Conclusions - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 274

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Conclusions

The geographical study of dunes and sand seas began in
earnest over a century ago, but it was the work of one man,
Ralph Bagnold, 70 years ago, that drew together a grand
synthesis of dune morphology with the physics of the
transport of sand, and brought quantitative measurements to
bear on how sand and dunes move.
The breadth and depth of dune studies today, even just
for the Earth, is now far beyond the compass of even the
most talented individual scientist, and the references to this
book (by no means comprehensive) attest to the vast
number of workers in the field, most of whom must spe-
cialize in just a few of the subject areas we have attempted
to survey. The space age has brought dune studies beyond
being a mere Earth science, and has challenged us with
familiar dune forms in exotic environments. Stimulated in
part by these findings, much important progress has been
made, especially in the last decade.
It is the nature of science to probe ever deeper and much
work of course remains, but our impression on compiling
this topic is that the major factors influencing dune scale,
morphology and evolution are now substantially under-
stood, and that given a sediment and a wind description, the
resultant dune landscape can be predicted. This may per-
haps offers a new challenge to producers of science
fiction—only a subset of all possible atmosphere-landscape
combinations is geologically or meteorologically plausible!
Dunes can be found, it seems, on any world with an
atmosphere thick enough to move granular material. On
Venus, with the thickest atmosphere of all, large dunes are
scarce because that world is starved of sand.
Mars' young dunes and ripples are generally larger than
young dunes on Earth, because the thin atmosphere is
associated with longer dynamic

trajectories and saturation length). Yet most of Mars' dunes
are barchans and transverse dunes—in motion, albeit
slowly. Except where dunes are confined in crater basins (as
many of them are) the sand has yet to find its final resting
place. In principle, Mars dunes could grow to be truly
massive, since the planetary boundary layer can be tens of
kilometers thick, but the sand has not accumulated to the
extent needed to build dunes of that scale.
Titan presents a fascinating counterpoint and shows that
the juxtaposition of seas of liquid and seas of sand is not a
combination unique to the Earth. Here, young dunes are so
small as to be invisible (to exploration so far) but the cir-
culation has allowed sand to accumulate in vast equatorial
sand seas, with strikingly large and regular linear dunes; the
regularity of the dune pattern attests to the maturity of the
sand seas. By coincidence, these dunes are the same height
as large dunes on Earth, consistent with Titan's atmospheric
structure. While Mars seems very much in flux, Titan's
aeolian evolution seems to have reached a conclusion,
perhaps with only minor astronomically-forced vacillations
about an end-state.
Dunes represent a history of previous conditions, the
small and uppermost features reflecting the recent past, but
often with a palimpsest of climate long ago in the larger
pattern underneath. This superposition and evolution (the
convolution of what are often intermittent or even rare
dynamic conditions) can now be explored in laboratory and
computer experiments. With these and other tools, we have
begun to learn the language of the dunes, and can decode
their whispers of the past.
We close, then, with an image (Fig. 25.1 ) that mirrors
the first image in this topic. While neither dunes nor ripples
are visible, the point is made. Planetary exploration brings a

length scales (saltation