The Interior of Mars
Let's compare the interior of the Earth with that of Mars. The Earth has a core with a radius of about 2,200 miles (3,500 kilometers) from the center to the surface. The core is made of iron and has two parts: a solid inner core and a liquid outer core. Radioactive decay in the core generates the heat. 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.
Mars, the more petite planet, probably has a core radius between 900 and 1,200 miles (1,500 kilometers and 2,000 kilometers). Its core (shown as red in the figure to the right) is probably made of a mixture of iron, sulfur and maybe oxygen. The outer part of the core may be molten, but it's unlikely, because Mars has only a weak magnetic field (less than 0.01 percent of Earth's magnetic field). Although Mars doesn't have a strong magnetic field now, it might have had a powerful 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 outer core is liquid, then the mantle is a paste, like toothpaste. The mantle is less dense than the core (which explains why it rests above the core). It's made of iron and magnesium silicates, and it stretches about 1,800 miles (3,000 kilometers) thick (remember that the next time you try to dig a hole to China). The mantle is the source of lava that spews and trickles from volcanoes.
Like Earth, the mantle of Mars (shown as brown in the figure) is probably made of thick silicates; however, it's much smaller at 800 to 1,100 miles (1,300 to 1,800 kilometers) thick. There must have been convective currents that rose up in the mantle at one time. These currents would account for the formation of the crustal upwarps, such as the Tharsis region, the Martian volcanoes and the fractures that formed Valles Marineris.
On Earth, the crust's continental 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 experience earthquakes and volcanoes. On Mars, the crust is also thin, but isn't broken into plates like the Earth's crust. There is no evidence of active volcanoes or earthquakes on Mars, but there must have been volcanic activity at one time because we can observe lava flows from orbit.
Do you want to see all this for yourself? You might have difficulty breathing on Mars. Find out why next.