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Fig. 14.2
Surface image from Venera 14 (note the distortion—the
image was scanned cylindrically, looking down at an oblique angle).
Most
representative of the surface as a whole. Some other landing site
images show more blocky terrain, and a few more patches of regolith,
some of which may be fine enough to saltate. Image NASA NSSDC
of
the
surface
here
is
platy
lava,
probably
somewhat
Fig. 14.3
A global mosaic of radar reflectivity measured by the Magellan spacecraft (cylindrical projection). The two major dunefields are
marked with circles. Image NASA/JPL
disappear. Further increases in windspeed caused the
'waves' (they may not meet the definition of dunes any
more) to grow longer, to 27 cm for a windspeed of 1.35 m/s.
At 1.5 m/s and higher no dunes or waves were formed.
Greeley et al. suggest that at the high wind speeds,
particles are able to jump across the lee of a wave onto
another, such that there is no longer a clear separation
between zones of deposition and removal of sand, and thus
the slip faces are lost. The transfer of material from one
structure to another resembles that in ripples, but the length
of the saltation path may be rather higher (compared to the
wavelength of the structures) than is the case for terrestrial
ripples.
Greeley et al. also found that a run of 90 s of wind at
1.25 m/s removed as much material as a 5 h run at 0.6 m/s,
near the threshold. Thus occasional 'high' winds (and 1 m/s
winds had been measured at the Venusian surface by the
Venera landers) could quickly obliterate small dune forms.
Although the Venus conditions replicated in the tunnel
are of most interest in understanding bedforms on Venus
itself, the range of pressures (and thus atmospheric density)
that can be explored in the tunnel is useful in understanding
aeolian processes more generally. Marshall and Greeley
(1992) mapped out the parameter space of windspeed,
atmospheric density and particle size, and found distinct
regions of behavior.
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