Mars' Hill, name given in the Bible for the Areopagus at Athens.

Mars, in astronomy, 4th planet from the sun, with an orbit next in order beyond that of the earth.

Physical Characteristics

Mars has a striking red appearance, and in its most favorable position for viewing, when it is opposite the sun, it is twice as bright as Sirius, the brightest star. Mars has a diameter of 4,200 mi (6,800 km), just over half the diameter of the earth, and its mass is only 11% of the earth's mass. The planet has a very thin atmosphere consisting mainly of carbon dioxide, with some nitrogen and argon. Mars has an extreme day-to-night temperature range, resulting from its thin atmosphere, from about 80°F; (27°C;) at noon to about -100°F; (-73°C;) at midnight; however, the high daytime temperatures are confined to less than 3 ft (1 m) above the surface.

Surface Features

A network of linelike markings first studied in detail (1877) by G. V. Schiaparelli was referred to by him as canali, the Italian word meaning "channels" or "grooves." Percival Lowell, then a leading authority on Mars, created a long-lasting controversy by accepting these "canals" to be the work of intelligent beings. Under the best viewing conditions, however, these features are seen to be smaller, unconnected features. The greater part of the surface area of Mars appears to be a vast desert, dull red or orange in color. This color may be due to various oxides in the surface composition, particularly those of iron. About one fourth to one third of the surface is composed of darker areas whose nature is still uncertain. Shortly after its perihelion Mars has planetwide dust storms that can obscure all its surface details.

Photographs sent back by the Mariner 4 space probe show the surface of Mars to be pitted with a number of large craters, much like the surface of our moon. In 1971 the Mariner 9 space probe discovered a huge canyon, Valles Marineris. Completely dwarfing the Grand Canyon in Arizona, this canyon stretches for 2,500 mi (4,000 km) and at some places is 125 mi (200 km) across and 2 mi (3 km) deep. Mars also has numerous enormous volcanoes—including Olympus Mons (c.370 mi/600 km in diameter and 16 mi/26 km tall), the largest in the solar system—and lava plains. In 1976 the Viking spacecraft landed on Mars and studied sites at Chryse and Utopia. They recorded a desert environment with a reddish surface and a reddish atmosphere. These experiments analyzed soil samples for evidence of microorganisms or other forms of life; none was found. In 1997, Mars Pathfinder landed on Mars and sent a small rover, Sojourner, to take soil samples and pictures. Among the data returned were more than 16,000 images from the lander and 550 images from the rover, as well as more than 15 chemical analyses of rocks and extensive data on winds and other weather factors. Mars Global Surveyor, which also reached Mars in 1997 and remained operational until 2006, returned images produced by its systematic mapping of the surface. The European Space Agency's Mars Express space probe went into orbit around Mars in late 2003 and sent the Beagle 2 lander to the surface, but contact was not established with the lander. The American rovers Spirit and Opportunity landed successfully in early 2004 and continue to explore the Martian landscape. In 2008 NASA's Phoenix lander touched down in the planet's north polar region.

Analysis of the satellite data indicates that Mars appears to lack active plate tectonics at present; there is no evidence of recent lateral motion of the surface. With no plate motion, hot spots under the crust stay in a fixed position relative to the surface; this, along with the lower surface gravity, may be the explanation for the giant volcanoes. However, there is no evidence of current volcanic activity.

There is evidence of erosion caused by floods and small river systems. The possible identification of rounded pebbles and cobbles on the ground, and sockets and pebbles in some rocks, suggests conglomerates that formed in running water during a warmer past some 2-4 billion years ago, when liquid water was stable and there was water on the surface, possibly even large lakes or oceans. Rovers have identified minerals that only form in the presence of liquid water. There is also evidence of flooding that occurred less that several million years ago, most likely as the result of the release of water from aquifers deep underground. However, other evidence suggests that the water would have been extremely salty and acidic. Data received beginning in 2002 from the Mars Odyssey space probe suggests that there is water in sand dunes found in the northern hemisphere, and the Mars Reconnaissance Orbiter, which went into orbit around the planet in 2006, collected radar data that indicates the presence of large subsurface ice deposits in the mid-northern latitudes of Mars. Most of the known water on Mars, however, lies in a frozen layer under the planet's large polar ice caps, which themselves consist of water ice and dry ice (frozen carbon dioxide).

