Data from Wikipedia and NASA (C)


Mercury is the first planet of the solar system in order of distance from the Sun and is the smallest in dimensions. It is a planet of modest dimensions, with an diameter inferior to the half of that earthling; many craterized, also because of the lack of an appreciable atmosphere that can mitigate the meteoric impacts, and for this its aspect remembers they of the Moon close.


It is the most close planet to the sun, not simply osservable, but however already famous to the ancient populations, like Egizi, Chinese, Sumeri (third millenium a.C.). The difficulties of seeing depend on the small distance from the Sun, than disturbs always the vision of it during the twilight or little before the dawn. The Sumeri called it Ubu-idim-gud-ud; the Babylonian, than has handed to us the first detailed observation of planets, used the names gu-to or gu-utu. The Greeks assigned to Mercury two names: Apollo, the star of the morning, and Hermes, the star of the evening. The realization of the fact that was be a matter of an only planet is attributed to Pitagora. Always because of the great osservative difficulties, until the ' 900 it was common opinion that in truth an other planet was most close of Mercury to the Sun still, Volcano, later on identified with the same celestial body. More recently, in the 1631 Pierre Gassendi was the first one to observe a transit of Mercury in front of the Sun, second the forecasts supplied from Giovanni Keplero. In the 1639 Giovanni Zupi Batiste, using a telescope, discovered the phases of Mercury, analogous to those of Venus and the Moon. This supplied the definitive test that Mercury orbits around to the Sun. Only in years Sixty of XX century, thanks to the observations radio and radar, the period of rotation of the planet has been estimated with precision, than before it was believed equal to that one of revolution.

Observation from Earth

Dealing of an inner planet regarding the Earth, Mercury appears always much neighbor to the Sun (the its maximum elongazione is of 28,3°), so that the Earth’s telescopes can observe it of only rarely. Its appearing magnitude oscillates between -0,4 and +5,5 according to its position regarding the Earth and to the Sun. During the day the solar brightness prevents every observation, and the direct observation is possible only after the sunset, on the horizon to the west, or little before the dawn towards east. Moreover the extreme brevity of its motion of revolution (only 88 days) allows its observation only for little consecutive days, after that the planet becomes inosservable from Earth. Like in the case of the Moon and Venus, also for Mercury is visible, from Earth, a cycle of phases.

Orbital parameters

The orbit of Mercury is elliptic in first approximation; it is in fact subject to the precession of the perihelion, effect that put in difficulty the astronomers of XIX century. It turns out explicable at the present moment only  through the theory of the general relativity, than just on this phenomenon it has had one of its test benches. Mercury moves on an orbit of eccentricity 0,2056, to a distance from the Sun comprised between 46 and 69 million km, with a medium value of 58 million km (respective 0,307, 0,466 and 0,387 astronomical units). The sidereal period of Mercury is 88 days, while the sinodic period is 115,9 days. The orbital plan is tilted on the ecliptic of 7º. The Mercury’s orbit is subject to variations, due to the perturbations from part of other planets; the phenomenon is particularly studied and known as far as the motion of the line of the apsidi, that it supplies one of the tests experiences of the theory of the general relativity. The sidereal average speed of the planet is equal to 48 km/s; it is the highest between planets of the solar system. The mercurian motion of rotation is much slow: it employs 58,6 days in order to complete a turn on if same, and completes therefore three rotations every two revolutions (a clear example of orbital resonance). This makes yes that the duration of the exposure to the solar beams for every point of its surface is many elevated (176 days): Mercury is the only planet of the solar system on which the duration of the day, understanding like insolation, is greater of a period of revolution. A particular phenomenon relative to Mercury is the so-called precession of the perihelion, that is the perihelion of its orbit rotates progressively around the Sun in the same direction of the planet; the entity of the movement exceeds of 43 second of arc for century according to the Newtonian mechanics and can be justified with discreet approximation applying the relativistiche corrections, but it is still surveying object. The Mercury inclination regarding the plan of its orbit, is only 2°, and renders minims the variations of insolation during the seasons, rendering the Equator particularly warm and considerably cold the polar regions in whichever period of the year, without appreciable differences between the two hemispheres. In polar craters perennially protected from the beams of the Sun it could be contained iced water. Mercury is a decidedly small planet, being characterized from a mass equal to 0,055 earth’s masses and from a diameter equivalent to approximately 0,38 times that earthling; its medium density is of 2% inferior to that one of the Earth.


