Venus Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 163

Fig. 14.4 Magellan radar mosaic of the Algaonice dunefield. The
dunes form a roughly V-arrangement in the center of the image, just
above the circular pts. More obvious than the small, bright dunes

themselves are some background volcanic features and wind streaks
that permeate the region. Credit NASA/JPL

14.5 Aeolian Transport Under Venus

Conditions: Theory

As discussed in Sect. 4.6 , Claudin and Andreotti (2006)
have advanced a scaling theory for the scale of an 'ele-
mentary' duneform, the wavelength at which a flat bed of
sediment will destabilize most quickly. This scale k is found
to be k * 53(q s /q a )d wherein a particle of size d, density q s
is accelerated to a velocity approaching that of the wind in
air of density q a . In the Venus atmosphere, where q a is quite
large, this scale is rather small. Specifically, for particles of
q s * 3000 kgm -3 and d * 0.1 mm k is predicted to be
*22 cm. This is in rather good accord 2 with the experi-
ments by Greeley in the Venus wind tunnel which ranged
from 8 to 27 cm, depending on windspeed.
At present, little is known about the Venusian boundary
layer and so the success of the theory that the size of dunes is
limited by the boundary layer thickness cannot be assessed.
However, if it is to hold, the *1 km wavelength of the
Fortuna-Meshkenet dunes is the benchmark to be met.

The formulae for predicting the windspeed threshold of
motion of particles can of course be applied to Venus
conditions and, unsurprisingly, given the much denser
atmosphere than any other world, the required threshold
windspeed is low (see Chap. 4 ) . Experiments in the Venus
wind tunnel largely bear out the predictions (see Fig. 14.9 ).
The thick atmosphere essentially forces the momentum
of a grain to rapidly couple to that of the freestream airflow.
White (1981) predicted the shape of Venusian saltation
trajectories using relatively simple equations of motion, and
found that for the likely 1-2 m/s surface winds, saltation
trajectories would be likely to be only 2-8 mm long, with
similar height. Under the assumption (common at the time)
that ripple length scales might relate to the saltation length,
White (1981) predicted that ripple wavelengths would be
small—likely too small to affect radar remote sensing by
Bragg scattering.
Perhaps counterintuitively, the saltation flux on Venus
under dynamically similar conditions (same ratio of friction
speed to threshold) would be 10 times less than on Earth
(White 1981). This is largely because the threshold speeds
are so low on Venus, and flux scales roughly as the third
power of friction speed. Measurements in the Venus wind
tunnel appear to support these predictions (Greeley et al.
1984a).

14.6 Venus Winds

Venus has a thick, torrid atmosphere. Although winds at
altitude sweep the cloudtops rather swiftly in the sense of
the planet's rotation (much as there is a superrotating flow

2

One must be careful to avoid circular reasoning—the Venus wind
tunnel data was one of the datapoints used to compute this

relationship.