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