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 130 km (80 mi). Beneath the landing site of the United States spacecraft Viking 2, it may be as thin as 15 km (9 mi).
The interior of Mars is known only by inference from data about the surface and the bulk statistics of the planet. The most likely scenario is a dense core about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth's and a thin crust. Mars' relatively low density compared to the other terrestrial planets indicates that its core probably contains a relatively large fraction of sulfur in addition to iron (iron and iron sulfide). The mean density of the planet is less than that of Earth. In other words, Mars weighs 3.9 times as much as an equal volume of water would weigh. In contrast, the specific gravity of Earth is 5.5. Since a planet becomes less dense from its core towards its outer layers, the mean density of Mars is roughly equal to that of the outer layers of Earth.
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.
Like Mercury and the Moon, Mars does not, and probably did not ever, have active plate tectonics, or a crust made up of separate sections that move about and sometimes crash into each others. There is no evidence of horizontal motion of the surface such as the folded mountains so common on Earth. Because Mars is so much smaller than Earth, it cooled quickly after formation and the crust thickened, forming one solid piece and eliminating any possibility of plate tectonics as is seen on Earth. Though the Martian crust is not broken into separate plates, Mars’s liquid mantle has sculpted the planet’s surface. The molten rock has broken through the crust to form volcanoes and its motion has cracked the crust to form large rifts. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This, along with the lower surface gravity, may account for the Tharis bulge and its enormous volcanoes. There is no evidence of current volcanic activity, however. And though Mars may have been more volcanicly active in the past, it appears to never have had any plate tectonics.
Large, but not global, weak magnetic fields exist in various regions of Mars. This unexpected finding was made by Mars Global Surveyor just days after it entered Mars orbit. They are probably remnants of an earlier global field that has since disappeared. This may have important implications for the structure of Mars's interior and for the past history of its atmosphere and hence for the possibility of ancient life.
Due to the difficulty of measuring the internal mechanics of Mars, all the information we know has been inferred from data collected on the surface or in orbit. Therefore, there is no way to tell how accurate the data is.