Titan Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 153

Fig. 13.5 Elsewhere in the T8 image, the topographic glints were not
obvious, but the same features appeared as dark streaks above a
brighter substrate. In this image (again, 100 km high) the bright
interdune areas are visible, as is a streamlined inselberg at center. This

showed the features to be longitudinal in character and suggested a
depositional origin. The case for dunes was generally convincing.
Credit NASA/JPL/R. Lorenz/Cassini Radar Team

of 30-70 m. They also noted that the strong contrast between
dunes and interdunes suggested the latter were completely clear
of dark sand (whereas radar might 'see through' some centi-
meters of sand, the near-IR light would not) and this might
suggest that the dunes were still actively saltating.
Lorenz and Radebaugh (2009) mapped the direction of
all the dunes observed with radar through the nominal or
Prime mission of Cassini (Fig. 13.9 ). By this time, coverage
of Titan's surface by radar imaging was around 30 % (and
distributed enough to exclude large sampling biases). The
dunes are typically aligned close to the E-W direction, and
while in some cases the direction along the dune axis is
ambiguous, the overall pattern is overwhelmingly one of net
sand transport from west to east: dunes terminate abruptly at
the western edge of obstacles (Fig. 13.10 ), and pick up
gradually thereafter. Some obstacles have 'tails' in this
downstream direction—some of these morphological clues
are discussed in Radebaugh et al. (2010).
The growing Cassini coverage (radar imaging with res-
olution adequate to resolve dunes now covers about 40 % of
the surface) permits some global trends to emerge
(Fig. 13.11 ). Savage et al. (2013) have documented in detail
the widths and spacings of the dunes and their variation with
latitude: a pattern analysis shows that there is really only one
major population. One notable variation is the general
increase in radar backscatter from north to south, and a
corresponding decrease in microwave brightness tempera-
ture. This has been interpreted (Le Gall et al. 2011, 2012) as
a progressive increase in interdune area from south to north
(see Fig. 13.12 ). This may be connected with the general
configuration of Titan's seasons; in the present epoch (as
coincidentally is the case for both Mars and the Earth,
although the effect on Earth is very small), the perihelion of
Saturn's eccentric (e = 0.09) orbit around the sun occurs
close to the southern summer solstice, which has the effect of
making southern summers more intense, but shorter, than
those in the north. Although the total solar energy delivered
to each hemisphere is the same, nonlinear effects can cause

Fig. 13.6 Radarclinometric profile of the Titan dunes in Belet, shown
in Fig. 13.4 . This profile shows well the typical *3.3 km crest-crest
dune spacing on Titan, and the remarkable [100 m height

It may be noted also that the Doppler tracking (e.g., Bird
et al. 2005) of the Huygens probe during its parachute
descent (given independent support from some optical and
thermal measurements) indicated near-surface winds of the
order of 0.3 m/s. This is of a comparable magnitude to the
saltation threshold estimated in pre-Cassini work.
Dunes were detected in the T20 VIMS multispectral image
strip (or 'noodle') in Fensal (Barnes et al. 2008). Up to this point,
VIMS data had been of too low spatial resolution to measure
individual dunes. In this instance, however, the observations
were made close enough to Titan that the resolution was about
250 m. A challenge in interpretation is that, while long, the
image is only 64 pixels wide (Fig. 13.8 ), but analysis here was
facilitated by context from a high-resolution radar image of this
location (*50 W, 10 N) acquired a few months earlier on T17.
observation by radar. The dune material spectroscopically
appears to contain less water ice than other units on Titan, and
various organic materials would be consistent with the data.
Barnes et al. (2008) measured a typical dune spacing of 2 km,
and used photoclinometry to determine a dune height in this area