Real science: Why is northern hemisphere of Mars so different from southern one?

Emily Lakdawalla offers some tidbits in this post titled "Why is only half of Mars magnetized?" at The Planetary Society Blog. Her post also has elevation & magnetic maps from 70 degree N to 70S.

Observed facts.

1. "Mars' northern hemisphere is low (in elevation) and flat, while the southern hemisphere is high and rugged."
2. "where Mars does have a magnetic field, it's mostly in the south, not the north."

Constraint.

1. "evidence suggests that the [magnetic] dichotomy is a truly ancient feature, that should have formed before the dynamo shut down."

Explanation.

"a Pluto-sized body may have slammed into Mars early in its history, erasing the cratering record of half the planet and leaving behind the low-lying, flat volcanic plains that now form the northern lowlands".

With the internal Martian dynamo still operating at the time of impact (now it's down), assume that "northern hemisphere core-mantle boundary hotter than the southern hemisphere core-mantle boundary, a reasonable initial condition to impose if you very suddenly remove a huge amount of crust atop that part of the planet with a giant impact." And you get something resembling the current magnetic distribution.

[via James Nicoll]