Basic Types of Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 87

Fig. 6.27 Star or akle dunes on Mars. HiRISE image PSP_008323_1735, NASA/JPL/U.Arizona

orientation, depending on the most recent orientation of a
sand-driving wind. Star dunes tend to grow vertically through
sand accretion from the sides rather than to show much
movement laterally.The tallest dunes on Earth tend to be star
dunes, and the best examples are to be found in the Algerian
Sahara, where they are called ghourds or rhourds (Fig. 6.26 ).
Recent numerical simulations of star dunes (Zhang et al.
2012) suggests that in fact star dunes only grow extensive
arms when the wind comes from an odd number of equal-

spaced directions (typically three or five), in which case the
arms grow upwind in the respective directions. When four
or six wind directions are involved, a square or hexagonal
pyramid—without long arms—results.
The star-like duneforms identified on Mars appear to be
in general linked in a trellis-like pattern (i.e., akle,
Fig. 6.27 ), perhaps as a result of forming as linear or
transverse ridges that have then been subjected to a shift in
wind direction.