Dunes and Climate - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 244

build up and be violently released, hosing out a spray of
dark sand onto the frost. Another mechanism is to deposit
dissolved minerals or salts onto sand. This happens quite
naturally when the dunes have appreciable amounts of
gypsum, but salt spray from the sea, or acid vapors from
volcanos can also cause similar crusts to form on terrestrial
sands. Both water ice and salt or acid induration had been
suggested as reasons why Martian dunes had not been seen
to move (although movement in at least a few places has
now been documented—see Chap. 8 ) .
On Titan so far, the character of the interaction between
climate modeling and aeolian geomorphology has been a
different one. Sufficiently little is known about weather on
Titan that the dune pattern has actually been one of the main
pieces of information with which circulation models can be
refined. That has stimulated something of a resurge in
interest in linear dunes and the wind regimes that form them
(e.g., Reffet et al. 2010; Rubin and Hesp 2009). As confi-
dence in these models progressively improves, and as
mapping coverage has expanded to identify some 'outliers'
in the dune population as possible fossils of a paleoclimate
regime, Titan studies may become more like the Mars and
terrestrial ones—attempts to diagnose the past, rather than
dunes being diagnostic of the tools we use to simulate the
wind.
Venus is probably doomed to remain in that first Titan
stage, in aeolian paleoclimate terms. The couple of dune-
fields known, the streaks seen on the surface, and perhaps
(in future) the orientation of microdunes, serve as con-
straints on circulation models. But dunes are so rare overall,
and so generally small (such that they may reflect only the
last hours or days of wind action, rather than the tens of
thousands of years) that there is little to be learned about the
past from them. Its retrograde-but-equatorial rotation means
that Venus likely does not see the same orbit-obliquity
forced Croll-Milankovich climate change that Earth, Mars
and Titan do. While Venus' climate may have convulsed in
the deep past (millions to billions of years ago, as true for
Titan and Earth) by volcanic forcing of its greenhouse, its
few dunes will be of little help in understanding such
change.
As for exoplanets, we are unlikely to see maps of their
dune patterns unless they happen to have inhabitants who
send or bring them to us! But we can at least speculate in a
more informed manner about what scenarios are likely or
possible (see Chap. 24 ) . We might conceivably learn (from
detailed study of how light from the parent star is reflected
by the planet from the advanced astronomical observatories
planned in coming decades) whether a world has continents
and oceans, but the experience of our own watery planet,
and with Titan (which the first author expected not to have
dunes, because it was expected to have seas) is that planets
are complex places, and seas do not rule out deserts.

Fig. 21.1 Sand trap at La Jornada Experimental Range, New Mexico,
used to evaluate the agricultural impact of aeolian processes. This
simple but effective vane/pivot arrangement points three flared two-
part boxes upwind at different height: the triangular flare decelerates
the air, which escapes through a wire mesh at top while the sand
collects in the closed bin. After some sampling period (usually weeks)
the boxes are opened and the time-integral soil transport is evaluated
by weighing. Photo R. Lorenz

that many challenges remain. The wide obliquity changes in
Mars recent past (100,000 year to million year timescales)
set an interesting range of alternate climate possibilities,
and modelers (e.g., Fenton and Richardson 2001) have
explored the implications for dune formation and

orientation.
Although Mars, for now, lacks the complication of
vegetation, one such challenge is the extent to which sand
has been immobilized by cementation ('induration'). One
mechanism is to freeze material on the dunes. The giant
Martian polar dune field, Olympia Undae, has dark basaltic
dunes sitting on a brighter plain, but during winter the
whole area is made white with carbon dioxide frost up to
some tens of centimeters thick, which fixes the dunes in
place. In spring, vapor released as the CO 2 sublimates can