Figure 7-15: Relic Magnetism on Mars For almost a billion years after Mars formed, molten material with electric currents generated a planetwide magnetic field. Most of the surface was magnetized during this time, which is the crustal field we measure today. Regions without magnetism probably experienced higher temperatures that can remove magnetism, or were formed by lava flows after the planetwide field shut down. Some magnetic stripes are visible in the lower middle of the image, where the field alternately points up and down. These stripes suggest that Mars went through a period with plate tectonics.

(NASA)