Mars Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 146

the channel form still seems to be an effective concentrator
of the mobile sand deposits. If the channel is large and
oriented radial from a large topographic high (something
like the Tharsis ridge), then it might funnel dense nighttime
air somewhat analogous to katabatic winds emanating from
over an ice sheet on Earth. Another example of the inter-
action between relief and mobile sand deposits is climbing
or falling dunes (Fig. 7.4 ), where wind indicators around
the dunes should confirm which way the dominant wind is
blowing with regard to the cliff against which the dunes are
banked. For example, in Fig. 7.4 the falling dunes break
into barchans, where the barchans clearly indicate the
direction of the sediment-driving wind. Dunes can also
collect in the wind shadow behind a topographic obstacle,
such as the elevated rim of a fresh impact crater, although
this situation appears to be less common on Mars than it is
in deserts on Earth. Interestingly, small (\100 m scale)
dune features have been imaged everywhere from the
summits of the Tharsis volcanoes to the deepest portion of
the floor of the Hellas impact basin, covering a range in
elevation of more than 30 km, so that there is no apparent
elevation control on where dunes can form on Mars,
although Lorenz et al. (2012) show that the bedform
wavelength does appear to vary systematically with eleva-
tion (see Sect. 4.6 and Chap. 5 ) .

general correlation between the areas of the greatest pre-
dicted shear at the surface and regions of low albedo, but
the global trends were not applied to individual dune fields.
More localized (mesoscale) climate models, using a higher
spatial density of grid points for the modeling, may have
better success relating observed dune orientations to the
more spatially constrained wind predictions that could come
out of such regional scale modeling efforts (e.g., Fenton
et al. 2005).
A more recent analysis by Gardin et al. (2012) examines
the morphology of some 550 dune fields in some detail
(they find 62 % are barchan and barchanoid, 18 % are
transverse, 11 % are linear and the rest unclassified). They
then use the slip face orientation and morphology to esti-
mate the wind regime at a number of locations. They find
that most are consistent with the global circulation esti-
mated by present models, but some are not. This may be due
to local topographic forcing unresolved in the models, by
fossilized dunes recording a prior wind configuration, or to
dunes having transient geometries not in equilibrium with
the present regime (see Sect. 7.10 ) . In this connection, it is
estimated that the turnover time of Martian dunes (i.e., the
timescale on which dunes should adapt to a new wind
regime) is in the range of 10,000-100,000 years, which is
the same timescale on which obliquity changes may cause
climate variations, so some dunes might always be in

transition, never quite reaching steady-state.
Sand dunes hold the potential to preserve evidence of
past climates within their deposits. The interaction between
active aeolian deposition and annual snow deposits means
that some dunes might preserve internal ice deposits for
extended periods of time, something that has been observed
to be the case in some polar dunes on Earth. HiRISE holds
the potential to look for possible ice exposures in the dunes
that comprise the north polar erg, although great care would
need to be exerted to demonstrate that the snow or ice was
not the product of a recent winter. Whenever coring or
detailed ground-penetrating radar sounding becomes widely
available for Mars, the polar dunes would be a good place to
apply such techniques.

12.7 Links Between Dunes and Climate?

It is very difficult to establish the relationship between dune
occurrences or orientations and past climates on Mars.
Unlike on Earth, where field studies can be used to constrain
the timing for the emplacement of the dunes, it is difficult to
obtain age constraints for dunes on Mars, except that they
postdate the surfaces on which the dunes are found. One
exception to this situation is the TARs that cover the floor of
Nirgal Vallis (Fig. 5.13 ); at this location, enough impact
craters could be identified superposed on the features to
provide an upper limit of about 1.4 million years on the age
of the dune field (Reiss et al. 2004). Recently, HiRISE
repeat imaging of dunes is providing abundant evidence that
Martian dunes are active under present wind regimes, so
that 12.8 Dune and Ripple Migration

the dunes should be very useful as local wind

indicators.
The Hayward et al. (2007, 2009) global database of sand
dunes was compared to surface winds predicted from GCM
runs, but only limited success was achieved in relating
observed dune patterns to the varying predictions from the
GCMs. No general conclusions could be drawn from the
few examples where dunes might possibly reflect paleo-
winds rather than the current wind regime (see also Fenton
and Hayward 2010). An earlier analysis of surface wind
shear obtained from a GCM (Anderson et al. 1999) found a

An enduring puzzle for many years after the MGS/MOC
images began to yield coverage with a fine enough resolu-
tion and a long enough time separation to detect Earth-like
dune migration, was that no such migration was observed.
This prompted several workers to speculate that perhaps
most Martian dunes were immobilized somehow, via some
sort of induration (with ice, salts or the action of volcanic
acid fog, perhaps). In fact, as described in Chap. 9 , slipface
migration has now been seen in some locations on Mars, so