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(Bourke et al. 2009)), given the size of the dunes, this
corresponds to a respectable sand flux.
9.4
Ripple Migration Rates
Ripples are small enough that their migration can be noted
in real time in the field. It is easy to mark the crest of a
ripple with a stake like a pencil, and see that even just some
minutes later the pencil is no longer at the crest. Measure-
ments of ripple migration of the order of *1 mm/min have
been documented by examining a marker after some hours.
Timelapse imaging (e.g., Lorenz 2011; Lorenz and Valdez
2011) has also been used (which can easily allow a range of
ripples of different sizes at a locality to be measured) and
has detected even faster motions—of the order of a few
cm/min for the smallest ripples.
As with barchan observations, smaller ripples move the
fastest, with an approximately reciprocal relationship
between ripple height (or width) and migration rate (Fig. 9.9 ).
Note that the migration rate of granule ripples in par-
ticular may not completely capture the magnitude of sal-
tation flux, since the granules that define the ripple crest
only move slowly when nudged by saltating sand grains.
Much sand may fly through the system without causing
observable motion of the bedform.
The migration rates of mm-cm/min of course only
pertain when the wind is fast enough to cause saltation,
which may be only a small fraction of the time. For
example, the site where the data in Fig. 9.9 were acquired
saw no motion at all on 59 out of the 70-day observation
period. Thus the average migration rate that might be
inferred by comparing only two images widely separated in
time would be considerably lower.
On Mars, ripples have been observed to move in such
temporally-separated orbital imaging. For example, Silvestro
et al. (2010) report ripple migration over the stoss side of dark
barchan dunes in Nili Patera (Fig. 9.10 ). The measured aver-
age migration of *1.7 m in four (terrestrial) months clearly
indicates that saltation is active. Bridges et al. (2012) noted that
the ripple migration rate is faster towards the top of the dunes,
presumably due to the speed-up of wind towards the crest.
Fig. 9.10 Two pairs of before and after images from the High
Resolution Imaging Science Experiment (HiRISE) camera on NASA's
Mars Reconnaissance Orbiter illustrate movement of ripples on dark
sand dunes in the Nili Patera region of Mars. The three images on the left
are excerpts from a June 30, 2007, observation (late autumn at the site).
The three on the right are of the same ground observed 15 weeks later,
on October 13, 2007 (winter at the site). Ripple crests discernable in the
central portion of each image are diagrammed in the lower right portion
of each image, with blue lines highlighting the largest changes. White
scale bars in the bottom right of each of the images are 20 m (66 ft.)
long. North is toward the top. http://www.nasa.gov/mission_pages/
MRO/multimedia/pia12860.html . Image credit NASA/JPL/U.Arizona
Whereas for many years no quantitative measurements
of aeolian change existed on Mars, higher resolution
imaging from the HiRISE camera on the Mars Reconnais-
sance Orbiter has now found a number of sites where
aeolian change can be observed. Chojnacki et al. (2011)
report several dome dunes at the Endeavour crater (visited
by the Opportunity rover) which had deflated away and/or
migrated by 10-20 m. HiRISE imaging has also revealed
displacements of the lee and stoss margins of several north
polar dunes (Hansen et al. 2011), finding position differ-
ences (relative to the conveniently patterned ground beneath
them) of between 2.2 and 4.7 m, corresponding to about
3 m/year. Most recently, Bridges et al. (2012b) documented
both ripple and slip face movement in the barchans of Niili
patera (Fig. 9.7 ), finding migration rates of the dune slip
faces of the order of 1 m/year (Fig. 9.8 ). Although this is
small (and comparable with some of the slowest migrations
documented on Earth, in the Victoria Valley of Antarctica
 
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