Remote Sensing - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 206

18

Remote Sensing

In this section, we review the various remote sensing
methods used to study dunes. The quality, extent and, most
especially, the accessability of satellite remote sensing data
have transformed studies of dunes on Earth. And, of course,
it is remote sensing that has for the most part made studies
of dunes on other worlds possible. We devote some par-
ticular attention to radar remote sensing, in part because this
is less familiar to most readers than are optical pictures, but
also because it is the means by which dunes on two of the
four dune worlds in the solar system were discovered. We
begin the discussion with satellite imaging's predecessor,
aerial photography.

enhancement in post-processing (e.g., Photoshop) can help
substantially.
More impressive views may be had from lower altitude,
typically from a light plane (e.g., Fig. 18.2 ); usually images
are clearer as well as of larger scale. Some spectacular
commercial photography has been obtained from paramo-
tors (powered paragliders) which fly both low and slow, and
can be conveyed by truck into the middle of sand seas for
sustained operations.
Due to the same factors (gravity vs. atmospheric density)
worlds without dunes are worlds on which one can fly, and
in terms of ease of flying near the surface are Titan, Earth,
Mars and Venus. From an aeronautics standpoint Titan is
easy, albeit cold and distant. Mars' thin atmosphere presents
aerodynamic and thus structural challenges, although viable
aircraft and balloons have been proposed. While the Venus
lower atmosphere is dense, it is so hot as to be difficult to
operate in.
Vehicles such as landers or rovers on their way down to
the surface sometimes have the opportunity to image the
surface from the air: descent cameras of one sort or another
have been carried on all the Mars landers and rovers in the
last decade, and a spectacular recent example, which happens
to show some dunes, is Fig. 18.3 . Interestingly, although it
was not realized at the time (see Chap. 13 ) the first images
showing Titan dunes were those taken by the Huygens
probe's camera during its parachute descent (see Fig. 13.7 ).
A higher altitude and wider view inevitably means
longer paths through the atmosphere, which in dusty envi-
ronments can lead to poor contrast in images. This can be
ameliorated somewhat by using longer wavelengths, such as
by using an infrared filter. Aerial images are often attained
at a range of times of day so, unlike most orbital images,
shadows can often be present. These can be dramatic and
useful in showing the shape of dunes, which are usually
covered in sand of a uniform color and are thus 'washed
out' near noon.

18.1 Aerial Photography

A fundamental limitation with field work is the limited
perspective afforded by a viewpoint only a couple of meters
above the ground. Sometimes there is a convenient moun-
tain nearby from which a good regional view of a dune or
dunefield can be obtained, but usually there is not. Thus a
wider view needs a higher viewpoint. This can be obtained
from kites, balloons or aircraft (we discuss kite imaging in
the field in Chap. 16 ). Jet airliners, cruising at what the pilot
will call 30,000 ft, are 10 km above the ground, so a typical
unzoomed point-and-shoot camera through the window will
show an area on the ground about 20-30 km across;
although the smallest dunes may be hard to see, nonetheless
some impressive views can be had by the alert airline
passenger (Fig. 18.1 ; also other examples are of the Algo-
dones dunes, Fig. 24.3 ; barchans in Egypt, Fig. 6.5 ; the
Moenkopi dunes, Fig. 7.3 ; and the Badain Jaran, Fig. 18.1 ).
Usually the best pictures are taken on the side of the plane
away from the sun (for most readers, this will mean the
north side), to avoid glare and scattering by dirty or scrat-
ched windows. A good zoom lens is useful, as may a
near-infrared filter (see below). Sometimes, some contrast