MARS

Mosaic of a martian hemisphere. The light regions of the South part are covered with frozen carbon dioxide. In the centre you can notice the Schiaparelli impact crater, with a 450 Km diameter.   (USGS)

Mars is the fourth planet of the Solar System.  It is also called "the red planet" because of its typical colour due to the iron oxide which is abundant on its surface.  The planet, visible even to the naked eye, is well known since ancient times and was named after the Roman god of war, maybe because of its vivid colour.  Mars has always been indicated as the most likely planet to host life on its surface, because its environmental conditions are the most similar, among all the planets of the Solar system, to the terrestrial ones.

In the last century some dark streaks, called canals, were observed on the surface of Mars, along with some dark spots at their intersection. Besides, it was discovered that these spots had seasonal variations. Some thought that the canals could be riverbeds surrounded by vegetation, and, therefore, that Mars hosted life. Soon, such belief gave rise to a very strong interest in the red planet, which originated the myth of Martians.
In actual fact, the streaks are an optical effect and the seasonal variations of the aspect of the spots are due to the sedimentation and the movement of sand under the action of the strong martian winds. 
 

The "Face on Mars", which was imaginatively interpreted by some as evidence of the existence of intelligent life on the planet, is probably a hill eroded by the strong martian winds.  (Calvin J. Hamilton)

The chemical-biological experiments carried out by the Viking probes, which were the first to land on the martian surface, had controversial results, but they never revealed the existence of any form of macroscopic life on the planet.  In August 1996, the discovery of organic components in a meteorite with a definite martian origin, was announced. This could indicate the existence of micro-organisms on Mars in the past. Nevertheless it is too soon to say if such fact implies the existence of extraterrestrial life.

 

The red planet was explored for the first time in 1965 by the Mariner 4 probe, which transmitted the first photographs of the martian surface. In 1971, the Mariner 9 probe allowed to put together its complete cartography. In 1976 the American probes Viking 1 and Viking 2 succeeded in landing on Mars, and sent back to Earth a lot of data, among which those on the atmospheric composition and temperature. 
In the future, there will be explorations of Mars by Mars GlobalSurveyor and by Mars Pathfinder.
 

The diameter of Mars measures 6,794 Km, it is the seventh planet of the Solar System as far as the size is concerned. 
The mass equals 6,4219 x 10²³ Kg, that is 64 billion billion tons.
Mars rotates around the Sun at a mean distance of 227 million 940 thousand Km, which is equal to 1.52 U.A.; the distance at the aphelion is 249.23 million kilometres; perihelion it is 206.65 million kilometres. The eccentricity of the martian orbit is therefore quite high: 0.093.

The planet has two small satellites, Deimos and Phobos.
 

Deimos in a picture taken by the Viking, in 1977.  (NSSDC/NASA)

Phobos photographed by the Viking probe in 1977. (NASA)

Mars has, like all telluric planets, a dense iron nucleus with a radius that measures 1,700 Km, surrounded by a mantle of rocks.
The density of the planet (3.96) is lower compared to that of the other planets of the same type, and this indicates that the nucleus is composed by lighter elements: it probably contains a large fraction of iron sulphide.
Besides, the absence of a magnetic field indicates that such nucleus can be solid. A fluid nucleus would in fact produce an even weak magnetic field due to the dynamo effect, caused by the differential rotation of the planet and its nucleus.

The crust is thicker than the terrestrial one, and it can reach in some points 150 Km. Its composition is similar to that of the terrestrial crust, but it contains two times less Silicon and three times more Iron; this is the cause of its red colour. 
 
 
 

The martian surface has a large morphological variety: there are mountains, valleys, craters, basins and volcanos. 
From the morphological point of view, Mars can be subdivided in two large regions. The Southern Hemisphere is geologically older, and characterized by uplands and craters with a flat bottom. The Northern Hemisphere is younger and is dominated by the region of the Tharsis massif, full of volcanos. 
 

Some kinds of dunes in the North polar region of the planet. (Calvin J. Hamilton)

Probably, among all the planets, Mars is the one whose inner layers cooled down less rapidly, and this fact allowed the formation of superficial structures up to very recently.  Like Mercury, Mars does not have the horizontal motion of the plates that characterizes the surface of the Earth, nor the formation of mountain ranges from the corrugation of the soil. On the contrary, the mountains of Mars are all of volcanic origin, even though the volcanos are inactive at present.

