QUESTION:
We are told that the Martian surface we see in the Pathfinder images is
several billion years old. We are also told that there are dust storms, and
wind. I would have thought that these latter processes would have completely
transformed the surface features. Billions of years is a long time even
considering that erosion must be slow due to the atmospheric density being
1/100th of Earth's. I would have thought that exposed rocks would have been
weathered down to nothing and all we would see would be vast expanses of
dust like some Earth deserts. What is wrong with my thinking?

ANSWER from Mary Urquhart on August 13, 1997:
The area where Pathfinder landed is between and a few and about 1 billion
years old based on the impact cratering record.  The thin atmosphere is just
one factor in the slow erosion rate we observe on Mars.  On the Earth, water
is by far a more effective eroding agent than wind.  In desert areas on the
Earth, such as our own desert southwest, features that have been wind eroded
have also been greatly affected by water erosion.  Much of the sand we see
in desert areas is brought in by flowing water or created during the
occational heavy rainfalls and associated flooding events.

Water is a very effective mechanical erosion agent. In addition, water
chemically erodes rocks as well as mechanically eroding them. Without
rainfall or flowing surface water, even with an atmosphere as dense as our
own, the erosion rate will be *very* slow compared to the typical erosion
rates we see here on the Earth. Slow erosion rates on Mars as indicated by
the age of surfaces are one of the major arguments that climate on Mars
hasn't been able to support rainfall for most of the history of the planet.
Wind velocity, the ability of the wind to carry abrasive agents such as sand
(a function of velocity and density), the availability of abrasive agents
such as sand, and topography each also play a role.  Typical wind speeds on
Mars are not very high (despite the occasional large scale dust storm).

Mary Urquhart                                    
Laboratory for Atmospheric and Space Physics  
University of Colorado at Boulder