Mars

Mars

**Mars** is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.

The rotational period
 and seasonal cycles of Mars are likewise similar to those of Earth, as
is the tilt that produces the seasons. Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature.

Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Mars trojan.

## Physical Characteristics

Mars is approximately half the diameter of Earth, and its surface area
is 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.

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 show 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.

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). The polar caps at both poles consist primarily (70%) of water ice.
Frozen carbon dioxide accumulates as a comparatively thin layer about
one metre thick on the north cap in the northern winter only, whereas
the south cap has a permanent dry ice cover about eight metres thick.
This permanent dry ice cover at the south pole is peppered by flat floored, shallow, roughly circular pits, which repeat imaging shows are expanding by meters per year; this suggests that the permanent CO2 cover over the south pole water ice is degrading over time.

## Satellites

Mars has two relatively small natural moons, Phobos (about 22 km (14 mi) in diameter) and Deimos
 (about 12 km (7.5 mi) in diameter), which orbit close to the planet.
Asteroid capture is a long-favored theory, but their origin remains
uncertain. Both satellites were discovered in 1877 by Asaph Hall; they are named after the characters Phobos (panic/fear) and Deimos (terror/dread), who, in Greek mythology, accompanied their father Ares, god of war, into battle. Mars was the Roman counterpart of Ares.

From the surface of Mars, the motions of Phobos and Deimos appear different from that of the Moon. Phobos rises in the west, sets in the east, and rises again in just 11 hours. Deimos, being only just outside synchronous orbit
 – where the orbital period would match the planet's period of rotation –
 rises as expected in the east but slowly. Despite the 30-hour orbit of
Deimos, 2.7 days elapse between its rise and set for an equatorial
observer, as it slowly falls behind the rotation of Mars.