The current rate of small impacts on Mars is informed by more than one
thousand impact sites formed in the last twenty years, detected in
images of the martian surface. More than half of these impacts produced
a cluster of small craters formed by fragmentation of the meteoroid in
the martian atmosphere. The spatial distributions, number and sizes of
craters in these clusters provide valuable constraints on the properties
of the impacting meteoroid population as well as the meteoroid
fragmentation process. In this paper, we use a recently compiled
database of crater cluster observations to calibrate a model of
meteoroid fragmentation in Mars' atmosphere and constrain key model
parameters, including the lift coefficient and fragment separation
velocity, as well as meteoroid property distributions. The model
distribution of dynamic meteoroid strength that produces the best match
to observations has a minimum strength of 10--90 kPa, a maximum strength
of 3--6 MPa and a median strength of 0.2--0.5 MPa. An important feature
of the model is that individual fragmentation events are able to produce
fragments with a wide range of dynamic strengths as much as ten times
stronger or weaker than the parent fragment. The calibrated model
suggests that the rate of small impacts on Mars is 1.5--4 times higher
than recent observation-based estimates. It also shows how impactor
properties relevant to seismic wave generation, such as the total impact
momentum, can be inferred from cluster characteristics.