  Model Production Function chronology uses dynamic models of the Main Belt
Asteroids (MBAs) and Near Earth Objects (NEOs) to derive the impactor flux to a
target body. This is converted into the crater size-frequency-distribution for
a specific planetary surface, and calibrated using the radiometric ages of
different regions of the Moon's surface. This new approach has been applied to
the crater counts on Mariner 10 images of the highlands and of several large
impact basins on Mercury. MPF estimates for the plains show younger ages than
those of previous chronologies. Assuming a variable uppermost layering of the
Hermean crust, the age of the Caloris interior plains may be as young as 3.59
Ga, in agreement with MESSENGER results that imply that long-term volcanism
overcame contractional tectonics. The MPF chronology also suggests a variable
projectile flux through time, coherent with the MBAs for ancient periods and
then gradually comparable also to the NEOs.
