Mars: Surface and Interior 323
6. Canyons
On the eastern flanks of the Tharsis bulge is a vast system
of interconnected canyons. They extend just south of the
equator from Noctis Labyrinthus at the crest of the Tharsis
bulge eastward for about 4000 km until they merge with
some large channels and chaotic terrain. The characteristics
of the canyons change from west to east. Noctis Labyrinthus
at the western end consists of numerous intersecting closed,
linear depressions. The depressions are generally aligned
with faults in the surrounding plateau. Further east, the
depressions become deeper, wider, and more continuous to
form roughly east–west trending canyons. Still further east,
the canyons become shallower; fluvial features become
more common, both on the canyon floor and on the
surrounding plateau; and finally the canyons end as the
canyon walls merge into walls containing chaotic terrain.
The canyons almost certainly formed largely by faulting and
not by fluvial erosion, as is the case with the Grand Canyon,
Arizona. Faulting is indicated by the partial merger of
numerous closed depressions in the western end of the
canyons and by straight walls in the east. While faulting
created the initial relief, the canyons have been subse-
quently enlarged by failure of the walls in huge landslides
and by fluvial action. The faulting was on such an enormous
FIGURE 4 The middle section of the canyons. In the upper left
is the completely enclosed Hebes Chasma, within which a
mound of layered sediments is clearly visible. The main part of
the canyon consists of three parallel canyons each 200 km across,
also partly filled with mounds of sediments. The sediments are
believed to have been deposited in lakes which drained
catastrophically to the east. Candor Chasma (see Fig. 5) is the
middle canyon. (Mars Orbiter Laser Altimeter.)
scale that it probably involved the entire lithosphere. (See
Figs. 4 and 5.)
Thick sequences of layered (and unlayered) deposits are
present in many places throughout the canyons, including
some closed canyons completely isolated from main depres-
sion Fig. 5. The consensus is that the canyons formerly con-
tained lakes and that the layered sediments were deposited
in these lakes. The lakes drained to the east, hence the con-
tinuity eastward from the canyons into several large flood
channels. Orbital detection of sulfates within the canyonsFIGURE 5 Detail of the layered sediments in Candor Chasma
as seen from orbit. Such sediments are common throughout the
canyon and in some of the adjacent depressions. (Mars Orbiter
Camera.)