QUESTION:
What kind of surface features did the water leave on Mars' landscape?

ANSWER from Bruce Jakosky on December 22, 1997:
There are many different types of features that appear to have been formed
by the actions of water.  Let me discuss some of them:

1) Dendritic valley networks.  These occur on the ancient surfaces and show
the dendritic pattern that is similar to valleys carved on Earth by the
runoff of liquid water.  As such, these are inferred to have formed slowly,
and are therefore thought to have required a climate in which water was more
stable than it is at present.  Even at their densest, these valleys are much
less abundant than in almost any region on Earth.

2) Eroded ancient craters.  The impact craters on these same old surfaces
are very heavily eroded and degraded, and those that are smaller than about
15 km across have been removed entirely.  Some partially degraded craters
have gullies on their interior walls that suggest that erosion by liquid
water may have played a role.  Again, this suggests that water was more
stable prior to about 3.5 billion years ago.

3) Catastrophic flood channels.  These are large, water-carved features that
formed when tremendous amounts of water were released from within the crust
and flowed over the surface.  Individual flood regions can be more than 100
km across, and flowed for up to 1000 km length.  These formed more or less
throughout geologic time.  Because of their catastrophic nature, forming in
a matter of weeks or months, they would not have required a climate
different from today's climate--water can flow a long ways even in
sub-freezing temperatures before it freezes.

There are a large number of additional features that formed from the actions
of water or ice, including what appear to be rock glaciers (cemented or
underlain by ice), the polar ice caps, and a type of patterned ground
similar to what is seen in permafrost regions on the Earth.