The Viking landers carried seismometers to probe the internal structure of the planet. However, one failed to work, and the other was unable to clearly distinguish seismic activity from the buffeting of the Martian wind. As a result, no seismic studies of the Martian interior have yet been carried out. On the basis of studies of the stresses that occurred during the Tharsis uplift, astronomers estimate the thickness of the crust to be about 100 km.

During its visit to Mars in 1965, Mariner 4 detected no planetary magnetic field, and for many years the most that could be said about the Martian magnetic field was that its strength was no more than a few thousandths the strength of Earth's field (the level of sensitivity of Mariner's instruments). The Viking spacecraft were not designed to make magnetic measurements. In September 1997, Mars Global Surveyor succeeded in detecting a very weak Martian field, about 800 times weaker than Earth's; this is probably a local anomaly, not a global field. Because Mars rotates rapidly, the weakness of the magnetic field is taken to mean that the planet's core is nonmetallic, or nonliquid, or both. (Sec. 7.4)

The small size of Mars means that any radioactive (or other internal) heating of its interior would have been less effective at heating and melting the planet than similar heating on Earth. The heat was able to reach the surface and escape more easily than in a larger planet such as Earth or Venus. The evidence we noted earlier for ancient surface activity, especially volcanism, suggests that at least parts of the planet's interior must have melted and possibly differentiated at some time in the past. But the lack of current activity, the absence of any significant magnetic field, the relatively low density (3900 kg/m³), and an abnormally high abundance of iron at the surface all suggest that Mars never melted as extensively as did Earth.

The history of Mars appears to be that of a planet where large-scale tectonic activity almost started but was stifled by the planet's rapidly cooling outer layers. The large upwelling of material that formed the Tharsis bulge might have developed on a larger, warmer planet into full-fledged plate tectonic motion, but the Martian mantle became too rigid and the crust too thick for that to occur. Instead, the upwelling continued to fire volcanic activity, almost up to the present day, but geologically much of the planet apparently died 2 billion years ago.