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| flattening = 0〔 | surface_area = | volume = | mass = | density = 〔 | surface_grav = | moment_of_inertia_factor = 〔 | escape_velocity = 4.25 km/s〔 | sidereal_day = | rot_velocity = | axial_tilt = (to orbit)〔 (0.034°)〔 | right_asc_north_pole = | declination = 61.45°〔 | albedo = | magnitude = −2.6〔 to 5.7〔〔 | angular_size = 4.5–13″〔 | temperatures = yes | temp_name1 = 0°N, 0°W | min_temp_1 = 100 K | mean_temp_1 = 340 K | max_temp_1 = 700 K | temp_name2 = 85°N, 0°W〔 | min_temp_2 = 80 K | mean_temp_2 = 200 K | max_temp_2 = 380 K | pronounced = | adjectives = Mercurian,〔 Hermian | atmosphere = yes | surface_pressure = trace | atmosphere_ref=〔 | atmosphere_composition = }} Mercury is the smallest and closest to the Sun of the eight planets in the Solar System, with an orbital period of about 88 Earth days. Seen from Earth, it appears to move around its orbit in about 116 days, which is much faster than any other planet in the Solar System. It has no known natural satellites. The planet is named after the Roman deity Mercury, the messenger to the gods. Because it has almost no atmosphere to retain heat, Mercury's surface experiences the greatest temperature variation of the planets in the Solar System, ranging from at night to during the day at some equatorial regions. The poles are constantly below . Mercury's axis has the smallest tilt of any of the Solar System's planets (about of a degree), but it has the largest orbital eccentricity. At aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercury's surface is heavily cratered and similar in appearance to the Moon, indicating that it has been geologically inactive for billions of years. Mercury is gravitationally locked and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun.〔(【引用サイトリンク】url=http://sciencenetlinks.com/interactives/messenger/or/OrbitRotation.html )〕 As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years. Because Mercury orbits the Sun within Earth's orbit (as does Venus), it can appear in Earth's sky in the morning or the evening, but not in the middle of the night. Also, like Venus and the Moon, it displays a complete range of phases as it moves around its orbit relative to Earth. Although Mercury can appear as a bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus. Two spacecraft have visited Mercury: flew by in the 1970s; and ''MESSENGER'', launched in 2004, orbited Mercury over 4,000 times in four years, before exhausting its fuel and crashing into the planet's surface on April 30, 2015.〔http://messenger.jhuapl.edu/news_room/details.php?id=284〕 == Internal structure == Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of . Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.〔 Mercury's density is the second highest in the Solar System at 5.427 g/cm3, only slightly less than Earth's density of 5.515 g/cm3.〔 If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm3 versus Earth's 4.4 g/cm3. Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron. Geologists estimate that Mercury's core occupies about 42% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten core.〔 Surrounding the core is a 500–700 km mantle consisting of silicates.〔Gallant, R. 1986. ''The National Geographic Picture Atlas of Our Universe''. National Geographic Society, 2nd edition.〕 Based on data from the mission and Earth-based observation, Mercury's crust is estimated to be 100–300 km thick.〔 One distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had already solidified. Mercury's core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass.〔 Early in the Solar System's history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass and several thousand kilometers across.〔 The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.〔 A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of the Moon.〔 Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. It would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and possibly even as high as 10,000 K. Much of Mercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" that could have been carried away by the solar wind.〔 A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material and not gathered by Mercury. Each hypothesis predicts a different surface composition, and two space missions, ''MESSENGER'' and ''BepiColombo'', both will make observations to test them. ''MESSENGER'' has found higher-than-expected potassium and sulfur levels on the surface, suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because potassium and sulfur would have been driven off by the extreme heat of these events. The findings would seem to favor the third hypothesis; however, further analysis of the data is needed.〔(【引用サイトリンク】url=http://www.rsc.org/chemistryworld/News/2011/September/30091103.asp )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Mercury (planet)」の詳細全文を読む スポンサード リンク
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