Mars
After Earth, we find Mars at an average distance of 1.5 astronomical units from the Sun, or 230 million kilometers. Unlike other planets, Mars has a very similar rotation period of the Earth, the alternation between day and night is therefore in line on our planet.
The inclination of the axis of rotation relative to the orbital plane also has a similar value, which leads the planet to be subjected to a similar cycle of seasons to that of the Earth, slightly slower since the year Mars is longer than ours.
View of the Earth, the planet appears generally reddish, with some dark areas and whitish polar regions. With the changing seasons, its appearance changes a lot. In summer, the polar regions are shrinking and the dark areas are expanding. In winter, the white cover on the poles is very marked and the dark areas are more discreet.
These dark areas are darker rocks regions and their appearance change is probably due to a layer of variable thickness dust with the seasons. The red appearance of the planet is in turn due to the presence of iron oxide.
The atmosphere of Mars
Mars has a diameter of 6800 km. It has a very thin atmosphere with a pressure less than a percent of the land value. The atmosphere consists of carbon dioxide greater than 95 percent, a small amount of nitrogen, argon and oxygen, and trace amounts of other gases. There are also some water vapor, sufficient to give birth to ice clouds or fog.
Images taken from the surface show that the sky appears orange, which is likely due to fine dust particles in the atmosphere.
The temperature at the surface of Mars varies between a minimum of about -140 degrees Celsius at night and daytime maximum of 0 degrees in winter and 20 degrees in summer.
The atmosphere of Mars is sometimes animated formidable storms that cover the entire planet and can last several months. The surface is then completely hidden by dust raised by the wind. This is for example the product early in the Mariner 9 probe had been unable to observe the surface for several weeks.
The surface of Mars
The probes revealed a fascinating and rich surface different types of formations: extinct volcanoes, craters, canyons and dry river beds. These curricula are not uniformly distributed over the planet, but rather consolidated in a given hemisphere.
The northern hemisphere of Mars is dominated by volcanic formations. It includes in particular two areas where many volcanoes are concentrated: the dome of Tharsis, with 3 volcanoes whose height exceeds 20 kilometers, and on the other side of the planet, Elysium Planitia.
Tharsis the dome is near Olympus Mons, the largest volcano in the solar system that culminates at 26 km altitude with a base 600 km in diameter.
Martian volcanoes are the same type as the volcanoes of Hawaii, with flanks very gentle slope. They are due to the presence of a hot spot in the coat which ejects the washer outwardly through the crust.
Their sheer size is probably related to the lack of plate tectonics. On Earth, because of tectonics, the crust moves relative to the hot spot, producing a succession of small volcanoes. On Mars, the crust is set and lava accumulation in a point form gradually huge volcanoes.
From a study of the number of these craters in volcanic areas, it has been possible to date them approximate: the Tharsis dome is such relatively young with only a few hundred million years.
The southern hemisphere is very different, dominated by old impact craters several billion years. Unlike lunar craters whose appearance does not change with time, the Martian craters are subject to erosion from the atmosphere, which alters the shape, rounded edges and covers them inside a thick layer of dust.
Un panorama de la surface martienne pris lors de la mission Pathfinder en 1997
The satellites of Mars
Let's end this quick overview of Mars, noting that the planet has two small satellites, Phobos and Deimos, with a dimension of about 10 kilometers. These satellites appear very irregular and are covered with craters. Due to the proximity of the asteroid belt and their irregular appearance, astronomers suspect them to be asteroids captured by Mars.
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