Remote Sensing - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 215

Fig. 18.13 ASTER DEM of the
Namib from stereo optical
images, with 30 m postings.
Compare this with the Landsat
image (Fig. 18.6 ) and the DEM
generated by radar interferometry
during the SRTM mission
(Fig. 18.26 ). Profiles 1 and 2 in
Fig. 18.14 were extracted from
horizontal (E-W) lines about
25 % from the top and bottom of
this image, respectively

inertia', I, which is defined as I = (kqc) 0.5 . This is measured
in the rather ungainly aggregate units of Jm -2 K -1 s -0.5 ,
which are mercifully abbreviated as 'SI units' or 'tiu—
Thermal Inertia Units'. Rocky surfaces typically have ther-
mal inertias of several hundred. A value of 200 tiu is typical
for sand, although measurements of I for the north polar erg
on Mars are lower, 75 tiu, perhaps indicating a smaller grain
size. Fine-grained sand can have a high tiu value if the grains
are cemented together ('indurated') and thus the mobility of
a dune may be connected to its observed thermal inertia.
Studies of several Mars regions have been made, e.g., the
dunes in Proctor crater (Fenton and Mellon 2006) and the
north polar erg (Putzig et al. 2008).
Plots of surface temperature through the course of a day
clearly show that there are times in both the early morning
and the late afternoon when curves for different thermal
inertias tend to cross (Fig. 18.8 ). Near these daily crossover
points, it is the most difficult time to determine the thermal
properties of geologic materials, so measurements obtained
during both day and night are best for constraining the
thermal properties. The minimal variation in qc among
geologic materials means that most thermal inertia varia-
tions on Mars can be attributed to changes in the thermal

conductivity; for Earth, the effects of the thick atmosphere
and the potential presence of surface water make such an
assertion far more problematic. Albedo differences on the
surface can shift the daily temperature curves up or down
without greatly altering their basic shape.
On Mars, many areas (and especially the north polar erg)
are covered for part of the year in a seasonal CO 2 frost
which pins the surface at a uniform value (day and night) of
about 150 K. It goes without saying that sand sealed under
such a frost layer cannot saltate.
On Earth, the Landsat Thematic Mapper has a thermal
channel—often dunes may appear to have a quite different
contrast in the thermal images compared with the optical
reflectivity. In orbit around Mars, a series of instruments has
flown starting with a radiometer on Mariner 9 with varying
spectral and spatial resolutions. The highest spatial resolu-
tion instrument (and thus arguably the most useful for dune
studies—e.g., Fig. 18.9 ) is THEMIS, whose thermal imag-
ing resolution is about 100 m.
At Titan, Cassini's Composite Infrared Spectrometer
(CIRS) has very modest spatial resolution, that nonetheless
has been able to detect that the large dark sand seas on Titan
are about *1 K warmer than their surrounds, and that there