  The silhouette cast by the horizon of the supermassive black hole in M87 can
now be resolved with the emerging millimeter very-long baseline interferometry
(VLBI) capability. Despite being ~2000 times farther away than SgrA* (the
supermassive black hole at the center of the Milky-Way and the primary target
for horizon-scale imaging), M87's much larger black hole mass results in a
horizon angular scale roughly half that of SgrA*'s, providing another practical
target for direct imaging. However, unlike SgrA*, M87 exhibits a powerful radio
jet, providing an opportunity to study jet formation physics on horizon scales.
We employ a simple, qualitatively correct force-free jet model to explore the
expected high-resolution images of M87 at wavelengths of 1.3mm and 0.87mm
(230GHz and 345GHz), for a variety of jet parameters. We show that future VLBI
data will be able to constrain the size of the jet footprint, the jet
collimation rate, and the black hole spin. Polarization will further probe the
structure of the jet's magnetic field and its effect on the emitting gas.
Horizon-scale imaging of M87 and SgrA* will enable for the first time the
empirical exploration of the relationship between the mass and spin of a black
hole and the characteristics of the gas inflow/outflow around it.