 

Mosaic of images of Mars. In the centre, the region of the canyons in the Vallis Marineris. West, the three dark red spots are the Tharsis volcanos. (USGS)

The martian craters were partly eroded by the atmosphere, even though there are some young ones: Mars is still bombarded by meteorites. Some craters, on the contrary, seem to have a volcanic origin.   The basins have a size of up to 2,000 Km. The volcanos of Mars are divided in two main regions, the Tharsis upland, 4,000 Km large, and 10 Km high, and Arsia Silva.  The most important fault, known as Vallis Marineris, is actually a huge system of canyons, 4,500 Km long, up to 120 Km large and up to 7 Km deep.  The martian soil is covered by many rocks and by a layer of sand of the depth of a few centimetres.

Spring on the martian northern hemisphere: the ice of the polar cap sublimated, and the cap withdrew. (HST)

An other important feature of Mars are its polar caps: they are sediments of carbon dioxide alternated with sand, that cover the poles of the planet. The sizes of the two caps are different, and they vary during the martian seasons.  During the summer of the Northern Hemisphere, the carbon dioxide sublimates due to the insolation, in other words it changes from the solid phase directly to the gaseous, and the cap vanishes leaving a residue of water ice.   The southern cap, on the other hand, withdraws but does not disappear completely.

 
 

Image of the South Pole at the minimum of its extension (the diameter is approximately 400 Km). The ice appears reddish due to the incorporated sand.  (NASA)

The martian North Pole. (Calvin J. Hamilton)

On Mars, there are some ancient dry riverbeds with evident signs of erosion: their age is approximately 4 billion years. In the past, therefore, there seems to have been plenty of water on the planet. At present, on the contrary, there is no water in the liquid form on its surface: at the average pressure of 6 millibar water evaporates at 0 degrees centigrade, in other words it sublimates. Probably, if there is water on Mars, it is trapped in the frozen subsoil of the planet in the form of permafrost, that is frozen soil.
 

System of dry "rivers" and "canals" that seem to have been gradually eroded by water. It probably existed on Mars, in the past. (Calvin J. Hamilton)

Island in the martian soil, probably formed by water erosion. The craters that are seen in the picture have diameters 8-10 Km long, the height of the slope is approximately 500 metres. (Calvin J. Hamilton)

 

Portion of the Ravi Vallis canal. As many other canals, it is part of a region with a very indented and chaotic land, that of one of the oldest uplands of Mars. The structure of the canals indicates that in the past they contained flowing water. (Calvin J. Hamilton)

 

The martian atmosphere is very thin: the pressure on the planet ranges from a maximum of 9 millibar on the bottom of the depressions, to a minimum of 1.5 on the peaks of the mountains; on average, it is 6.1 millibar. 
Despite the very low atmospheric density, Mars is swept by very strong winds, that cause real sandstorms able to darken its surface even for months.
In such atmosphere there are yellowish clouds, which actually are sand vortices risen by the wind. They can reach the height of 50 Km, and hide the entire surface of the planet. Besides, between 10 and 20 Km from the ground, there are blue clouds composed of ice crystals.

A so rarefied atmosphere has a very low thermal inertia, in other words a low capacity of retaining heat: for this reason, the temperature changes very rapidly on the planet. The daily temperature range is therefore high: approximately 60 degrees.
The temperatures detected by the probes range from -123 to +22 °C, according to the zone of the planet and the season. 
 

Carbon dioxide is the main constituent of the martian atmosphere (95.3%); the other gases present are Nitrogen (2.7%), Argon (1.6%), Oxygen and other gases (0.4%), while aqueous vapour is less than 0.04%. 
Despite the abundance of carbon dioxide, the green house effect it produces is able to increase the temperature of the planet by only 5 degrees.
 
 
 
 

ANIMATIONS

The Marineris Valley,  AVI, 1.5 Mb (NASA/JPL)

The martian poles,   AVI, 1.4 Mb (NASA)

The Olympus Mons,  AVI, 3.5 Mb (NASA)