(♂) The fourth planet from the Sun. It appears distinctly reddish to the naked eye. Its mean opposition magnitude is −2.0, although at perihelic oppositions it can reach −2.9, brighter than all planets except Venus and Jupiter. It is slightly ellipsoidal in shape (equatorial diameter 6792 km, polar diameter 6752 km).
Mars has a thin atmosphere composed (by volume) of about 95% carbon dioxide, 2.7% nitrogen, 1.6% argon, 0.1% oxygen, 0.1% carbon monoxide, and small variable traces of water vapour. The average atmospheric pressure at the surface is about 6 mbar, with a seasonal variation of over 1 mbar. Surface temperatures range from 20° C to −140°C, averaging about −60°C. White clouds of condensed water vapour or carbon dioxide are relatively common, particularly near the terminator and in polar latitudes. There are two permanent water-ice caps at the poles which never melt. In winter these are overlain by caps of frozen carbon dioxide several metres thick, extending to latitude 60°. Dust storms occur from time to time, particularly just after perihelion when they may spread to cover the entire planet in a yellow haze, hiding the familiar surface markings.
Mars's surface is a volcanic basalt with a high iron content. Oxidation of this iron gives Mars its distinctive rust-red colour. Dark and bright markings can be seen through telescopes, but these do not always correspond to topographic features or terrain types; dark patches appear to be due to areas of dark surface dust. These may slowly change over the years as the dust is transported by winds. The most prominent dark marking, Syrtis Major, is an unremarkable east-facing slope with a gradient of less than 1°. There are many areas of sand dunes; the largest surround the polar caps, and constitute the largest dune field of the Solar System.
Extensive volcanic activity has occurred on Mars. Tharsis Montes is the largest volcanic region, with Olympus Mons to the northwest and the vast collapsed structure of Alba Patera to the north. Together, these volcanic areas make up nearly 10% of the planet's surface. No volcanoes are presently active on Mars, but in the past they produced plains of lava stretching for hundreds of kilometres.
Impact craters are widespread on Mars, but there is an almost continuously cratered upland area, similar to the lunar highlands, which makes up about half the planet's surface, mainly in the southern hemisphere. Many of the fresher impact craters, known as rampart craters, have steep slopes at the edges of their ejecta blankets, suggesting that the surface was damp or muddy when the impacting bodies struck. The best-preserved large impact basins are Argyre and Hellas (see Argyre Planitia; Hellas Planitia). Although there is now no liquid water on the surface, there are signs that rivers and lakes once existed when the atmosphere was presumably denser, warmer, and wetter than at present. Dried-up water channels include Ma'adim Vallis, over 800 km long and several kilometres wide. Direct evidence that liquid water existed at the surface in the distant past was found by the Mars Exploration Rovers in the form of deposits of salts associated with the weathering of rocks by water, plus the identification of jarosite, a mineral that forms on Earth by prolonged exposure to water. The likely presence of liquid water in the past opens the possibility that complex organic chemistry, perhaps leading to simple forms of life, once occurred on Mars.
Subjects: Astronomy and Astrophysics.