324 Encyclopedia of the Solar System
and some of the outlying depressions supports the lake hy-
pothesis. If climatic conditions were similar to present-day
conditions, such lakes would have frozen over, although the
lake beneath the ice could have been sustained for extended
periods if fed by groundwater. Even though the lake hypoth-
esis is plausible, there are many unanswered questions, such
as where the sediment in the layered deposits came from
and what caused the layering.
7. Erosion and Deposition
7.1 Water
Water-worn features present some of the most puzzling
problems of martian geology. Valley networks likely formed
when the climate was significantly warmer than at present,
yet how the climate might have changed is unclear. Huge
floods have episodically moved across the surface, yet there
is little trace left of the vast amounts of water that must
have been involved, and gullies are forming on steep slopes
during the present epoch despite the cold conditions. Per-
haps most puzzling of all is whether there were ever oceans
present, and, if so, how big they were, when they formed,
and where all the water went.
7.1.1 BRANCHING VALLEY NETWORKS
Much of the ancient cratered uplands is dissected by
branching valley networks that superficially resemble ter-
restrial river valleys. (See Fig. 6.) They are mostly 1–4 km
across and tens to hundreds of kilometers long, although a
FIGURE 6 Valley networks in the ancient cratered terrain
northeast of Hellas. The regional slope is to the southwest down
into Hellas. The complex branching patterns indicate that the
valleys formed by surface runoff following precipitation rather
than seepage of groundwater. (Mars Orbiter Camera WA.)
few extend for thousands of kilometers. Large parts of the
cratered uplands are heavily dissected, but other parts are
sparsely dissected. Most of the younger plains are not dis-
sected, although there are a few exceptions. A few volcanoes
are also very heavily dissected. The distribution suggests
that the rate of valley formation was high prior to about
3.8 billion years ago, and that it declined rapidly about that
time. Seepage of groundwater has clearly contributed to
formation of some of the valleys; however, most appear to
have formed as a result of precipitation followed by surface
runoff, which requires significantly warmer conditions than
prevail at present.
No satisfactory explanation has been proposed for how
early Mars could have been warmed to allow precipitation
and stream flow. The output of the Sun is thought to have
been less than at present during this early era. Greenhouse
models suggest that even a very thick CO 2 —H 2 O atmo-
sphere could not warm the surface enough. The lack of
detection of carbonates from orbit also appears to rule out
massive amounts of CO 2 as a cause. The possible role of
other greenhouse gases is being explored. One possibility is
that large impacts injected massive amounts of water into
the atmosphere, which precipitated out as hot acid rain.
The idea is attractive in that it might explain why the valley
networks formed mainly in the old terrains when impact
rates were high, but it may also explain the rarer localized
occurrence of younger valleys.
7.1.2 OUTFLOW CHANNELS
Outflow channelsare very different from valley networks.
(See Figs. 7 and 8.) They are tens of kilometers wide and
thousands of kilometers long, have streamlined walls and
scoured floors, and contain teardrop-shaped islands. Most
start full-size and have few if any tributaries. They closely
resemble large terrestrial flood features and have almost
universally been accepted to be the result of massive floods.
Most start around the Chryse basin, emerging either from
the canyons or from closed rubble-filled depressions and
extending northward for thousands of kilometers until all
traces are lost in the northern plains. The largest flood fea-
tures are in the Chryse region, but others occur in Elysium,
Hellas, and elsewhere, commonly starting at faults. As al-
ready indicated, the channels that merge with the canyons
may have formed by catastrophic drainage of lakes within
the canyons. Other outflow channels appear to have formed
by massive eruptions of groundwater. Groundwater stored
under pressure beneath a kilometers-thick permafrost may
have been released when the permafrost seal was broken
by impact, volcanic activity, or faulting. Most of the outflow
channels formed in the middle of Mars’ history, well after
the time that the valley networks formed; some networks
have formed much more recently. Cold surface conditions
and a thick permafrost were probably required for their
formation.