Mercury, definition and study. A-Z index of Cognitio.
It is the smallest and its orbit is also the most eccentric (ie, the least circular) of the eight planets. Known since the time of the Sumerians, its name, taken from Greek mythology, derives from that of the messenger of the Gods, probably because of its speed of movement in the sky.
The Romans, however, considered him as the God of commerce and the protector of thieves among the twelve Gods.
Being an inner planet with respect to the Earth, Mercury always appears very close to the Sun, to the point that terrestrial telescopes can rarely observe it.
During the day the solar brightness prevents any observation, and direct observation is possible only immediately after sunset, on the horizon to the west, or just before dawn towards the east. Moreover, the extreme brevity of its revolutionary motion (only 88 days) allows observation only for a few consecutive days, after which the planet becomes undetectable from the Earth. To avoid damage to instruments, the Hubble Space Telescope is never used to capture images of the planet.
As in the case of the Moon and Venus, even in the case of Mercury from the Earth a cycle of phases is visible, although it is quite difficult to realize it with amateur instruments.
Mercury was first visited in 1974 by the US spacecraft Mariner 10, which televised photos recorded during three successive over flights.
The anomalies observed in the orbit of the planet made Urbain Le Verrier hypothesize in 1859 the existence of another planet, which he called Vulcan; it was assumed that the orbit of Vulcan took place entirely within that of Mercury. The first to give a correct explanation of the anomalies of the procession of the perihelion of the orbit of Mercury was Albert Einstein thanks to general relativity in 1915. A demonstration of the “bizarre” orbit of Mercury is the fact that the Sun, seen by Mercury, follows a very unusual route: it climbs to the zenith, stops, backs away a little, stops again and finally lowers towards sunset.
Geologists estimate that the core of Mercury occupies about 42% of its volume, while for the Earth this percentage is 17%. A research published in 2007, combined with the presence of a weak magnetic field, suggests that Mercury possesses an electrically conductive fused metal core surrounded by a 500-700 km thick layer of silicates. Based on the data from the Mariner 10 and observations made by the Earth, the crust of Mercury is believed to be 100-300 km thick. A distinctive feature of the surface of Mercury is the presence of numerous narrow ridges, which extend up to several hundred kilometers in length. These are believed to have formed from the cooling and contraction of the core and mantle, following the solidification of the crust. The core of Mercury has a higher iron content than any other great planet in the solar system, and several theories have been proposed to explain this feature. The most reliable theory is that originally Mercury had a metal-silicate ratio similar to the common chondrite meteorites, which constitute the typical rock material present in the solar system, and had a mass of about 2.25 times the current one. When the solar system was forming, Mercury may have been hit by a planet of about 1/6 of its mass and several thousand kilometers in diameter.
The impact would have wiped out much of the crust and mantle present at that time, leaving the core as the predominant component of the celestial body. A similar process, known as the giant impact theory, has been proposed to explain the formation of the Moon.
The first photographs of the surface are due to the Greek-French astronomer Eugène M. Antoniadi (1870 – 1944) who at the beginning of the twentieth century drew maps of this planet.
Similarly to the Moon, Mercurian soil is largely cratered due to the numerous impacts of asteroids that have marked its past and presents basins filled with old lava flows, still evident due to the almost absolute lack of an atmosphere.
Some craters are surrounded by rays. The presence of tectonic plates on the planet is excluded. Because of its low gravitational attraction, Mercury is devoid of a real atmosphere like that of Earth, except for slender traces of gas probably the result of the interaction of the solar wind with the surface of the planet.
The sky of Mercury would be black even during the day, since the planet does not have an atmosphere around it.
The orbit of Mercury is indeed rather eccentric, and the distance of the planet from our star varies considerably during “his” year. The name Mercury derives from Roman mythology, and although it was of Etruscan origin (Turms), it was the correspondent of the Greek God Hermes, who according to Greek mythology was born of a fleeting relationship between Zeus and Maia, the most beautiful of the Pleiades. On the surface of Mercury is a mysterious substance whose nature scientists can not yet decipher. This was revealed by the images collected by the NASA Messenger probe during its close passage to the first planet in the solar system. The substance is bluish in color and Mark Robinson of Arizona State University named it «mysterious dark blue material».
It is hypothesized that it was ejected from the heart of the planet during volcanic eruptions which were numerous in an age between 3.8 and 4 billion years ago by distributing lava in large quantities.
Mercury, definition and study. A-Z index of Cognitio.
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