Mars is differentiated, which—for a terrestrial planet—implies that it has a central core made up of high density matter (mainly metallic iron and nickel) surrounded by a less dense, silicate mantle and crust. Like Earth, Mars appears to have a molten iron core, or at least a molten outer core. However, there does not appear to be convection in the mantle. Presently Mars shows little geological activity.
The elemental composition of Mars is different from Earths in several significant ways. First, Martian meteorite analysis suggests that the planets mantle is about twice as rich in iron as the Earths mantle. The planets distinctive red color is due to iron oxides on its surface. Second, its core is richer in sulphur. Third, the Martian mantle is richer in potassium and phosphorus than Earths and fourth, the Martian crust contains a higher percentage of volatile elements such as sulphur and chlorine than the Earths crust does. Many of these conclusions are supported by in situ analyses of rocks and soils on the Martian surface.
Much of what we know about the elemental composition of Mars comes from orbiting spacecraft and landers. (See Exploration of Mars for list.) Most of these spacecraft carry spectrometers and other instruments to measure the surface composition of Mars by either remote sensing from orbit or in situ analyses on the surface. We also have many actual samples of Mars in the form of meteorites that have made their way to Earth. Martian meteorites (often called SNCs, for Shergottites, Nakhlites, and Chassignites—the groups of meteorites first shown to have a martian origin) provide data on the chemical composition of Mars crust and interior that would not otherwise be available except through a sample return mission.
Based on these data sources, scientists think that the most abundant chemical elements in the Martian crust are silicon, oxygen, iron, magnesium, aluminium, calcium, and potassium. These elements are major components of the minerals comprising igneous rocks. The elements titanium, chromium, manganese, sulfur, phosphorus, sodium, and chlorine are less abundant but are still important components of many accessory minerals in rocks and of secondary minerals (weathering products) in the dust and soils (the regolith). On September 5, 2017, scientists reported that the Curiosity rover detected boron, an essential ingredient for life on Earth, on the planet Mars. Such a finding, along with previous discoveries that water may have been present on ancient Mars, further supports the possible early habitability of Gale Crater on Mars.
Hydrogen is present as water (HO) ice and in hydrated minerals. Carbon occurs as carbon dioxide (CO) in the atmosphere and sometimes as dry ice at the poles. An unknown amount of carbon is also stored in carbonates. Molecular nitrogen (N) makes up 2.7 percent of the atmosphere. As far as we know, organic compounds are absent except for a trace of methane detected in the atmosphere. On 16 December 2014, NASA reported the Curiosity rover detected a "tenfold spike", likely localized, in the amount of methane in the Martian atmosphere. Sample measurements taken "a dozen times over 20 months" showed increases in late 2013 and early 2014, averaging "7 parts of methane per billion in the atmosphere." Before and after that, readings averaged around one-tenth that level.
On 25 October 2023, scientists, helped by information from the InSight lander, reported that the planet Mars has a radioactive magma ocean under its crust.