The Interior
Let's compare the interior of the Earth with that of Mars. The Earth has a core that has a radius of about 2,100 miles (3,500 km). The core is made of iron and has two parts, a solid inner core and a liquid outer core. Heat is generated from radioactive decay in the core. This heat is lost from the core to the layers above. Convective currents in the liquid outer core along with the rotation of the Earth produce the Earth's magnetic field. In contrast, the core of Mars (shown as red in the figure below) is probably made of a mixture of iron, sulfur, and maybe oxygen.
 Photo courtesy NASA/JPL Diagram showing the interior of Mars |
The core probably has a radius between 900 to 1,200 miles (1,500 to 2,000 km). The outer part of the core may be molten, but it is unlikely, because Mars has only a weak magnetic field (less than 0.01 percent of Earth's magnetic field). Although Mars does not have a strong magnetic field now, it might have had a strong one long ago.
Surrounding Earth's core is a thick layer of soft rock called the mantle. What do we mean by soft? Well, if the core is liquid, then the mantle is a paste like toothpaste. The mantle is less dense than the core (which is why it is above the core), made of iron and magnesium silicates, and about 1,800 miles (3,000 km) thick. The mantle is the source of lava from volcanoes. Like Earth, the mantle of Mars (shown as brown in the figure) is probably made of thick silicates; however, it is much smaller, 800 to 1,100 miles (1,300 -1,800 km) thick. There must have been convective currents in the mantle at one time to account for the formation of the crustal upwarps (e.g. Tharsis region), the Martian volcanoes, and fractures that formed Valles Marineris.
Surrounding the mantle is the crustcontinental plates. The plates float over the underlying mantle and rub against each other (continental drift). The areas where they rub are filled with cracks or faults such as the San Andreas fault in California. These areas of contact between plates have earthquakes and volcanoes. On Mars, the crust is also thin, but is not broken into plates like the Earth's crust. There is no evidence of active volcanoes or earthquakes, but there must have been volcanic activity at one time because we can observe lava flows (e.g. Northern Plains) from orbit.
