phological diversity, with river channels, polar seas of
hydrocarbon liquids, and dunefields. It is therefore of con-
siderable interest for future exploration, and Titan's unique
thick atmosphere opens many possibilities: not only is it
fairly straightforward to deliver landers by heat-shield and
parachute, but the atmosphere can be used to decelerate
vehicles into an orbit by 'aerocapture', a much more fuel-
efficient approach than using retrorockets. A future Titan
orbiter could use a radar or near-infrared imager to obtain
global mapping with a resolution 5-10 times better than