Mars Crustal Magnetism

The Mars Global Surveyor spacecraft carried a Magnetometer, the MAG/ER.
During MGS's aerobraking procedure, the MAG/ER found that Mars does not have a global Magnetic Field like the Earth does, but large areas are strongly magnetic.

There are few good ideas of what this magnetism means, although plate tectonic processes do not seem likely. The area of Magnetic Anomalies seems to make a girdle around Mars, and this is (at least partly) because that's where MGS's orbit took it closest to Mars during aerobraking.