Analogous to the Moon, because of its low gravitational attraction Mercury is unprovided of atmosphere, made exception for exiles gas traces probably fruit of the interaction of the solar wind with the surface of the planet. The atmospheric composition has been determined like follows: potassio (31.7%), sodio (24.9%), atomic oxygen (9.5%), argon (7.0%) helium (5.9%), molecular oxygen (5.6%), nitrogen (5.2%), carbon dioxide (3.6%), water (3.4%), hydrogen (3.2%). The atmospheric pressure to the ground, measured from the probe Mariner 10, is in the order of thousandth of pascal. Because of the absence of a mechanism of distribution of the heat received from the Sun and of its extremely slow rotation, that the same hemisphere is exposed to the directed solar light for long periods, the thermal excursion on Mercury is the most elevated in the entire solar system; the illuminated hemisphere catches up the 600 K (700 K in the equatorial zones), that one in shadow often comes down until 90 K.


Similarly to the Moon, the mercurian ground is wide craterized because of the numerous asteroid impacts that have marked its past and has river basins filled up from old lavic taps, still obvious because of the nearly absolute lack of an atmosphere. Some craters are encircle from radii. The presence of tettonic plates is excluded on the planet. In truth, not only Mercury and the Moon have had hit with meteorites; it is however normal that the planets in possession of an consisting atmosphere resent in much smaller measure of the effect of the impacts, since the incidents bodies come strongly eroded from the atmospheric friction. Moreover the same atmosphere slowly erodes the surface of the planet, cancelling the traces of the collision. The smaller craters of  Mercury have diameter smaller of 10 km, those largest ones exceed the 200 km and take the name of river basins. In the center of many craters, often filled up from ancient still obvious lavic taps, they raise small mountly formations. The larger and more famous river basin is Caloris Sea, from the diameter of approximately 1300 km: it is a great circular plain encircled from mount rings. This river basin must its name to the fact that is always exposed in the light of the sun during the passage of Mercury to the perihelion and therefore is one of the warmer points of the planet. The Mercury surface has breaking that cross the edge of craters: these last ones have been probably provoked from the contraction of the crust. The reduced distance of Mercury from the Sun and the absence of atmosphere renders the planet with a great thermal excursion, with temperatures to 350° in the zone exposed to the sun, while in the part in shadow they arrive to -170°. the medium insolation of the mercurian surface is equal to approximately 6 times that one of the Earth; the solar constant has a value of 9,13 kW/m².


Beyond to craters, the more important structures of the surface are wide plainy zones, similar to the seas of the Moon. The greater one is Planitia Caloris (from the Latin planitia, that it means plain). The plains of Mercury, analogous to the lunar seas but with a greater albedo, come said planitiae. With the exception of Planitia Caloris, the most extended plainy region is Borealis Planitia, that it occupies part of the northern hemisphere, all the other plains take the Mercury name in the various languages. The four valley (valles) take to the name of the radio installations on the Earth thanks to which it has been determined the effective period of rotation of mercury. The names of slopes (rupes) derive from the ships used for the great explorations of XV century, with reference to the Mercury role like God of the travels and the commerce. Two slopes take the name of dorsa and are entitle to astronomers who observed Mercury from the Earth. The names of craters derive from the great exponents of the arts and the umanesimo, except the crater Kuiper, that it was the first one to be discovered being in how much visible one from far away and was entitled to the astronomer, and the crater Hun Kal, from the Mayan term "20", since conventionally is crossed from the twentieth meridian of longitude and is the point of reference for the mercurian system of coordinates.

