Magnetic fields of Mars
Unlike Earth, Mars has no strong global magnetic field, only weak local ones are present.
Surveys in the 1990s of magnetic fields on Mars, by the orbiting Mars Global Surveyor, detected the signatures of relatively intense magnetism in some of the planet's more modern surfaces. But the fields were found to be very weak in two large and old impact basis, Hellas and Argyre. Each basin, carved out by a colossal space asteroid, is more than 3 billion years old. The data implied that Mars had a weak magnetic field back then.
New research calls into question the validity of measuring magnetism from an orbital perch. It has been found that rocks in the 2-billion-year-old Vredefort impact crater in South Africa -- the oldest such structure on Earth -- are highly magnetized, yet from above the magnetism appears weak. Two other ancient craters reveal similar differences.
The basic reason is simple: While magnetism is strong in individual rocks, the direction varies from rock to rock in these impact craters, so when examined from a distance, they cancel each other out.
So, viewed from a high altitude of 100-400 km, certain areas of Mars might appear magnetically featureless if the magnetic vectors of their source rocks vary in direction over distances of a few kilometers or less.
Magnetic fields on Mars are "striped", a feature that clearly shows that Mars had crustal plates movement in its history. Stripe patterns like these are widespread on Earth – particularly along the Mid-Atlantic Ridge.