In astronomy, fourth major planet from the Sun, named after the Roman god of war because of its reddish colour.
A brief treatment of Mars follows.
Mars has an elliptical orbit, and so its distance from the Earth varies considerably. Its mean distance from the Sun is 228 million km (141 million miles), about half as far again as is the Earth. A Martian day, or sol, is 24.6 Earth hours, and the Martian year is approximately 687 Earth days. The planet has two small satellites, Phobos and Deimos.
Like the Earth, Mars has seasons because of an oblique axis of rotation and the presence of an atmosphere. It is, however, much colder: the mean surface atmospheric temperature is only -23 C (-9.4 F). Mars is a small planet, having a mean diameter of 6,790 km, approximately half that of the Earth. Also, its density, 3.933 grams per cubic centimetre, is lower than that of Earth. No magnetic field has been detected on Mars, and this, together with the low density, suggests the absence of a substantial metallic core. Mars's thin atmosphere is composed predominantly of carbon dioxide, with some nitrogen and argon. Traces of water vapour have also been detected. The perennial part of the ice caps consists of water ice and the seasonal parts of frozen carbon dioxide.
Studies conducted with Earth-based telescopes established the existence but not the nature of Mars's atmosphere, polar caps, albedo markings, and satellites. The first spacecraft images of the planet were obtained by the flyby missions of the U.S. planetary probes Mariner 4 (1964) and 6 and 7 (1969); they showed a cratered surface, similar to that of the Moon, in the southern hemisphere. The first orbiter spacecraft was Mariner 9 (1971), which obtained photographs of much of the Martian surface. These images revealed a dramatic difference between the southern and northern hemispheres. The southern hemisphere is old and cratered, whereas the northern hemisphere is less cratered because most of the terrain has been buried by younger materials, probably of volcanic and wind-blown origin. During the late 1970s two U.S. Viking spacecraft photographed the Martian surface in greater detail. The Viking landing craft performed various experiments on the surface, including tests designed to detect the presence of life forms such as microbes, but these yielded no positive results.
Among the surface features of the planet that were revealed by the Mariner 9 and Viking probes were volcanoes, extensive lava plains, various types of channels and canyons, and remnants of landslides. Many of these features are large by terrestrial standards. Olympus Mons, for example, is the largest known volcano in the solar system, having a diameter of 550 km and a height of roughly 27 km. Other large volcanoes occur in the same region as Olympus Mons, on the 2,000-kilometre-long Tharsis rise, which is a region up to 10 km higher than the surrounding northern plains. The Vallis Marineris, a gigantic equatorial rift more than 4,000 km long, comprises the equatorial canyon lands. This rift has been interpreted as the beginning of crustal plate separation that was not sustained. Although faulting is common on the Martian surface, there is no evidence that plate tectonics (crustal movement) has operated further than the early stages. This may be attributed to the great thickness of the Martian lithosphere (crust and upper mantle).
Some of the craters on Mars resemble those found on Mercury and the Moon, but a new type of impact crater was discovered on the Martian surface. Structures of this kind are called rampart craters because the ejecta blanket (i.e., the material thrown out by a meteorite colliding with the surface) has raised edges. This characteristic indicates that after the ejecta was emplaced, it flowed outward and thus extended over a greater area than expected from ballistic considerations. Such action may be attributed to the presence of ground ice.
The major types of channels observed on Mars are classified as runoff, outflow, and fretted channels. Runoff channels are dendritic networks similar to some terrestrial river valleys. Their morphology is consistent with erosion by some sort of liquid. The outflow channels are much larger, originate in closed depressions called box canyons, and decrease in size downstream. Their formation is not fully understood; current theories propose erosion by melting of ground ice, by solid ice, or by wind. Fretted channels are a network of wide valleys that have been modified by landslides along their walls.
Wind is an important element on Mars. Wind-formed deposits such as dunes and crater streaks are common. The largest dune field so far detected forms an annulus around the north polar cap. Crater streaks, flame-shaped accumulations of material, build up in the protected areas to the lee of obstacles to the wind. These landforms may be craters or small isolated hills.
The satellites of Mars, Phobos and Deimos, were photographed by the Mariner 9 and Viking spacecraft. Closeup photographic images show that they are irregular, cratered lumps of rock. Both moons may have originated from residual material yielded during the formation of Mars, or they may be asteroids that were captured by the planet's gravity early in its history.
©Copyright Andrew Do