Winds and Atmospheres - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 28

Fig. 3.3 Astronomical variations in the rotational and orbital param-
eters can drive climate changes—the so-called Croll-Milankovich
cycles. The variations on Mars are particularly strong, as shown here

Fig. 3.4 A mountain ridge (with the small town and airport of
Sharurah near its tip) intrudes into a linear dunefield in the Rub' Al
Khali. The accumulation of sand is prevented by the topography and
the disrupted airflow causes a sand sheet to form in the lee (right)of
the ridge before the flow stabilizes and linear dunes re-form further
downwind. Note also that the linears (and thus presumably the wind)
are diverted slightly north around the ridge: similar topographic
interactions can be seen in the linear dunes of the Simpson desert (see
Fig. 12.12 ) and the Namib and on Titan (Figs. 1.3 , and Chap. 13 )A
couple of small clouds and their shadows can be seen. NASA Earth
Observatory image using USGS Landsat 7 data

can be a window into paleoclimate. Croll-Milankovich
changes on similar timescales are also suggested to be
responsible for the intricate Martian polar layered terrain
and, perhaps, for the concentration of liquid hydrocarbons
at Titan's northern polar regions. On Mars, the effects are
stronger than on Earth, in part because the Earth's large
moon gives some 'gyroscopic' stability to the system. As
shown in Fig. 3.3 , Mars' obliquity has seen major excur-
sions: 500,000 years ago, the equatorial tilt varied from
about 17 to 32 over a *50,000 year period. Its orbital
eccentricity also underwent significant change—all these
factors drive the climate significantly.
The second major influence on wind patterns is the dis-
tribution of surface materials which affect the temperature
response to the sun. On the Earth, the principal factor here is
the oceans which have a much higher thermal inertia, and
thus much less temperature variation than the land, on both
seasonal and diurnal timescales. This leads to persistent
onshore flows in warm lands, and offshore in cold places
(and at night). On Mars, dust-covered areas have low
thermal inertia and warm up during summer days, whereas
more rocky areas are slower to respond to the sunshine (see
Chap. 18 ) . Not only the thermal inertia of the surface, but
also its reflectivity or albedo, can influence the thermal
response. For example, Titan's dark sand seas appear to be
persistently warmer than their surrounds, which doubtless
influences the wind pattern (and of course there may be an
interesting feedback at work, in that the sand seas influence
the winds, which may influence the transport of sand into
the sand sea…).
A third effect is topography. Topography can play a
blocking role, as wind diverts around obstacles (Figs. 3.4 , 3.5
and 3.6 ). The conservation of mass in a flow (i.e., the product
of the density, area and speed of a flow—which for subsonic

Fig. 3.5 Linear dunes in the Namib desert sweeping around insel-
bergs ('island mountains'). The local modification of the wind
directions causes the orientation of the dunes to resemble streamlines.
ASTER image, courtesy NASA/JAXA