Mars (planet)

Article

August 15, 2022

Mars (pronounced in English: /maʁs/) is the fourth planet in the Solar System in ascending order of distance from the Sun and the second in ascending order of size and mass. Its distance from the Sun is between 1.381 and 1.666 AU (206.6 to 249.2 million kilometers), with an orbital period of 669.58 Martian days (686.71 days or 1.88 Earth years). It is a telluric planet, like Mercury, Venus and the Earth, about ten times less massive than the Earth but ten times more massive than the Moon. Its topography presents analogies both with the Moon, through its craters and impact basins, and with the Earth, with formations of tectonic and climatic origin such as volcanoes, rifts, valleys, mesas , dune fields and polar caps. The highest volcano in the Solar System, Olympus Mons (which is a shield volcano), and the largest canyon, Valles Marineris, are found on Mars. Mars has now lost almost all of its internal geological activity, and only minor events would still occasionally occur on its surface, such as landslides, probably CO2 geysers in the polar regions, possibly earthquakes , even rare volcanic eruptions in the form of small lava flows. The period of rotation of Mars is of the same order as that of the Earth and its obliquity gives it a cycle of seasons similar to that which we know; these seasons are, however, marked by an orbital eccentricity five and a half times higher than that of the Earth, hence a markedly more pronounced seasonal asymmetry between the two hemispheres. Mars can be observed with the naked eye, with a brightness much weaker than that of Venus but which can, during close oppositions, exceed the maximum brightness of Jupiter, reaching an apparent magnitude of -2.91, while its apparent diameter varies from 25.1 to 3.5 seconds of arc depending on whether its distance from the Earth varies from 55.7 to 401.3 million kilometres. Mars has always been visually characterized by its red color, due to the abundance of amorphous hematite — iron(III) oxide — on its surface. This is what has made it associated with war since antiquity, hence its name in the West after the god Mars of war in Roman mythology, equated with the god Ares of Greek mythology. In French, Mars is often nicknamed "the red planet" because of this particular color. Before the flyby of Mars by Mariner 4 in 1965, it was thought that there was liquid water on the surface and that forms of life similar to those existing on Earth could have developed there, a very fruitful theme in science fiction. Seasonal variations in albedo on the planet's surface were attributed to vegetation, while rectilinear formations seen in astronomical telescopes and telescopes of the time were interpreted, notably by the American amateur astronomer Percival Lowell, as irrigation canals crossing desert expanses with water from the polar ice caps. All these speculations were swept away by the space probes which studied Mars: as early as 1965, Mariner 4 made it possible to discover a planet devoid of a global magnetic field, with a cratered surface reminiscent of that of the Moon, and a tenuous atmosphere. Since then, Mars has been the subject of more ambitious exploration programs than for any other object in the Solar System: of all the stars we know, it is in fact the one that presents the environment with the most similarities to that of our planet. This intensive exploration has given us a much better understanding of the Martian geological history, revealing in particular the existence of a remote era — the Noachian —