Dune Basics - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 14

Part II

Dune Basics

In this section, we outline the basic physics behind dunes,
and the different forms dunes and ripples may take, and how
they move. This naturally begins with a discussion of the
particulate material from which they are made. This is
usually—but not always—sand, sand being a size category
rather than a composition. Dune-forming material, even on
Earth, can have a wide range of origins and compositions,
and

them may simply not exist, but microdunes form on a scale
that we associate only with ripples on Earth.
After exploring this distinction, we note the different
morphologies of dunes and how they relate to wind regime
and sand supply, a relationship that appears to be essentially
universal (and is explored in numerical models in Part IV).
We then introduce a discussion of the factors that control
the scale of dunes, notably the saltation path length, the
saturation length over which a variation in sand transport
will adjust to imposed conditions, and the thickness of the
atmospheric boundary layer.
Dunes are not, in general, static landforms. We summa-
rize the rates at which dunes and ripples are observed and
predicted to move on different planets. On Earth, megab-
archans have been documented moving at *0.1m/yr, while
small barchan and dome dunes can scoot along at tens of
meters per year. Ripples can move at centimeters per
minute. On Mars, until very recently it was puzzled why
dune movement had not been observed, but observations
with higher resolution and longer timespans have now
detected ripples and dune evolution and migration. On
Venus and Titan, migration rates are predicted to be too
small to detect with current (relatively meager) observa-
tions. However, even though present-day motion is not
observable, computed sand transport rates are important in
considering how long the observed landscape may have
taken to form.
Finally, we will reward the reader's patience with all this
physics on a lyrical note, with a discussion of singing sand
and booming dunes.

this range is broader still when considering other

planets.
We then describe the principal features of planetary
atmospheres and their winds, both on the global scale, and
how winds at a given location may be statistically described.
The movement of sand by wind depends on a balance of
forces: wind drag and lift pulling particles up, and gravity
and cohesion holding them down. This classical balance,
which depends on environmental factors such as the particle
size, gravity, and the densities of the air and sand, is what
once dominated discussion of sediment mobility (e.g.,
Bagnold 1942; Greeley and Iverson 1987). However, there
has been a growing recognition of the importance of
unsteady dynamics, the difference between sand starting to
move and continuing to move (Kok et al. 2010; Zheng
2009).
It is this difference that leads to the distinction between
two broad classes of bedforms on Earth: ripples and dunes.
Whereas ripples are defined largely by the pseudoballistic
trajectories of individual particles accelerated by the wind,
dunes are controlled more by the airflow itself and how the
airflow is modified by the dune. Such distinctions merit
reconsideration in other planetary environments—for exam-
ple, in the thick Venus atmosphere ripples as we consider