Geoscience Reference
In-Depth Information
13
Titan Dunes
13.1
Pre-Cassini Expectations
Greeley and Iversen calculated the windspeeds required for
particle motion as a function of size, assuming that particle
cohesion was the same as for sand on Earth. They found that
the optimum size for saltation was 180 lm diameter, with a
friction speed of about 0.04 m/s—note that this assumed a
particle density of 1900 kg/m 3 (the average density of
Titan) which may be a factor of almost 2 too high for most
organic and ice surface materials.
As plans to explore Titan matured around 1990 with the
start of the development of the Cassini mission, others
began to consider the problem. In an analysis of likely
winds on Titan in a 1991 conference on Titan, Allison
(1992) suggested winds might be of the order of 0.3 m/s,
and that this was comparable with that required for particle
motion and thus 'wind-blown material might be an occa-
sional feature of Titan meteorology'. Grier and Lunine, in
an unrefereed abstract (DPS conference 1993) also noted
the possibility of dune formation, in both subaerial and
subaqueous environments.
Motivated by the possibility that Titan's atmosphere
might have changed in density over time, and ease of
aeolian transport would depend on the density, Lorenz et al.
(1995) reconsidered aeolian transport. They used scaling
arguments to assess likely near-surface windspeeds, based
on estimated surface solar fluxes and the thickness of the
atmosphere, and determined that, on average, winds would
be lower than the threshold. However, the same was noted
to be true for the Earth, prompting consideration of the
statistical variation in windspeeds.
Specifically, expectations of thermally-driven winds near
Titan's surface were extremely low, due to the low solar
flux, the large column mass of the atmosphere and the small
radius of Titan. This kind of energy-flux argument, which
correctly predicts windspeeds of a few meters per second on
Earth, suggests windspeeds on Titan of only about 1 cm/s.
In contrast, despite Titan's thick atmosphere and low
gravity, which both favor transport of material by wind, the
threshold windspeeds required to move sand are of the order
of 0.5-1 m/s.
Titan is Saturn's largest moon, at 5150 km diameter is
larger than the planet Mercury, and is unique among the
satellites of the Solar System in having a dense atmosphere,
four times denser than our own. The next-thickest atmo-
sphere, that of Triton (see Chap. 7 ) is 100,000 times thin-
ner. Titan's atmosphere was hinted at by the dark edges of
its disk seen in telescopic observations in 1908, but gaseous
methane was detected spectroscopically in 1944. This made
Titan an object of interest, not only planetologically, but
also from the astrobiological point of view, since the action
of sunlight on methane was known to create solid (and
liquid) organic compounds, which were expected perhaps to
accumulate to a depth of 1 km on Titan's surface; accord-
ingly Titan was made a target of the Voyager 1 spacecraft in
1980 (see Fig. 13.1 ).
This early study of Titan is summarized in Lorenz and
Mitton (2008) and Coustenis and Taylor (1999, updated in
2008); a popular post-Cassini account is Lorenz and Mitton
(2010). The most comprehensive research-level book is
Brown et al. (2010). The Titan scientific literature has
exploded in recent years—from a few dozen papers a year
in the 1990s, to over 100 a year in the Cassini era. Recent
reviews of saltation physics on Titan, and of the dunes
overall,
are
by
Lorenz
(2013)
and
Radebaugh
(2013),
respectively.
Atmospheric conditions on Titan—and specifically the
density of the atmosphere at ground level—were deter-
mined by passing Voyager 1's radio signal through the
atmosphere and measuring how much the atmosphere bent
it: a 'radio occultation' experiment. Although the surface
was speculated to perhaps be wet with liquid methane (the
surface temperature is 94 K), the atmospheric pressure is
1.5 times that of Earth, leading to an air density of 4 times
that of Earth. It was realized that in such a dense atmo-
sphere, and in Titan's low gravity, particles of material on
the surface could be blown around with rather low wind-
speeds. In their book Wind as a Geological Process (1985),
 
 
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