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Fig. 14.7 Close-up view of a microdune formed in the Venus wind tunnel (see also Sect. 4.6 ). The bedding planes in the sand are visible, as is
the steep slipface. Image courtesy of Ron Greeley
in Titan's upper atmosphere), the near-surface winds are
almost unknown except for a handful of radio tracking
measurements of descent probes, some brief wind mea-
surements on the surface, and indirect indication of winds
such as the orientation of wind streaks.
Venus orbits the Sun in 224.7 terrestrial days, but
because of the planet's slow retrograde rotation, a solar day
(i.e., the time between successive solar noons, and thus the
period with which atmospheric motions are forced) is 116.8
terrestrial days. The planet's orbital eccentricity is only
0.007, and the obliquity is 177.4 (i.e., a tilt of 2.6 with
retrograde rotation), so seasons are effectively nonexistent.
The cloud-tops can be tracked, especially at ultraviolet and
infrared wavelengths, and the predominant motion is zonal,
as noted above. The uneven deposition of sunlight drives a
slow meridional (Hadley) circulation, and the angular
momentum balance of this circulation is a key problem in
Venus meteorology. Because the atmosphere is so massive,
it has a large inertia or 'memory', which makes it rather
challenging to simulate numerically.
Dobrovolskis (1993) suggests that slope winds on Venus
may be significant, as on Mars, and that such winds may be
strong enough to transport sand. This picture does seem to
be borne out in part by the Magellan mapping of some 5700
wind streaks (Greeley et al. 1995) which show a preference
in the downhill direction where slopes are observed (about
20 % of the total being biased downhill), even though the
Fig. 14.8 Schematic (adapted from Greeley et al. 1984b) showing
the progression of bedforms in the Venus wind tunnel at Venus
densities as a function of speed (see also Fig. 4.18 ) . At the lower
windspeeds, sand is pushed up the stoss slopes and avalanches down
to form a slipface. At higher speeds, the slipface disappears and a
wave or ridge morphology appears, lengthening as windspeed
increases. At higher winds yet, the waves become shallow and a plane
surface results
 
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