The magnetic field is a key to the understanding of the planet's history. The presence of a global magnetic field is related to the existence of motion in the liquid iron-composed core. One can identify two source for this phenomenon :

• the internal heat is strong enough to not be removed by thermal conduction,
• or the planet has reached the necessary temperature and pressure for the solidification and the precipitation of iron that will form the solid inner core.

Today, there is no global magnetic field on the surface of Mars, but a magnetic field, fossil, unlike the Earth. This tells us that the planet's core does not have enough motion, or no motion at all, in its core could be completely solid. Because of the fossil magnetic field of Mars, we can still assume that the planet had a fluid core.