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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 about1794 ± 65 km 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, with a maximum thickness of 125 km.Earth's crust, averaging 40 km, 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.
Mars has two permanent polar ice caps. During a pole's winter, it lies in continuous darkness, chilling the surface and causing the deposition of 25–30% of the atmosphere into slabs of CO2 ice (dry ice).When the poles are again exposed to sunlight, the frozen CO2 sublimes, creating enormous winds that sweep off the poles as fast as 400 km/h. These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth-like frost and large cirrus clouds. Clouds of water-ice were photographed by the Opportunity rover in 2004.