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with log-normal or, more typically, Weibull functions being
used to describe them, in the wind energy literature at least.
Lorenz (1996) showed that Martian winds, measured by the
Viking landers, could be well described by a Weibull dis-
tribution, and some studies on dust-lifting at Mars are now
using this statistical approach.
However, Titan has the additional factor of a significant
gravitational tide in its atmosphere, due to its eccentric orbit
around the massive planet Saturn: while unique in this solar
system, this may in fact be an important effect on many
tidally-locked extrasolar planets. The tidal potential height
difference associated with the tide is several hundred times
larger than that exerted by the Moon on the Earth. Tokano
and Neubauer (2002) explored the influence of atmospheric
tides on Titan's windfield, and found that it could be a
significant contributor, especially to near-surface winds,
causing flows of the order of 0.5 m/s.
Little was known about Titan's surface prior to Cassini's
arrival: the thick organic haze produced by the action of
sunlight on methane made the atmosphere all but opaque to
Voyager's visible-light camera. In the near-infrared, how-
ever, the atmosphere is clearer, and the first maps of Titan's
surface were made from Hubble Space Telescope data in
1994, with a resolution of around 300 km. As 10 m tele-
scopes with adaptive optics systems (like Keck and Gemini)
became available around the year 2000, the maps improved
by about a factor of 2, but were still about as detailed as
naked-eye views of our own moon. Bright and dark features
could be discerned, but their nature was unclear. It was
tempting to speculate (as for our own moon, where the dark
lava plains were named 'Mare' or seas) that the dark areas
were seas of liquid hydrocarbons. Indeed, when a remark-
able radar experiment using the giant 300 m radio telescope
at Arecibo detected a mirror-like glint from Titan, sug-
gesting patches of surface that were dead flat, it was a
natural interpretation that the dark areas were lakes. Nobody
suspected that they would turn out to be seas of sand.
Fig. 13.1
A Cassini image of Titan, with Saturn and its rings (Titan
orbits Saturn very close to the ring plane, so they are always seen
edge-on). The shadow of the rings is cast on Saturn at lower right. This
rather featureless view of Titan is comparable with (but prettier than)
the Voyager view. The fuzzy edge of the day-night boundary is due to
the same scattering/absorption effect (limb-darkening) that gave the
first telescopic clues in 1907 that Titan had an atmosphere. Credit
NASA/JPL/SSI
The second issue is of sand supply. Because of the weak
sunlight, the hydrological cycle was expected to be weak
overall, corresponding to about 1 cm of liquid methane per
Earth year. Furthermore, raindrops would fall relatively
slowly, making them only weakly erosive. The freeze-thaw
action that serves to break down bedrock on Earth would
not occur on Titan since the heat capacity of the atmosphere
buffers the surface against large diurnal temperature chan-
ges. It had been further conjectured that seas of liquids on
Titan's surface might act as traps for any sand that was
formed (for example, the sand-sized fraction of ejecta from
impact craters—this is believed to be the principal source of
sand on Venus). Because of both the low expectation of
sand abundance and the low expected windspeeds, Lorenz
et al. (1995) were rather pessimistic about the prospects of
detecting dunes on Titan, and little work was done on the
topic thereafter while Cassini was en route.
It was noted in Lorenz et al. (1995) that measures of
average windspeeds may be of only limited utility in
assessing sand transport processes, in that winds above the
transport threshold may in fact be quite rare, and thus the
controlling factor in aeolian feature formation is the prob-
ability distribution of various windspeeds, and how long
and fat is the high-speed 'tail' of the skewed distribution.
Windspeeds are characterized by nonGaussian statistics,
13.2
Cassini Observations
The Cassini-Huygens mission to Saturn and Titan, con-
ceived shortly after the Voyager encounter, began serious
development in 1990 and was launched in 1997. It entailed
the most massive planetary spacecraft built in the West, a
5-ton orbiter supplied by NASA, and a 2.7 m-diameter
probe named Huygens, supplied by the European Space
Agency. Scientific instruments for both orbiter and probe
came from the USA and Europe, including several that
would study Titan's surface. These included a camera
(extending further into the near-infrared than Voyager's,
allowing it to peer murkily through the haze), a mapping
spectrometer ('VIMS') which would yield compositional
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