Mars probably has a core composed of iron, nickel, and sulfur. The density of Mars gives some indication of the size of the core. Unlike the core of the Earth, which is partially molten (melted), the core of Mars probably is solid. Scientists suspect that the core is solid because Mars does not have a significant magnetic field. A magnetic field is an influence that a magnetic object creates in the region around it. Motion within the molten core of a planet makes the core a magnetic object. The motion occurs due to the rotation of the planet. Data from Mars Global Surveyor show that some of the oldest rocks of Mars formed in the presence of a strong magnetic field. Thus, in the distant past, Mars may have had a hotter interior and a molten core.