Scientists believe that Mars’s interior consists of a crust, mantle, and core as if Earth’s interior but they do not know the relative sizes of these components. Because no spacecraft has ever brought instruments that can study Mars’s interior to the planet, the only real data that scientists have about the planet’s structure are its mass, size, and the structure of the gravity field. From that data, scientists can learn some things about density in different parts of the planet.
Compared to Earth, Mars probably has a relatively thick crust. Beneath the Tharsis bulge, an area of volcanic activity in the Northern Hemisphere, it may be as thick as 80 mile. Beneath the landing site of the United States spacecraft Viking 2, it may be as thin as 15 km (9 mi.).
The core is probably mostly iron, with a small amount of nickel. Other light elements, particularly sulfur, could exist in the core as well. If so, the core may be quite large. From studying the earth’s magnetic field and core, scientists theorize that the motions of the liquid rock in the earth’s core generate its magnetic field. Mars does not have a significant magnetic field, so scientists believe that Mars’s core is probably solid.
It is unknown whether plate tectonics certainly exist on Mars or a crust made up of separate sections that move about and sometimes crash into each other. Because Mars is so much smaller than Earth, it cooled quickly after formation and the crust thickened, forming one solid piece. Though the Martian crust is not broken into separate plates, Mars’s liquid mantle has sculpted the surface of the planet. The molten rock has broken through the crust to form volcanoes and its motion has cracked the crust to form large rifts.