Mechanics of Sand Transport - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 43

Fig. 4.10 Fluid threshold friction speeds

Fig. 4.9 Wind tunnel measurements of fluid threshold as a function of
size compared with various analytic expressions. The scatter in the
data is comparable with much of the difference between the different
models. (from Kok et al. 2012, with permission)

So, we now have the theory, backed by experiment, to
support prediction of the fluid threshold on other planets,
with other sands and atmospheres. This overall picture is
laid out in Greeley and Iversen (1985) with a more updated
discussion in Kok et al. (2012); investigations for specific
planetary bodies are called out in the respective chapters of
part 3 of this topic. Figure 4.10 shows the results, from
Lorenz et al. (1995), and quite similar results are shown by
Greeley and Iversen (1985). The density of the atmosphere
is unsurprisingly the dominant effect, as the labels on the
respective curves follow the order of the surface pressures
on the bodies shown—the threshold friction speeds in the
thin barely-an-atmosphere of Triton are colossal (and likely
supersonic), are still high for Mars, and get lower for Earth,
Titan and Venus. Titan and Venus are not too different, the
low gravity of Titan partly compensating for the thicker
Venus atmosphere.
The effect of gravity can be discerned on the plot. On
lower-gravity worlds like Triton and Titan, weight becomes
significant with cohesion at a larger diameter than on the
terrestrial planets, and thus the optimum size for saltation is
at larger diameters. All this assumes, of course, that the
cohesion is the same for Titan/Triton materials as for ter-
restrial sands. The effect of varying cohesion is readily seen
by varying the moisture content of sand—measurements
summarized in Greeley and Iversen (1985) suggest that even
0.3-0.6 % moisture can double the threshold friction speeds
for sand; see also McKenna-Neuman and Nickling 1989.
As an amusing aside, it is pertinent to note that any future
human presence on the moon may experience somewhat
Titan-like mechanics. The moon's gravity is about the same
as Titan (1.3 m/s 2 ), and a pressurized moonbase or landed
spacecraft will have an air density of perhaps 0.9 kg/m 3 (if
filled with air, like an airliner) or 0.5 kg/m 3 (if enriched with

cohesion are buried in the same parameter, it is difficult to
judge what the effects might be. The discussion above
exposes the factors a little more clearly.
We have in all this discussion neglected possible elec-
trostatic forces on particles. These could be quite important,
especially on Titan. One easy experiment is to place your
hand inside a 'beanbag' chair, filled with small polystyrene
foam spheres. These will stick quite well to your hand,
attesting to the electrostatic attraction. The triboelectric
processes that generate electrical charges (Zheng 2009; Kok
et al. 2012; Merrison et al. 2012) discuss these effects at
some length: they could cause saltation thresholds to go up,
or go down. Electric fields generated in blowing snow or
sand (or dust) can cause radio interference, and may offer a
means of measuring the transport of granular materials.
Figure 4.9 shows a compilation of these wind tunnel
results; it is seen that indeed the overall behavior is rather as
indicated in 2.3.9, with a minimum threshold. One way of
collapsing data made with different materials (and Greeley
used everything from lead to sugar to walnut shells) of dif-
ferent densities is to scale the particle diameter. In other
words, for the 'normal' quartz sand with qp = 2650 kg/m 3 ,
the scaled diameter is just the diameter. But for gypsum
(say) particles of 300 lm diameter with density 1700 kg/m 3 ,
the diameter to use on these plots is 300*1700/
2650*195 lm. In other words, denser materials behave like
bigger quartz particles. This approach is a bit of a fudge, but
is reasonably effective for the so-called 'fluid threshold', the
speed at which particles begin to move. As we shall see, this
is not the whole story.