QUESTION:	
What is used to make the heat shield?

ANSWER from R. Manning on December 9, 1996:
The heatshield is made out of an aluminum honeycomb structure that is
covered with a graphite-epoxy sheets. The outer part (the part the feels
the heat during Martian entry) is made out of phenolic honeycomb which was
filled with an ablative material called SLA-561V. This stuff is pressed by
hand into the honeycomb as a paste. Soon after it dries hard like wood
putty does. 

The SLA-561 is in fact made out of cork wood and epoxy resins
that are mixed with very tiny gas-filled silica glass spheres. These
spheres are the secret to making the ablator work. They chemically react
with the Martian atmoshere allowing the chemical by-products to take the
heat energy away from the entry vehcle into its wake so that the
heatshield does not burn up.