QUESTION:
Would the walls of a flood channel or Vallis Marineris be a good place to
look for life and/or learn more about Martian geology?  In theory, one could
land a controlled-landing spaceship (as compared to Pathfinder) near the
canyon and have a camera or probe rappell down the side using the lander
itself (presumably much heavier) as a counterweight.

It would seem that the geologists would be able to learn a lot about Mars's
geologic history by watching the various layers of rock go by on the way
down.  Moreover, such a mission could be useful in searching for fossilized
life, if it existed.  First, the probe would be able to search for fossils
in rocks of various ages, and squashed bacteria mats and stuff like that
could stand out from the surrounding rock (and be dated).  Second, the probe
may be able to find where the groundwater is (the water table could be high
enough to penetrate the canyon walls though not necessarily the surface) --
any telltale ice would be a good place to look for life as well (and bring a
drill to get through the crust of ice on the outside to look for liquid
water and with it the possibility of extant life).  Third, if fossilized
life is found, it MAY be possible to track its evolution from its first
foothold in the area to its eventual demise and know when it all happened. 
Finally, if no fossils are found in the side of the canyon, there's always
the floor (assuming the probe can get down there safely)

Are any missions like this in the works?  Obviously you'd have to pack lots
of sensors and gadgets into the probe rappelling down the side of the canyon
and be able to position the lander very precisely (and make sure it doesn't
fall in with the probe...), but it sounds like it should be theoretically
possible.

ANSWER from Steve Wall on July 30, 1997:
Good idea, especially for the geology-related reasons.  The Grand Canyon has
given us terrific insight into the strata that exist below the surface, and
it is likely that we could get equally important information from landing in
Valles Marineris.

It would be harder, of course, just like landing a helicopter in the Grand
Canyon is harder than normal landings.

I think the changes in temperature and pressure at the bottom would not be
large enough to allow liquid water, though.

ANSWER from Steve Wall on September 25, 1997:
This is a wonderful idea.  Good science is what NASA is all about, and this
would be VERY good science.  We have to think about how much science we can
accomplish for every dollar we spend, too, and unfortunately in these days
of lower-cost missions, the mission you outline just isn't in the cards.  A
major reason that Pathfinder succeeded on the budget it had was its low-cost
way of landing.  By contrast, the Viking mission did a controlled landing at
around 10 to 15 times the cost, but even the Viking system would not be able
to do real controlled descent at rates where one could study stratigraphy
(that's the layering that you can observe as you go deeper into the
surface).

ANSWER from Mark Adler on November 26, 1997:
From a science point of view, the answer is definitely yes, especially
considering recent images taken by MGS of the walls.  From an engineering
point of view, a rappeling robot is pretty darned risky.  However, you might
not need to rappel, since the walls are not necessarily steep like our Grand
Canyon.  Even without rappeling, it's still risky, since you are targetting
to land as close as possible to highly sloping terrain or even cliffs.  This
is exactly the opposite of how landing sites are usually selected: to be as
flat and safe as possible.  We do not currently have any plans to mount such
a mission.

But it sure sounds fun!

Mark Adler