Even though Mars has a metallic core, it does not have a global intrinsic magnetic field. Early magnetic field measurements from the Mars 2, 3, and 5 spacecraft were interpreted to imply that Mars had a small magnetic field with a dipole moment 3× 10−4 times the Earth’s magnetic dipole moment. Data from the Mars Global Surveyor have settled the question of the existence of a global Martian magnetic field – there is none, but the crust of Mars has strong concentrations of remanent magnetism implying that Mars had a global magnetic field in the past. Crustal
magnetization on Mars is mainly confined to the ancient highlands of the southern hemisphere and it is largely organized into east-west-trending linear features of alternating polarity extending over distances as large as 2000
km. The magnetization features are reminiscent of the magnetic stripes on the Earth’s seafloor, and suggest the possibility of a plate tectonic regime with seafloor spreading early in the history of Mars. The absence of crustal
magnetism near large impact basins such as Hellas and Argyre implies that the early Martian dynamo ceased to operate before about 4 Ga. The major evidence for an initially hot and differentiated Mars is the acceptance of
Mars as a parent body of the SNC meteorites. This is a class of meteorites found on Earth that apparently escaped from the Martian gravity field after one or more large impacts. The radiometric ages for SNC meteorites are
about 4.6 Ga, the U/Pb isotopic composition of SNC meteorites require core formation at about 4.6 Ga, and the old age (≥4 Ga) of the southern highlands suggests early crustal differentiation. Other evidence for a hot early
Mars includes water-carved features on the Martian surface suggesting early outgassing and an early atmosphere.