Deposits of Sand: Ripples Versus Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 53

Fig. 5.4 Small conical piles of dirt (arrowed) created by the Viking
Lander 1 sampling arm. The pile was made to estimate the angle of
repose of the sediment, and to observe any changes. The image on the
left was taken on Sol 921; the composite image on the right (where the

sandpiles have been blown away) was assembled from images taken
on Sols 2068 and 2209. This excellent comparison was assembled by
Phil Stooke

Fig. 5.5 The slip face of a dune can be steep and loose enough to
permit rapid descent, as in this case (Jani Radebaugh dune-surfing on a
megabarchan near Liwa in the United Arab Emirates). However, the

friction is still rather higher than for snow, and the experience is not
always totally exhilarating. Photo R. Lorenz

dropping one grain width, to the entire slip face sliding. It is
found, both in reality and with computer simulations (see
Chap. 19 ) that the relative frequency of avalanches varies as
an inverse power law, with avalanches that are 10 times

bigger occurring 10 times less often. The same sort of power
law statistics are seen with earthquakes, forest fires, moun-
tain avalanches and so on, and are studied under the physics
umbrella of 'Self-Organized Criticality', e.g., Bak (1999).