The interior of Mars is poorly known. Planetary scientists have yet to conduct a successful seismic experiment via spacecraft that would provide direct information on internal structure and so must rely on indirect inferences. The moment of inertia of Mars indicates that it has a central core with a radius of 1,300-2,000 km (800-1,200 miles). Isotopic data from meteorites determined to have come from Mars demonstrate unequivocally that the planet differentiated—separated into a metal-rich core and rocky mantle—at the end of the planetary accretion period 4.5 billion years ago. The planet has no detectable magnetic field that would indicate convection (heat-induced flow) in the core today. Large regions of magnetized rock have been detected in the oldest terrains, however, which suggests that very early Mars did have a magnetic field but that it disappeared as the planet cooled and the core solidified. Martian meteorites also suggest that the core may be more sulfur-rich than Earth's core and the mantle more iron-rich.
Mars is almost certainly volcanically active today, although at a very low level. Some Martian meteorites, which are all volcanic rocks, show ages as young as a few hundred million years, and some volcanic surfaces on the planet are so sparsely cratered that they must be only tens of millions of years old. Thus, Mars was volcanically active in the geologically recent past, which implies that its mantle is warm and undergoing melting locally.
Mars's gravitational field is very different from Earth's. On Earth, excesses and deficits of mass in the surface crust, corresponding to the presence of large mountains and ocean deeps, respectively, tend to be offset by compensating masses at depth (isostatic compensation). Thus, the pull of gravity on Earth is the same on high mountains as it is over the ocean. This is also true for Mars's oldest terrains, such as the Hellas basin and the southern highlands. The younger terrains, such as the Tharsis and Elysium domes, however, are only partly compensated. Associated with both of these regions are gravity highs—that is, places where the measured gravity is significantly higher than elsewhere because of the large mass of the domes. (These areas are similar to the mascons that have been detected and mapped on Earth's Moon.)
Because the gravity over the southern highlands is roughly the same as that over the low-lying northern plains, the southern highlands must be underlain by a thicker crust of material that is less dense than the mantle below it. Estimates of the thickness of the Martian crust range from only 3 km (2 miles) under the Isidis impact basin, which is just north of the equator and east of Syrtis Major, to more than 90 km (60 miles) at the south end of the Tharsis rise.
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