Inner structure

The density of Mercury, equal to 5,43 kg/dm³, is very different much from that lunar one and, to the contrary, is much neighbor to that earthling. This leaves to suppose that, in spite of the likenesses with the Moon, the inner structure of the planet is most near that one of the Earth, with a particularly massive nucleus (until 80% of the mercurian radius) formed from heavy elements. Searches published in 2007 on Science, lead with radar of high precision in the United States and Russia, have confirmed the idea of one liquid fraction in the ferronickel nucleus. It is therefore possible to distinguish a solid inner nucleus and a liquid external nucleus. The maintenance of a liquid nucleus for billions of years demands the presence of a lighter element, like sulfur, than lowers the temperature of fusion of the materials. The idea that the Mercury nucleus could be liquid already was advanced in order to explain the presence of a weak magnetic field around the planet (found for the first time from Mariner 10 and quantified in cent of that earthling). The field difficultly remains however explicable, dates the small dimensions of Mercury and its moderated speed of rotation. It supposes that the nucleus is encircled from a cape and one thick crust. The inner planets like Mercury, Venus, Earth and Mars are formed  from the condensation of the members of the primordial cloud of gas and powders that has originated the entire Solar System, in a zone rich of silicate and a ferrous zone, to the contrary of the external planets whose zone of formation was rich of water, hydrogen, methane and ammonia. Currently the geologic Mercury activity is stopped and of the presence of very many craters of impact is the test. In the inside of the planet exist a iron nucleus and nichel that it occupies seven parts on ten of its radius (1800 km). The external layer is similar to that earthling that is composed of silicati, and it extends little more in order than 500 km; density is much elevating, second to that one of the Earth. On the surface they are present breaking a lot emphasizes with advanced heights to 4000 m and lengths of the order of hundred of kilometers. The main developed theory in order to supply an explanation to such observations, famous like theory of the planetary contraction, previews that the planet would be being been compressing, contracting itselfwhile its nucleus is cooled off slowly.

Mercury exploration

Mercury is visited for the first time in the 1974-75 from the American probe Mariner 10, that it has televised to earth photographies recorded in the course of three successes to you overflights. Conceived for the observation of Venus and Mercury, Mariner 10 came launch on 3 November 1973 and caught up the planet in 1974. The American probe was approached until to some hundreds of kilometers from the planet, transmitting approximately 6000 photographies and mapping 40% of the mercurian surface. NASA has launch in 2004 probe MESSENGER, whose first drawn near Mercury passage, previewed for 14 January 2008, will be followed from others two encounter (October 2008 and september 2009) before income in orbit around the planet previewed for 18 March 2011. For 2009 instead is previewed the launch, from part of the ESA, the mission spaces BepiColombo, so called in honor of the scientist, mathematician and engineer Giuseppe  Columbus (1920-1984), for the exploration of the inner planet. The Mercury exploration has not been thought primary from the agencies spaces for two reasons: Mercury is a planet from the excessive temperatures, is not thought place adapted to the existence of life shapes, neither it appears simple to take advantage of the resources, and from one first analysis of our acquaintances was derived the idea that Mercury was the much most similar one to the Moon of how much does not join today and that therefore it would have been enough to study this last one for derivere information on the planet. An other important reason that it has contributed to reduce the number of missions towards Mercury is the high intrinsic cost dictated from the vicinity of the Sun, that it determines: the necessity that every probe is equipped of a heavy thermal shield , an elevated fuel consumption in order to contrast the increment in the attractive force of the Sun, an elevated fuel consumption in order to place and to maintain the probe in orbit around to the planet, without the aerobraking  or the parachute in order to help the landing of a lander.