  We calculate the present-day impact flux on Mars and its variation over the
Martian year, using the current data on the orbital distribution of known
Mars-crossing minor planets. We adapt the {\"O}pik-Wetherill formulation for
calculating collision probabilities, paying careful attention to the
non-uniform distribution of the perihelion longitude and the argument of
perihelion owed to secular planetary perturbations. We find that these
previously neglected non-uniformities have a significant effect on the mean
annual impact flux as well as its seasonal variation. The impact flux peaks
when Mars is at aphelion, but the near-alignment of Mars' eccentricity vector
with the mean direction of the eccentricity vectors of Mars-crossers causes the
mean annual impact flux as well as the amplitude of the seasonal variation to
be significantly lower than the estimate based on a uniform random distribution
of perihelion longitudes of Mars-crossers. We estimate that the flux of large
impactors (of absolute magnitude $H<16$) within $\pm30^\circ$ of Mars' aphelion
is about four times larger than when the planet is near perihelion.
Extrapolation of our results to a model population of meter-size Mars-crossers
shows that if these small impactors have a uniform distribution of their
angular elements, then their aphelion-to-perihelion impact flux ratio would be
as large as 25. These theoretical predictions can be tested with observational
data of contemporary impacts that is becoming available from spacecraft
currently in orbit about Mars.
