  Once the terrestrial planets had mostly completed their assembly, bombardment
continued by planetesimals left-over from accretion. Highly siderophile element
(HSE) abundances in Mars' mantle imply its late accretion supplement was 0.8
wt.%; Earth and the Moon obtained an additional 0.7 wt.% and 0.02 wt.%,
respectively. The disproportionately high Earth/Moon accretion ratio is
explicable by stochastic addition of a few remaining Ceres-sized bodies that
preferentially targeted Earth. Here we show that Mars' late accretion budget
also requires a colossal impact, a plausible visible remnant of which is the
hemispheric dichotomy. The addition of sufficient HSEs to the martian mantle
entails an impactor of at least 1200 km in diameter to have struck Mars before
ca. 4430 Ma, by which time crust formation was well underway. Thus, the
dichotomy could be one of the oldest geophysical features of the martian crust.
Ejected debris could be the source material for its satellites.
