Magnetic stripes cover much of Mars, as revealed by detailed new data from the Mars Global Surveyor spacecraft. The planet's former internal dynamo painted most of the planet's crust with magnetic stripes. The big "blank spots" — the northern lowlands and Tharsis — lost their magnetic signature when volcanism heated the rocks. Giant basins, such as Argyre and Hellas, likewise appear unmagnetized due to the heat of the impacts that formed them.
NASA / Goddard Space Flight Center