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Fig. 12.9
A map of dunefields on Mars assembled by the Mars Dune
Consortium (credit Hayward et al. US Geological Survey Open File
Report). Apart from the circumpolar erg in the north, most of the major
dunefields are in craters
equal but opposite wind patterns may be present, perhaps
driven by diurnal temperature and pressure variations,
especially where winds are likely to be confined in valleys.
For transverse features with wavelengths less than 100 m,
the non-generic term 'transverse aeolian ridge' (TAR) has
been applied, allowing for the smallest TARs to possibly be
either small sand dunes or large ripples (see
Chap. 5
).
Invariably, where TARs and dunes co-exist, the dunes are
usually dark and the TARs bright, and the dunes are
superposed on the TARs showing that they have been active
more recently (e.g., Figs.
12.6
and
6.11
).
True linear dunes (
Sect. 6.6
) are extremely rare on Mars
(Lee and Thomas 1995), particularly since it is usually
impossible to constrain local wind flow patterns to deter-
mine if the winds conform to what might be expected
around terrestrial linear dune fields. Linear dunes are found
near the margins of some intracrater dune fields, but these
features generally lack an obvious crest line or the presence
of a distinct slip face (Fig.
12.12
). The linear forms often
break up into individual dome dunes with increasing dis-
tance from the main portion of the dune field (again, Fig.
12.12
). Dome dunes also lack a crest line or slip face but are
small and are broadly elliptical to oval in planform.
Some dunes appear to be an amalgamation of dome and
T-shaped or teardrop dune form that might historically have
been simply dismissed as 'complex', but can now be
interpreted (as a result of numerical models and flume
experiments) as a barchan variant that results from an
intermittently bidirectional wind regime (see Fig.
6.8
). Both
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