198 Encyclopedia of the Solar System
FIGURE 8 (a) Classic view of the Grand Canyon of the Colorado River in Arizona USA. The massive layering records the
local geologic history for at least the last 500 Myr. Comparable layering has also been observed recently in canyons on Mars.
This simulated true color perspective view over the Grand Canyon was created from Advanced Spaceborne Thermal
Emission and Reflection Radiometer (ASTER) data acquired on May 12, 2000. The Grand Canyon Village is in the lower
foreground; the Bright Angel Trail crosses the Tonto Platform, before dropping down to the Colorado Village and then to the
Phantom Ranch (green area across the river). Bright Angel Canyon and the North Rim dominate the view. At the top center
of the image the dark blue area with light blue haze is an active forest fire. (Courtesy NASA/GSFC/METI/ERSDAC/JAROS,
and U.S./Japan ASTER Science Team). (b) Bhutan Glaciers, Himalayan Mountains, Asia. Classic Himalayan valley glaciers in
Bhutan, showing theater-like “cirque” source areas, long debris-covered ice streams, and terminal meltwater lakes. ASTER
data have revealed significant spatial variability in glacier flow, with velocities from 10–200m/yr. Meltwater volumes have
been increasing in recent years and threaten to breach terminal moraine deposits with consequent dangerous downstream
flooding. This ASTER scene was acquired 20 November 2001, is centered near 28.3 degrees north latitude, 90.1 degrees east
longitude, and covers an area of 32.3×46.7 km. (Courtesy NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER
Science Team).ranges) is provided by the tectonic activity of plates as they
collide or subduct.
Subaerial landscapes on the Earth are most generally
dominated by erosive processes, and subaqueous land-
scapes are generally dominated by depositional processes.
Thus, from a planetary perspective, it is the ubiquitous avail-
ability and easy transport of water, mostly in liquid form,
that makes it the predominant agent of sculpting terrestrial
landscapes on Earth. Based on the geologic record of an-
cient landscapes, it appears that this has been the case for
eons on the Earth. Such widespread and constant erosion
does not appear to have happened for such a long time on
any other planet in the solar system, although it appears that
Mars may have had a period of time when aqueous erosion
was important and even prevalent.
Fluvial erosion and transport systems (river and stream
networks) dominate the subaerial landscapes of the Earth,
including most desert areas. Even in deserts where aeo-
lian (wind-driven, e.g., sand dunes) deposits dominate the
current landscape, the bedrock signature of ancient river
systems, relict from more humid past climatic epochs, can
be detected in optical and radar images taken from orbiting
satellites. Surface runoff, usually due to the direct action of
rainfall occurs in nearly all climatic zones, except the very
coldest.
On the Earth, such network forms resulting from this
process tend to be scale-independent and take on a nearly
fractal character. That is, network patterns tend to be repli-
cated at nearly all scales, with regular geometric relation-
ships that tend to be similar, no matter what the physi-
cal size of the network. In contrast to the situation on the
Earth, the most visible and well-expressed Martian valley
networks tend to be highly irregular in their network ge-
ometries, probably reflecting very restricted source areas
of seepage or melt-driven runoff, rather than rainfall, and
strong directional control by fractures and faults that was
not overcome easily by river erosional processes. In addi-
tion, they are distributed very sparsely and are primitive in
their branching, very much like the canyon networks arid ar-
eas of the world like Northern Africa (Fig. 9) and the desert
Southwest of the United States (Fig. 8a). Thus, in contrast
to Mars, for most of its discernable history, the Earth’s land-
scapes have been distinguished, overall, by well-integrated
and complexly branched fluvial drainage networks driven
primarily by rainfall.
Uniquely on the Earth (within this solar system at least) it
is the competition between constant fluvial erosion and con-
stant tectonic uplift (and in some land areas, frequent vol-
canic eruptions) that is the predominant determinant of the
landscape’s appearance. For instance, the present terrestrial