  Recent observations have allowed the geometry and kinematics of the M87 jet
to be tightly constrained. We combine these constraints with historical Very
Long Baseline Interferometry (VLBI) results and the theory of synchrotron
self-absorbed radio cores in order to investigate the physical properties of
the jet. Our results strongly suggest the jet to be dominated by an
electron-positron (pair) plasma. Although our conservative constraints cannot
conclusively dismiss an electron-proton plasma, the viability of this solution
is extremely vulnerable to further tightening of VLBI surface brightness
limits. The arguments presented, coupled with future high-resolution
multi-frequency VLBI studies of the jet core, will be able to firmly
distinguish these two possibilities.
