QUESTION:
Are there tectonic plates on Mars? Is Mars differentiated?

ANSWER from Jeff Plescia on December 4, 1997:
You should look at the December 5 issue of Science, the Pathfinder mission
results will be published there.

Mars appears to be a one plate planet.  There are no indications of
spreading ridges or subduction zones that might indicate plate tectonics as
we understand it on the earth.  In addition the large size of the volcanoes
in Tharsis suggest that the crust has remained stationary with respect to
mantle plumes (at least over the lifetime of the volcanoes).

Mars is certainly differntiated.  The moment of inertia is such that there
is almost certianly an iron core.  The size and exact composition of that
core (e.g. pure iron versus an iron sulfur mixture or something else) is
uncertain.  Current estimates put the size at 1300 to 2000 km radius.