Although not much is directly known about the interior of Venus, its size and density give us hints about its composition. Venus has a density of 5.2 g/cm³, and probably a comparable composition and internal structure to the Earth's. The main difference between the two planets might be the thickness of the external, solid layer, i.e. the crust. From the absence of plate tectonics some speculate that Venus' crust is thicker than Earth's. The mantle and the core are assumed to be very similar in size to the Earth's. The core may be divided into a solid iron (Fe) portion at the center, surrounded by an iron (Fe) and sulphur (S) liquid layer. It is commonly thought that the currents within the liquid layer and the planet's rotation are responsible for the Earth's magnetic field. On Venus, however, we observe no substantial magnetic field, and that may be due to Venus' slow rotation around its axis.