Like Earth, this planet has undergone differentiation, resulting in a dense, metallic core region overlaid by less dense materials. Current models of the planet’s interior imply a core region about 1,794 km (1,115 mi) ± 65 km (40 mi) in radius, consisting primarily of iron and nickel with about 16–17% sulfur. This iron sulfide core is partially fluid, and has twice the concentration of the lighter elements that exist at Earth’s core. The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but now appears to be dormant. Besides silicon and oxygen, the most abundant elements in the Martian crust are iron, magnesium, aluminum, calcium, and potassium. The average thickness of the planet’s crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi). Earth’s crust, averaging 40 km (25 mi), is only one third as thick as Mars’s crust, relative to the sizes of the two planets. The InSight lander planned for 2016 will use a seismometer to better constrain the models of the interior.