Mars Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 145

Fig. 12.25 Portion of PanCam
mosaic of El Dorado dunes, with
Gusev crater rim visible in the
background. Note the local slope.
Image: NASA/JPL

Fig. 12.26 Mars Orbiter
Camera (MOC) image of
transverse aeolian ridges (TARs)
in the floor of a trough. MOC
image M12-00991 of Auqakuh
Vallis. Credit: NASA/JPL/MSSS

'typical' sand deposit yet examined in situ by a rover,
although eventually Curiosity may get a close examination
of dark sand deposits as it crosses the dune field that
surrounds the mound in Gale crater.
Transverse aeolian ridges (TARs). The generic term
'transverse aeolian ridge' has been applied to aeolian fea-
tures that may possibly be either large ripples (megaripples)
or small dunes. TARs have remarkably symmetric profiles
perpendicular to their crest, and when compared to mea-
sured profiles of terrestrial ripples and dunes, TARs [1m
in height are most similar to reversing dunes while TARs
\0.5 m in height are most likely granule-coated megarip-
ples (Zimbelman 2010).
Among locations with nice examples of TARs is Nirgal
Vallis (27.8 S, 316.7 E, see Fig. 5.13 ) with a field of TARs
that cover the floor of this long sinuous valley; crater counts
shows that the TARs on the floor of Nirgal Vallis are likely
only 140-380 ka in age, with an upper limit of \1.4 Ma
(Reiss et al. 2004). Another trough filled with TARs is
Auqakuh Vallis (29.2 N, 60.4 E), Fig. 12.26 .

12.6 Dunes in Relation to Topography

Dunes on Mars definitely do interact with the topography
beneath or around them. As discussed above, most dune
fields on Mars tend to be on the floors of impact craters, so
that the crater topography represents a one-way trap; sand
can move up the shallow slopes of the ejecta blanket, but
once it passes over the crater rim and falls inside the main
crater, it is unable to get out of the depression. For older,
more subdued craters, the interaction between the dunes and
the crater relief is less consistent, but the crater does still
influence dune locations; this situation is best illustrated by
dunes that have concentrated on and around the rim of one
subdued crater (Fig. 6.17 )—similar interactions can be seen
with ripples (Fig. 5.8 ).
Perhaps the next most common occurrence of dunes in
relationship to topography involves concentration of dune
forms along the floor of sinuous channels (e.g., Fig. 5.13 ).
Channels tend to have wall slopes that are often less steep
than the slopes inside the rims of large impact craters, but