QUESTION:
Two things amaze me.  First, isn't it early to conclude that the patterns of
deposition are necessarily caused by flooding?  What observations rule out
aerobic erosion and deposition?

Second, why are scientists so quick to conclude that there was a
catastrophic flood on Mars and so slow to conclude that the Earth could have
experienced the same kind of event?

ANSWER from Jeff Plescia on July 15, 1997:
The conclusion that the Pathfinder site was in the a large flood plain was
based on the Viking oribiter images.  In fact that is why the site was
chosen, because it was hoped that there would be a wide diversity of rock
types there (carried by the flood).  The surface features observed in the
Pathfinder images (e.g., the grooves and the imbricated rocks) are
consistent with such an interpretation.  The scale of the erosion is too
great to be caused by the wind and the vertical extent of the erosion (that
is above a certain altitude things are not eroded) require a fluid moving
along the surface.  The rocks in the Pathfinder scene could not be moved by
the wind.

The earth has experienced the same kinds of floods that are suggested for
the Pathfinder site.  The channeled scablands of the state of Washington are
a good analog.  There a large lake formed behind a glacier.  When the dam
broke, water washed across Washington to the Pacific ocean and eroded the
ground.

Jeff Plescia
Jet Propulsion Laboratory

ANSWER from Jim Murphy on July 15, 1997:
Yes, atmospheric processes can "weather" (wear down) rocks, but this occurs
at a much, much slower rate than does weathering under conditions of
liquid/flowing water.  Also, I believe, though I am no expert in the area,
that the chemical products from weathering due to water might be a bit
different than weatherung from wind processes, and that their might be a
signature in the chemical composition of the rocks and soil relating to
this.  One of the reasons Pathfinder is currently on Mars is to address such
questions, and if you are able to keep in touch with Pathfinder's results in
the weeks and years to come, you will probably receive a good answer to your
question.

Jim Murphy
Mars Pathfinder ASI/MET Science team