<P> Mars is approximately half the diameter of Earth with a surface area only slightly less than the total area of Earth's dry land . Mars is less dense than Earth, having about 15% of Earth's volume and 11% of Earth's mass, resulting in about 38% of Earth's surface gravity . The red - orange appearance of the Martian surface is caused by iron (III) oxide, or rust . It can look like butterscotch; other common surface colors include golden, brown, tan, and greenish, depending on the minerals present . </P> <P> Like Earth, Mars has differentiated into a dense metallic core overlaid by less dense materials . Current models of its interior imply a core with a radius of about 1,794 ± 65 kilometers (1,115 ± 40 mi), consisting primarily of iron and nickel with about 16--17% sulfur . This iron (II) sulfide core is thought to be twice as rich in lighter elements as Earth's . The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but it 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 averages 40 km (25 mi). </P> <P> Mars is a terrestrial planet that consists of minerals containing silicon and oxygen, metals, and other elements that typically make up rock . The surface of Mars is primarily composed of tholeiitic basalt, although parts are more silica - rich than typical basalt and may be similar to andesitic rocks on Earth or silica glass . Regions of low albedo suggest concentrations of plagioclase feldspar, with northern low albedo regions displaying higher than normal concentrations of sheet silicates and high - silicon glass . Parts of the southern highlands include detectable amounts of high - calcium pyroxenes . Localized concentrations of hematite and olivine have been found . Much of the surface is deeply covered by finely grained iron (III) oxide dust . </P> <P> Although Mars has no evidence of a structured global magnetic field, observations show that parts of the planet's crust have been magnetized, suggesting that alternating polarity reversals of its dipole field have occurred in the past . This paleomagnetism of magnetically susceptible minerals is similar to the alternating bands found on Earth's ocean floors . One theory, published in 1999 and re-examined in October 2005 (with the help of the Mars Global Surveyor), is that these bands suggest plate tectonic activity on Mars four billion years ago, before the planetary dynamo ceased to function and the planet's magnetic field faded . </P>

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