Our knowledge of Mars's interior is not very good because we do not have extensive seismic data from the surface of the planet. Some data from the Viking missions seem to indicate that Mars is not seismically very active. While future missions to Mars will yield more seismic data, what evidence we do possess today can give us some hints about the structure of the planet. This evidence falls into three categories: the average density of Mars, the morphology of its surface, and the almost complete absence of a magnetic field.

The average density of Mars is only 3.9 g/cm³, small in comparison to the 5.4 g/cm³ density of Earth. Mars probably has a smaller iron core, and its mantle makes up a greater percentage of the planet's mass than on Earth. Some calculations yield an estimate of about 1,300-2,400 kilometers for the diameter of the core, compared to the 3,500 kilometers estimated for Earth's core. This is a relatively large percentage of volume: the metallic iron and nickel making up the core are less dense and compressed than on Earth, because of the lower gravity on Mars.

The absence of plate tectonics and the relative volcanic inactivity seem to indicate that Mars's interior is cooler and less active than that of our planet. It probably also means that the crust is thicker on Mars. This fact is further supported by the near-absence of a magnetic field on Mars. A planet's magnetic field is believed to be linked to the presence of a liquid outer core of iron (Fe) and sulphur (S), surrounding the iron- nickel inner core. We have reasons to think that inner and outer cores are also present on Mars. However, the outer core may be too cool to be liquid. Other measurements seem to indicate that Mars's magnetic field might have been a lot stronger in the past, when its interior was more active.