New Data from Mars Hint at a Dynamic Past

As the Mars Global Surveyor orbits the Red Planet, magnetometers aboard the spacecraft have been sending home some astounding data. The instruments have confirmed indications from earlier missions that, if Mars has a global magnetic dipole at all, it is more than 10 000 times weaker than Earth’s. Nevertheless, the magnetometers find, Mars’s surface is dotted with local regions where the magnetic fields are over ten times greater than any perturbations in Earth’s global field. Most likely, the local fields stem from an early time when the planet’s core was still molten and a magnetic dynamo imprinted its field in the rocks. But the most intriguing discovery by far is the large, linear magnetic structure evident in Mars’s southern hemisphere: Along a band extending nearly 2000 km from east to west, the crustal magnetic moments are aligned in one direction; those in adjacent bands appear to be magnetized in the reverse direction, and so forth—for five or six flips of the magnetic field.