QUESTION:
The horizontal surface of the Mars rocks is being described as "oxidized" to
create the red color.  As an engineer in the semiconductor business, I view
the oxidation process as the reaction of oxygen with another atom to consume
the surface layers of the material leaving a layer of the oxide.  To my
knowledge, this process is not restricted to horizontal surfaces, but should
occur on all exposed surfaces.  How can this explain why the horizontal rock
surfaces are "oxidized" while the vertical rock surfaces are not.  The
process more closely resembles a deposition process.

The ANSWER comes from Jeff Plescia on July 15, 1997:
The general red color the material on Mars has been attributed to the
oxidation of iron in the soil and rocks.  It is indeed a process which
occurs independent of direction and particles exposed to the environment are
oxidized on all side.

The reason some rocks have red tops and darker sides is not because they are
only oxidized on one side, but rather because they are coated with red
(oxidized) dust which is falling out of the atmosphere.  You're right it is
a depositional process which is causing the red coloring of some of the top
of the rocks.

Jeff Plescia
Jet Propulsion Laboratory