Something bad happened to Mars.  It lost its top -- literally!  Tens of km of crust is missing from the northern hemisphere, and the best explanation an impact modeler can think of... is an impact.

Actually the explanation has a lot of merit, going back to a 1984 publication by Don Wilhelms and Steve Squyres that an impactor ~800 km diameter blased away the crust.

Our research, led by Margarita Marinova of Caltech (now at NASA Ames), shows how a moderately off-axis collision (at a typical impact angle of about 45°) can blast out a hole in Mars without producing a lot of non-escaping melt.  This solves the problem of flooding the hole with impact-generated lava.  It also solves the problem that the northern lowlands are somewhat ellipsoidal -- the crustal stripping from an oblique giant impact is of the right shape.  Read the paper here.