Titan Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 151

Fig. 13.2 A near-infrared map of Titan, based on early Cassini camera
observations. At this season (late southern summer) the northern high
latitudes were in darkness and so are not mapped. Groundbased and
HST observations since 1994 had detected the large dark areas (Fensal-

Atzlan, Senkyo, Belet and Shangri-La) and in particular the large bright
area Xanadu, but their nature was not known (and in fact names were
only formally given in 2004). Image credit SSI

information and, specifically for studying Titan's surface, a
Synthetic Aperture Radar (SAR) mapper. Additionally, the
Huygens probe carried instruments to measure the
mechanical properties of the surface if the probe survived
landing (not guaranteed) and a camera.
Cassini arrived at Saturn in July 2004, and made some
long-range observations of Titan a few days later. It would
then spend its nominal four year 'Prime' mission orbiting
Saturn, making 44 close encounters with Titan, and
releasing the Huygens probe prior to the third encounter, to
arrive in January 2005. (In fact, the spacecraft has per-
formed well, and the mission was given an extension 'XM'
or Equinox Mission through 2010, adding 26 further flybys
(to 'T70') and has begun a further extension ('Solstice
Mission') with the hope of operating through T126 near
northern mid-summer in 2017.)
It was noted (Porco et al. 2005) in optical imaging
(Fig. 13.2 ) from Cassini's first couple of flybys that there
existed some 'streaky' boundaries between light and dark
terrain. In particular, some streaks were identified and the
eastern boundaries of several features were noted to be
diffuse, while the western boundaries were sharp, suggest-
ing possible surface transport. It could not be determined
unambiguously whether these features were the result of
aeolian or fluvial transport.

The radar observations from the first pass (TA) were of a
rather inscrutable mid-latitude area, and no indications of
aeolian features were seen (or, at this point, expected).
However, the second SAR swath on Titan, T3, in February
2005, found many radar-dark subparallel features, nick-
named 'cat scratches' (Fig. 13.3 ), which were interpreted
(Elachi et al. 2006) as being possibly aeolian in origin, but
other processes were recognized as being impossible to
preclude. The features were purely dark streaks (with no
evidence of topographic shading) and some appeared to
emerge and fork from a common point. At the time, lon-
gitudinal dunes were less familiar to the radar team and so
other origins were still being entertained, even if in the
context of this topic, eight years later, their aeolian origin
seems now obvious. It was only on T3, for example, that
optically-dark areas were determined to be associated with
these radar-dark features; this led to some speculation that
perhaps the large dark areas like Shangri-La and Belet
might also have them.
It was the near-equatorial flyby T8 (Figs. 13.4 and 13.5 )
in October 2005 covering Belet that made the interpretation
clear (Lorenz et al. 2006). First, this flyby covered terrain
that was in many places completely covered in the sand
material; while on T3, the cat scratches were seen as some
dark material deposited above a brighter substrate, on places