Seasonal Changes

Because the axis of rotation is tilted about 25° to the plane of revolution, Mars experiences seasons somewhat similar to those of the earth. One of the most apparent seasonal changes is the growing or shrinking of white areas near the poles known as polar caps. These polar caps, which are are composed of water ice and dry ice (frozen carbon dioxide). During the Martian summer the polar cap in that hemisphere shrinks and the dark regions grow darker; in winter the polar cap grows again and the dark regions become paler. The seasonal portion of the ice cap is dry ice. When the ice cap is seasonally warmed, geyserlike jets of carbon dioxide gas mixed with dust and sand erupt from the ice.

Astronomical Characteristics

The mean distance of Mars from the sun is about 141 million mi (228 million km); its period of revolution is about 687 days, almost twice that of the earth. At those times when the sun, earth, and Mars are aligned (i.e., in opposition) and Mars is at its closest point to the sun (perihelion), its distance from the earth is about 35 million mi (56 million km); this occurs every 15 to 17 years. At oppositions when Mars is at its greatest distance from the sun (aphelion) it is about 63 million mi (101 million km) from the earth. It rotates on its axis with a period of about 24 hr 37 min, a little more than one earth day.

Satellites of Mars

Mars has two natural satellites, discovered by Asaph Hall in 1877. The innermost of these, Phobos, is about 7 mi (11 km) in diameter and orbits the planet with a period far less than Mars's period of rotation (7 hr 39 min), causing it to rise in the west and set in the east. The outer satellite, Deimos, is about 4 mi (6 km) in diameter.

Mars, in Roman religion and mythology, god of war. In early Roman times he was a god of agriculture, but in later religion (when he was identified with the Greek Ares) he was primarily associated with war. Mars was the father of Romulus, the founder of the Roman nation, and, next to Jupiter, he enjoyed the highest position in Roman religion. The Salii, his priests, honored him by dancing in full armor in the Campus Martius, the site of his altar. Chariot races and the sacrifice of animals were primary features of the festivals held in his honor in March (named for him) and October. Mars was represented as an armed warrior. His attributes include the spear and shield, and the wolf and woodpecker were sacred to him. He was frequently associated with Bellona, the Roman goddess of war.

Cinq Mars, Henri Coëffier Ruzé d'Effiat, marquis de, 1620-42, French conspirator. Introduced at court by Cardinal Richelieu at an early age, Cinq Mars rapidly rose in King Louis XIII's favor and was made master of the horse. He joined in a conspiracy with Frédéric Maurice de Bouillon and Gaston d'Orléans against the cardinal. The discovery of a secret treaty they had signed with Spain led to their arrest, and Cinq Mars and his friend, François de Thou, were executed. The conspiracy formed the basis of Alfred de Vigny's novel Cinq-Mars and Gounod's opera of the same name.

Champ-de-Mars, former parade ground of Paris, France, between the École militaire and the Seine River. There, at the Fěte de la Fédération (July 14, 1790), Louis XVI took an oath to uphold the new constitution. On its vast grounds several expositions were held, notably that of 1889, when the Eiffel Tower was erected there.

Fourth planet from the Sun, named after the Roman god of war. Its mean distance from the Sun is 141 million mi (227 million km). Its day is 24.6 Earth hours and its year about 687 Earth days. It has two small moons, Phobos and Deimos. Mars's equatorial diameter is 4,220 mi (6,792 km), about half that of Earth, and it is less dense than Earth. Its mass is about one-tenth of Earth's and its surface gravity about one-third as strong. No magnetic field has been detected on Mars, suggesting, as does its low density, the absence of a substantial metallic core. Like Earth, it has seasons and an atmosphere, but its average daytime surface temperature is only −10 °F (−20 °C). Mars's thin atmosphere is mainly carbon dioxide, with some nitrogen and argon and traces of water vapour. Spacecraft images show a cratered surface, with volcanoes, lava plains, flood channels, and canyons, many large by Earth standards; Olympus Mons, for example, is the largest known volcano in the solar system. Wind is an important element on Mars, sculpting features such as dunes and occasionally causing global dust storms. In the distant past Mars appears to have had a denser, warmer atmosphere and much more water than at present. Images from the Mars Global Surveyor spacecraft suggest that some liquid water may have flowed near the planet's surface in relatively recent times. No life has been detected on the planet.

Learn more about Mars with a free trial on Britannica.com.